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LIGO/Virgo/KAGRA S250818k

GCN Circular 42032

Subject
LIGO/Virgo/KAGRA S250818k: further MeerKAT monitoring
Date
2025-09-30T17:24:39Z (13 days ago)
From
Gabriele Bruni at INAF <gabriele.bruni@inaf.it>
Via
Web form
G. Bruni [1], L. Piro [1], G. Gianfagna [1] and A. L. Thakur [1] report:

We carried out a third epoch of MeerKAT observations on the field of AT2025ulz (Stein et al., GCN 41414), in the localization region of the subthreshold GW event S250818k (LVK Collaboration, GCNs 41437, 41440). Observations at 3 GHz (S4 band) started on 2025-09-13, 12:07 UT (26 days post-trigger) for a total of 4 hours. 

Given the previously reported discrepancies between the data reduction for this target carried out with the SARAO Science Data Processor (SDP) and other pipelines (GCN 41666), we reprocessed the three epochs using also oxkat (Heywood 2020, Astrophysics Source Code Library). While the results of the two pipelines agree for the first and third epochs, we find that the SDP pipeline image of the second epoch exhibited a flux density bias of up to 100% for field sources below ~200 μJy (S/N up to ~30-40), which led us to an incorrect conclusion about the variability of the target (GCN 41594). 

The reprocessed data for the three MeerKAT epochs at 3 GHz show emission at a position consistent with the host galaxy and compatible with a constant flux of ~80+/-10 uJy, thus constraining variability due to a transient. This suggests that the radio source is currently dominated by diffuse emission, likely star formation, by the host galaxy.


The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation.
—------------------------
[1] INAF-IAPS

GCN Circular 41837

Subject
LIGO/Virgo/KAGRA S250818k: HCT optical follow-up
Date
2025-09-16T08:43:46Z (a month ago)
From
V. Swain at IIT Bombay <vishwajeet.s@iitb.ac.in>
Via
Web form
V. Swain (IITB), G. C. Anupama (IIA), D.K. Sahu (IIA), S. Barway (IIA),  H. Das (IIA), J. Basu (IIA), S. Chamoli (IIA), S. P. Singh (IAO), A. Tiwari (IAO), T. Mohan (IITB), A.P. Saikia (IITB), V. Bhalerao (IITB):

We observed the optical transient AT2025ulz (Stein et. al, GCN #41414) in the localization region of a candidate binary neutron star merger S250818k (LVK, GCN #41437) using the 2.0m Himalayan Chandra Telescope (HCT) of the Indian Astronomical Observatory (IAO). We obtained single exposure in the SDSS r' filter starting at 2025-09-11T16:04:33.16 UT (~24.61 days post trigger). Image subtraction was performed using a reference frame from CFHT/MegaCam, and the transient was detected in the resulting difference image. The derived photometry is reported below:

| JD (mid)       | Filter | Exposure (s) | Mag (AB)     | Limiting Mag (AB) |
| -------------- | ------ | ------------ | ------------ | ----------------- |
| 2460930.169828 | r'     | 600          | 20.82 ± 0.11 | 21.9              |

The magnitudes are calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.

Our result is consistent with the photometry followed by (Busmann et. al, GCN #41421; Hall et. al, GCN #41433; Karambelkar et. al, GCN #41436; Nicholl et. al, GCN #41439; Srivastav et. al, GCN #41451; O’Connor et. al, GCN #41452; Gillanders et. al, GCN #41454; Mo et. al, GCN #41456; Liu et. al, GCN #41461; Ackley et. al, GCN #41468; Klose et. al, GCN #41474; Banerjee et. al, GCN #41476; Perley et. al, GCN #41480; D'Avanzo et. al, GCN #41489; Malesani et. al, GCN #41492; Smartt et. al, GCN #41493; Santos et. al, GCN #41501; Becerra et. al, GCN #41502; An et. al, GCN #41503; Passaleva et. al, GCN #41504; Taguchi et. al, GCN #41505; Troja et. al, GCN #41506; Freeburn et. al, GCN #41507; Angulo et. al, GCN #41518; Antier et. al, GCN #41519; Banerjee et. al, GCN #41532; Lipunov et. al, GCN #41534; Busmann et. al, GCN #41535; Kasliwal et. al, GCN #41538; Gillanders et. al, GCN #41540; Becerra et. al, GCN #41544; Pankov et. al, GCN #41548).

Further observations are planned.

These observations were carried out under the ToO program HCT-2025-C3-P43. We thank the HCT staff for their support during the observations. The Indian Astronomical Observatory is operated by the Indian Institute of Astrophysics, Bengaluru, India.

GCN Circular 41666

Subject
LIGO/VIRGO/KAGRA S250818k: No evidence for radio variability of AT2025ulz in MeerKAT 3 GHz data
Date
2025-09-03T04:23:58Z (a month ago)
From
Kunal Mooley at NRAO,Caltech <kmooley@caltech.edu>
Via
email
Lauren Rhodes (McGill U.), Oleg Smirnov (Rhodes U. / SARAO), Kunal Mooley (IIT Kanpur, Caltech), Patrick Woudt (U. Cape Town)

We reprocessed both MeerKAT S-band (3 GHz) datasets of the AT2025ulz (Stein et al., GCN 41414) field reported previously (Bruni et al. GCNs 41594, 41500) in the context of the subthreshold GW event S250818k (LVK Collaboration, GCNs 41437, 41440). Our reprocessing reveals that in both datasets, from Aug 28 (10 days post-GW-event) and Aug 21 (4 days post-GW-event), the source has a constant integrated flux density of ~90+/10 uJy. Thus, we do not find any evidence for radio variability. Further, the extended nature of the radio source indicates that the emission is host galaxy dominated.

We caution all users of the SARAO Science Data Processor (SDP) continuum pipeline that the pipeline results must be carefully assessed before dissemination across the astronomical community. In the Aug 28 SDP images, there were large-scale artefacts, likely RFI-induced, that may be responsible for the excess flux measurement reported in GCN 41594.

The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation.

GCN Circular 41594

Subject
LIGO/Virgo/KAGRA S250818k: MeerKAT detection of an increase in radio flux from AT2025ulz
Date
2025-08-29T18:07:17Z (a month ago)
From
Gabriele Bruni at INAF <gabriele.bruni@inaf.it>
Via
Web form
G. Bruni [1], L. Piro [1], G. Gianfagna [1] and A. L. Thakur [1] report:

We re-observed the field of AT2025ulz (announced by Stein et al., GCN 41414), which is proposed as a candidate counterpart for the subthreshold GW event S250818k (LVK Collaboration, GCNs 41437, 41440) with the MeerKAT radio telescope under the program SCI-20241101-GB-01 (PI: Bruni) at 3 GHz (S4 band) starting on Aug 28, 14:20 UT (at 10 days post-trigger) for a total of 4 hours (3.23 hours on source). J1939-6342 was used for flux scale calibration, and J1609+2641 for phase referencing. Data was processed with the SARAO Science Data Processor (SDP) continuum pipeline. The image RMS was 5.3 uJy/beam.

We detect a clear flux increase in the radio emission with respect to the first epoch at 3.6 days post trigger (GCN 41500), with an integrated flux density of 157 +/- 9 uJy. The increment with respect to the first epoch is thus ~80 uJy.

Further MeerKAT observations are planned.

The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation.
—------------------------
[1] INAF-IAPS

GCN Circular 41577

Subject
LIGO/Virgo/KAGRA S250818k: uGMRT 1.3 GHz observations of AT2025ulz
Date
2025-08-28T14:58:07Z (2 months ago)
From
pchandra@nrao.edu
Via
Web form
G. Bruni (INAF-IAPS), 
P. Chandra (NRAO), 
S. Chatterjee(Rhodes University)
C. Christy (Arizona),
R. Kale (NCRA-TIFR), 
T. Laskar (Utah), 
S. Mohnani (IIT Indore),
S. R. Das (IIST), 
L. Resmi (IIST), 
Roberto Ricci (URome)
and
E. Troja (URome)
report on behalf of multiple teams:

"We observed AT2025ulz (Stein et al. GCN 41414) with the Giant Metrewave Radio Telescope (GMRT) under programs 48_081 (PI: Mohnani), 48_090 (PI: Troja), 48_096 (PI: Chandra), 48_097 (PI: Bruni), 48_120 (PI: Laskar), and 48_151 (PI: Resmi) beginning on 2025 Aug 26 at 13:30 UT (~8.5 days after the LVK trigger) for 2 hours at Band 5 (1.3 GHz).

In preliminary analysis, we do not detect significant radio emission at the position of AT2025ulz or the host. The image rms is ~ 23 microJy/beam, resulting in a 5-sigma upper limit of 115 uJy, at an angular resolution of 2.5x2.0 arcsec.

Together with the VLA 3 GHz non-detection at the position of AT2025ulz (<40 uJy, GCN 41542), our GMRT 1.3 GHz upper limit suggests that the radio emission previously detected with MeerKAT at lower angular resolution may include a contribution from the host galaxy (as already noted in the GCN 41500).

We thank the GMRT staff for scheduling and executing these observations. GMRT is operated by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research.

GCN Circular 41548

Subject
LIGO/Virgo/KAGRA S250818k: SAO RAS BTA z-band observations of AT2025ulz
Date
2025-08-26T11:49:10Z (2 months ago)
Edited On
2025-08-26T13:37:50Z (2 months ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), A. Moskvitin (SAO RAS), A. Pozanenko (IKI), S. Kotov (SAO RAS), A. Volnova (IKI), P. Minaev (IKI) report on behalf of the IKI-GRB-FuN collaboration:

We observed the field of an optical transient AT2025ulz detected by ZTF (Stein et. al, GCN 41414) in the localization region of a candidate binary neutron star merger S250818k (LVK, GCN 41437). The observations with BTA 6-meter telescope of SAO RAS started on 2025-08-22 at 17:18:46 UT, i.e. ~4.67 days since LVK trigger. We obtained 2.4 ks exposure in the z'-band using the Scorpio-2 multi-mode focal reducer (Afanasiev & Moiseev,  2011, BaltA, 20, 363) installed in the prime focus of the telescope. We applied image subtraction using LS DR10 z-band image as reference. We clearly detect the optical source in the difference image at the position given by ZTF (Stein et. al, GCN 41414). The preliminary photometry of the source is presented below:

Date       UTstart  Exptime,s T-T0,days Filter OT    Err  UL(3sigma) FWHM,"
---------- -------- -------   -------   --     ----- ---- ----       ---
2025-08-22 17:18:46 2400.13   4.67963   z'     24.09 0.24 24.3       1.8

The photometry was calibrated using comparison stars from SDSS-DR12 and not corrected for the Galactic extinction. The OT was also followed by (Busmann et. al, GCN 41421; Hall et. al, GCN 41433; Karambelkar et. al, GCN 41436; Nicholl et. al, GCN 41439; Srivastav et. al, GCN 41451; O’Connor et. al, GCN 41452; Gillanders et. al, GCN 41454; Mo et. al, GCN 41456; Liu et. al, GCN 41461; Ackley et. al, GCN 41468; Klose et. al, GCN 41474; Banerjee et. al, GCN 41476; Perley et. al, GCN 41480; D'Avanzo et. al, GCN 41489; Malesani et. al, GCN 41492; Smartt et. al, GCN 41493; Santos et. al, GCN 41501; Becerra et. al, GCN 41502; An et. al, GCN 41503; Passaleva et. al, GCN 41504; Taguchi et. al, GCN 41505; Troja et. al, GCN 41506; Freeburn et. al, GCN 41507; Angulo et. al, GCN 41518; Antier et. al, GCN 41519; Banerjee et. al, GCN 41532; Lipunov et. al, GCN 41534; Busmann et. al, GCN 41535; Kasliwal et. al, GCN 41538; Gillanders et. al, GCN 41540; Becerra et. al, GCN 41544). We note that our measurements may be affected during image subtraction and should be considered as a lower limit on the source brightness.

GCN Circular 41544

Subject
LIGO/Virgo/KAGRA S250818k: GTC/OSIRIS Confirmation of Rebrightening of AT2025ulz
Date
2025-08-25T19:06:17Z (2 months ago)
Edited On
2025-08-26T13:38:07Z (2 months ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (U Rome), Yuhan Yang (U Rome), Alan M. Watson (UNAM), Eleonora Troja (U Rome) and William H. Lee (UNAM) report on behalf of a larger team:

We continued monitoring AT2025ulz/ZTF25abjmnps, initially reported by Stein et al. (GCN 41414) within the localization region of the candidate gravitational wave event S250818k (LVK Collaboration, GCN 41437, 41440), using the OSIRIS instrument mounted on the GTC telescope. Our observations commenced at 2025-08-24 21:18 UTC (6.8 days after the trigger) and were carried out in the g, r, i and z filters at an average airmass of ~1.3.

By comparison with our first epoch at T+3.9 days (Becerra et al., GCN 41502), we confirm a significant rebrightening in both the r and z bands, measuring: 

delta_r = 0.9 +/- 0.3
delta_z = 0.6 +/- 0.1

Our observations are in agreement with the rebrightening observed in i-band (Freeburn et al. GCN 41507; Angulo et al. GCN 41518; Busmann et al. GCN 41535; Gillanders  et al. GCN 41540), a behavior that is not expected for kilonova emission. 

Further observations are planned. 

We thank the staff at the GTC, especially Antonio García Rodríguez, Antonio Marante Barreto, and Antonio Cabrera for the rapid execution of these observations.

GCN Circular 41542

Subject
LIGO/Virgo/KAGRA S250818k: VLA upper limits on AT2025ulz
Date
2025-08-25T14:13:32Z (2 months ago)
From
muskan.yadav@students.uniroma2.eu
Via
Web form
R. Ricci, M. Yadav, E. Troja (U Rome) report on behalf of the ERC BHianca team:

We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O’Connor et al. GCN 41452) within the localization of the candidate gravitational wave event S250818k (LVKC GCN 41437; GCN 41440) with the Very Large Array in S-band and C-band at the centre frequency of 3 GHz and 6 GHz respectively with a bandwidth of 2 GHz and 4 GHz on Aug 24st 2025, 5.9 days after the GW trigger. 

A possible radio counterpart with flux 77 +/- 16 microJy/beam at 3 GHz was claimed by Bruni et al. (GCN 41500). 
After reducing the data using the latest version of the CASA VLA pipeline (v6.6.1) and standard imaging procedures, we found no detection at the transient position down to a 3-sigma flux density upper limit of about 40 microJy/beam and 25 microJy/beam for 3 GHz and 6 GHz respectively. 

We thank the VLA staff for promptly executing the observations.

GCN Circular 41541

Subject
LIGO/Virgo/KAGRA S250818k: nature of the AT2025ulz/ZTF25abjmnnps OT
Date
2025-08-25T13:17:47Z (2 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email

V.M. Lipunov, (Lomonosov MSU),


In several recent telegrams (GCN 41532, 41538) the nature of the AT2025ulz/ZTF25abjmnps source is discussed. An alternative between the KN and SN II models has emerged. The unusual observable properties of the optical flare are emphasized. This alternative seems strange. For example, where can SN II appear in an elliptical galaxy?

In this regard, I propose  to include a third possible model, namely the merger of two heavy white dwarfs (O-Ne-Mg dwarfs) turning into a neutron star - a dumbbell with powerful radiation of gravitational waves [1].

As was shown earlier, such a phenomenon can generate an unusual form of gravitational wave impulse, which can be taken for a phenomenon caused by a terrestrial artifact. Is this not related to the fact that the initial probability of a terrestrial nature turned out to be more than 70%?


[1] Lipunov V.M., Double O-Ne-Mg white dwarfs merging as the source of the powerful gravitational waves for LIGO/VIRGO type interferometers., New Astronomy, Volume 56, p. 84-85, 2017 https://doi.org/10.1016/j.newast.2017.04.013

GCN Circular 41540

Subject
LIGO/Virgo/KAGRA S250818k: Pan-STARRS imaging confirms re-brightening of SN2025ulz
Date
2025-08-25T13:07:11Z (2 months ago)
From
James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>
Via
Web form
J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith (Oxford/QUB), S. Srivastav, F. Stoppa, H. Stevance, J. Tweddle (Oxford), M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), T.-W. Chen (NCU, Taiwan), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G. Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs (Harvard).

We have been observing the optical transient SN2025ulz (Banerjee et al., GCN 41532), formerly AT2025ulz (Stein et al., GCN 41414), the candidate optical counterpart associated with the sub-threshold gravitational wave event S250818k (LVK Collaboration, GCNs 41437, 41440) using the Pan-STARRS twin telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560); see also Gillanders et al. (GCN 41454) and Smartt et al. (GCN 41493).

Our observations span grizy-bands, but here we report on the optical re-brightening evident in our i-band observations. On each of MJDs 60909.26, 60910.26, 60911.26 and 60912.26, we observed SN2025ulz for a total exposure time of 1800s. The images were processed with the Pan-STARRS pipeline, where, after astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). For this process, we utilise proprietary data to compile significantly deeper reference stacks (~3400s) than those that are publicly available through the STScI website (~300s) and our stacked target images (1800s).

From these difference images, we measure the following preliminary AB magnitudes:

MJD         t-t0 (days)    Filter    AB mag
60909.26    4.2            i         22.4 +/- 0.3
60910.26    5.2            i         22.5 +/- 0.3
60911.26    6.2            i         22.1 +/- 0.2
60912.26    7.2            i         21.6 +/- 0.1

Here, t0 corresponds to the GW trigger time (LVK Collaboration, GCN 41437).

SN2025ulz has risen in our i-band observations by ~0.5 mag (~0.9 mag) in the last one day (two days). While there are barriers to accurately measuring the photometry of this source (e.g., it is relatively faint; it lies close to the bright nucleus and body of the host), our photometry values, and the inferred re-brightening, are robust, given our access to deep reference stacks. We note that these are vital to accurately infer the true brightness of the transient, as performing difference imaging with a shallower reference image can lead to systematic errors (as much as ~0.7 AB mag, in the specific case of SN2025ulz utilising the public reference stack available through the STScI website).

Re-brightening of SN2025ulz has been previously noted by Liu et al. (GCN 41461), Freeburn et al. (GCN 41507) and Angulo et al. (GCN 41518).

Our observed re-brightening supports the interpretation of SN2025ulz being a young SN, based on the VLT spectral observations reported by ENGRAVE (Banerjee et al., GCN 41532). Our consistent i-band rise is likely to suggest it is now on a radioactively powered rise following an initial shock-cooling phase, and thus indicates that SN2025ulz is likely not the optical counterpart to S250818k. 

Further Pan-STARRS observations are continuing to determine if the colour and lightcurve shape remains consistent with a type II or IIb supernova, to compare with the conclusions of Kasliwal et al. (GCN 41538).

Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and NNX14AM74G, issued through the SSO Near-Earth Object Observations Program. Data processing is enabled by Queen's University Belfast and the University of Oxford, enabled through STFC grants ST/Y001605/1, ST/T000198/1 and ST/X001253/1, the Royal Society, and the Hintze Centre for Astrophysical Surveys.

GCN Circular 41538

Subject
LIGO/Virgo/KAGRA S250818k: Continued Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz)
Date
2025-08-25T10:04:54Z (2 months ago)
From
Mansi Kasliwal at Caltech <mansikasliwal@gmail.com>
Via
Web form
Mansi M. Kasliwal (Caltech), Viraj Karambelkar (Caltech), Christoffer Fremling (Caltech), Tomas Ahumada (Caltech), Xander J. Hall (CMU), Daniel A. Perley (LJMU), Shreya Anand (Stanford), Chang Liu (Northwestern), Kaustav Das (Caltech), Varun Bhalerao (IIT-B), Vishwajeet Swain (IIT-B), Aditya Saikia (IIT-B)

report on behalf of a larger ZTF and GROWTH collaboration

We continue to observe ZTF25abjmnps (AT2025ulz; Stein et al. GCN 41414) with the LRIS spectrograph on Keck I. Our latest epoch starts at UTC 2025-08-25 06:50. We reduce the data with lpipe (Perley et al. 2019) and subtract a spectrum at an earlier epoch to understand the evolution (Karambelkar et al. GCN 41436).

As reported by Banerjee et al. GCN 41532, we also see a feature with an absorption minimum at 6720A. If this is Halpha, this would imply a velocity of 17000 km/s at z=0.0848 (we measure a higher velocity than what was reported in Banerjee et al. GCN 41532). 

However, we posit the classification of this transient as a supernova or a kilonova currently remains ambiguous. If it is a supernova, it is an unusual supernova for at least four reasons: (i) There is no robust (rlap>5) spectral match in the SNID library, (ii) No supernova light curve in the ZTF supernova library matches especially the blue-to-red color evolution at this early phase (e.g. Busmann et al. GCN 41535), (iii) The feature at 6720A is not smooth in shape and has additional structure, (iv) There appear to be other weak, broad undulations in the spectra. If it is a kilonova, all photometric data in-hand continue to be consistent with this hypothesis, however, additional theoretical modeling may be needed to fully explain the spectra.    

Looking ahead, we encourage continued follow-up of the panchromatic light curve, including the radio and X-ray bands, to unambiguously classify this source.

GCN Circular 41535

Subject
LIGO/Virgo/KAGRA S250818k: FTW Observations Show Continued Reddening of AT 2025ulz
Date
2025-08-25T09:09:19Z (2 months ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Via
Web form
Malte Busmann (LMU), Xander J. Hall (Carnegie Mellon U.), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), Antonella Palmese (Carnegie Mellon U.), and Mansi Kasliwal (Caltech) report:

We observed the source AT 2025ulz reported by Stein et al. (GCN 41414)  possible counterpart to the LVK sub-threshold event S250818k (GCN 41437, 41440), with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, i, and J bands simultaneously starting at 2025-08-24T19:33:25 for 20 x 180 s with a few interruptions due to clouds. The difference imaging with templates from the Legacy Survey for the g-band and PS1 for the i-band, we report that AT 2025ulz has g - i ~ 1.7 AB mag.

We note our differenced i-band brightness is consistent with observations reported by Freeburn et al. (GCN 41507) and Angulo et al. (GCN 41518).

The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction.

Further analysis is underway.

We thank Christoph Ries from the Wendelstein Observatory staff for obtaining these observations.

GCN Circular 41534

Subject
LIGO/Virgo/KAGRA S250818k: MASTER predicovery limits of the AT2025ulz/ZTF25abjmnps KN candidate
Date
2025-08-25T08:21:58Z (2 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email


V.Lipunov, E.Gorbovskoy, A.Kuznetsov, N.Tiurina, D.Vlasenko, P.Balanutsa,
I.Panchenko, A.Sankovich, A.Chasovnikov K.Zhirkov, G.Antipov, I.Gorbunov, A.Sosnovskij, Yu.Tselik, Ya.Kechin, V.Senik, V.Topolev (Lomonosov MSU),
O.Gress, N.Budnev, O.Ershova (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix
Aguilar (OAFA),
D.Buckley (SAAO),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)

Global MASTER  robotic net [1] observed the field with AT2025ulz/ZTF25abjmnps,  reported by Stein et al. (GCN 41414) and possibly associated with the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440) before and after trigger time.

 At the location of AT2025ulz the twin MASTER600-Tunka telescope (Cmos QHY 6060, FOW = 18 square degress)   did not  detect a source with next 5-sigma limits:

 Date    UT   Tstart-Ttrig  Exp    Filter  tube    lim
       (middle)   (day)     (s)

250815 14:55:18  -3.12       60     Clear   East    19.8
250822 16:52:05   3.95     8x60     Clear   West    20.9
250823 15:37:06   4.90    38x60     Clear   West    21.8

The first observation was a lucky coincidence.
Since there was no automatic LVC alert,
subsequent observations had to be carried out in the Internet telescope mode, taking into account the specific quality of the weather.

[1] V.M. Lipunov, V.G. Kornilov, E.Gorbovskoy, N. Tiurina & A.Kuznetsov,
Astronomical Robotic Networks and Operative Multichanel Astrophysics,
Lomonosov MSU PRESS, 591pp.
http://www.pereplet.ru/lipunov/625.html#625

GCN Circular 41532

Subject
LIGO/Virgo/KAGRA S250818k: ENGRAVE observations of SN 2025ulz as a type II supernova
Date
2025-08-24T20:46:51Z (2 months ago)
From
Antonio Martin-Carrillo at UCD,Space Science Group <antonio.martin-carrillo@ucd.ie>
Via
Web form
Smaranika Banerjee (Stockholm University), Maria-Teresa Botticella (INAF - Capodimonte Obs.), Seán J. Brennan (MPE), Enrico Cappellaro (Padua Obs.), Ting-Wan Chen (NCU Taiwan), Paolo D'Avanzo (INAF - Brera Obs.), Valerio D’Elia (ASI-SSDC), Massimiliano De Pasquale (Univ. Messina), Rob A. J. Eyles-Ferris (Leicester), Morgan Fraser (UCD), James H. Gillanders (Oxford), Ben Gompertz (Birmingham), Nusrin Habeeb (Leicester), Luca Izzo (INAF/OAC and DARK/NBI), Peter G. Jonker (Radboud), Andrew J. Levan (Radboud and Warwick), Daniele Bjørn Malesani  (DAWN/NBI and Radboud), Antonio Martin-Carrillo (UCD), Matt Nicholl (QUB), Sam Oates (Lancaster U.), Silvia Piranomonte (INAF-OAR), Luigi Piro (INAF-IAPS), Andrea Rossi (INAF - Bologna), Om Sharan Salafia (INAF - Brera Obs.), Nikhil Sarin (Cambridge), Steve Schulze (Northwestern), Avinash Singh (Stockholm University), Stephen J. Smartt (Oxford), Albert Sneppen (DAWN/NBI), Jesper Sollerman (Stockholm), Danny Steeghs (Warwick), Nial R. Tanvir (Leicester), Aishwarya L. Thakur (INAF-IAPS), report for the ENGRAVE collaboration:

We report preliminary analysis from follow-up ESO VLT observations obtained by the ENGRAVE collaboration of SN 2025ulz (Stein et al., GCN 41414), which was discovered within the 3D localization region of the GW alert S250818k (LVK Collaboration, GCNs 41437, 41440).

Starting on 2025-08-24 00:03:40 UT, we observed the field of SN 2025ulz using the ESO VLT UT4 (Yepun) equipped with the MUSE integral-field spectrograph. A series of 4x700 s exposures were taken for this observation.

From a preliminary reduction, extraction of a spectrum at the position of SN 2025ulz reveals a continuum well detected over the full wavelength range of 5000 to 9000 Å. The spectrum shows a prominent broad feature which we interpret as a P-Cygni profile of the H-alpha line, similar to that observed in young type II or IIb supernovae. The peak of the emission and the absorption trough is consistent with the host galaxy redshift of z = 0.0848 (ENGRAVE GCN 41476,  Karambelkar et al., GCN 41436) and an expansion velocity of around 12,000 km/s. There is weaker evidence for an H-beta P-Cygni profile. Plausible matches to young type II and IIb SNe are found with SNID-SAGE (Stoppa et al. in prep, a new enhanced, python version of SNID), although the transient object continuum is contaminated by the host and the absorption line strengths are therefore diluted. We suggest that SN 2025ulz is a type II supernova (of unconfirmed subtype at this point) and is therefore unrelated to S250818k.

We acknowledge the excellent support from the ESO staff in Paranal, in particular Camila de Sa Freitas, Israel Blanchard and Sam Kim.

GCN Circular 41528

Subject
LIGO/Virgo/KAGRA S250818k: Swift Observations of AT 2025ulz - Second Epoch
Date
2025-08-24T16:00:28Z (2 months ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (U Rome), Eleonora Troja (U Rome) and Simone Dichiara (PSU) report:

We requested a second epoch of ToO observations of AT2025ulz/ZTF25abjmnps (Stein et al.; GCN 41414) discovered within the localization of the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440).

The observations started on 2025-08-22 at 21:56 UT, approximately 4.9 days after the trigger, with a total exposure of 3.2 ks. At the transient position, we detect no X-ray source down to a 3-sigma upper limit of 0.004 cts/s (0.3-10 keV), consistent with the result reported by Hall et al. (GCN 41453). Using a power-law with nH=2.2E+20 and a photon index=2, we convert the count rate into an unabsorbed flux of 1.5e-13 erg/cm2/s, which corresponds to a luminosity of 3e42 erg/s at z=0.0848 (Karambelkar et al. GCN 41436). 

In simultaneous UVOT observations performed with the u filter, we identify some faint diffuse emission likely related to the host galaxy and place 3-sigma upper limit u > 21 AB at the transient’s position, in agreement with the faint detection reported by Troja et al. (GCN 41506).

We thank the Swift team and the PI for promptly scheduling and making these observations possible.

GCN Circular 41519

Subject
LIGO/Virgo/KAGRA S250818k: CFH/MegaCam follow-up observation for AT2025ulz
Date
2025-08-24T07:14:08Z (2 months ago)
From
Sarah Antier at OCA <sarah.antier@oca.eu>
Via
Web form
S. Antier (OCA/IJCLAB), M. Pillas (Uliege), D. Akl (NYUAD), S. Karpov (FZU), M. Coughlin (UMN), P. Hello (IJCLAB), A. Jacquesson (IJCLAB) report on behalf on ZTF collaboration:

We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al., GCN 41414) over several nights, which has been reported by Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493; Bruni et al., GCN 41500; Santos et al., GCN 41501; Becerra et al., GCN 41502; An et al., GCN 41503, Angulo et al., GCN 41518 as a possible source to the Gravitational Wave Event S250818k (LVK collaboration GCN 41437; 41440). The observations were carried out using MegaCam on the CFHT.

We report here our preliminary measurement in r-band taken around 2025-08-21 06:50 UTC by co-adding several images and analyzed with STDPipe (Karpov 2025) with photometric calibration against Pan-STARRS. The quality of the sky during that night was not ideal.

r = 22.85 +/- 0.15 (AB)

Differential image analysis against archival CFH data has been performed. 

Further observations are planned.

We would like to thank the CFHT staff enabling these timely observations, in particular Nadine Manset and Heather Flewelling.

GCN Circular 41518

Subject
LIGO/Virgo/KAGRA S250818k: COLIBRÍ confirmation of rebrightening of AT2025ulz
Date
2025-08-24T06:21:08Z (2 months ago)
Edited On
2025-08-25T13:30:13Z (2 months ago)
From
Alan Watson at UNAM <alan@astro.unam.mx>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Alan Watson at UNAM <alan@astro.unam.mx>
Via
Web form
Camila Angulo (UNAM), Alan M. Watson (UNAM), Damien Dornic (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Fredd Sánchez Álvarez (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM) report:

We imaged the field of the AT2025ulz (Stein et al., GCN Circ. 41414) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed at two epochs, from 2025-08-22 03:19 to 05:29 UTC (4.13 days after the trigger) and from 2025-08-24 03:29 to 05:41 UTC (6.14 days after the trigger). At both epochs we obtained 96 minutes of exposure in the i filter.

The data were reduced and coadded with custom software and then analyzed in STDWeb (Karpov 2021). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

After subtracting a Pan-STARRS DR2 template image from our first epoch image (4.13 days after the trigger), we detect a source at the reported position of AT2025ulz with

i = 23.1 +/- 0.25

After subtracting a Pan-STARRS DR2 template image from our second epoch image (6.14 days after the trigger), we detect a source at the reported position of AT2025ulz with

i = 22.0 +/- 0.1

Subtracting the first epoch image from the second epoch image, we find a brightening that corresponds to

Delta i = 22.7 +/- 0.2

We caution that these analyses are preliminary, especially the photometry after subtracting the shallower PS template images. Nevertheless, our results would appear to confirm the rebrightening reported by Freeburn et al. (GCN Circ. 41507).

Further observations are planned.

We thank the staff of the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir and the COLIBRÍ and DDRAGO engineering teams.

COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.

GCN Circular 41507

Subject
LIGO/Virgo/KAGRA S250818k: Rebrightening detected with Gemini/GMOS
Date
2025-08-23T15:18:41Z (2 months ago)
From
James Freeburn at University of North Carolina at Chapel Hill <jamesfreeburn54@gmail.com>
Via
Web form
J. Freeburn (UNC), B. O’Connor (CMU), X. J. Hall (CMU), M. Busmann (LMU), I. Andreoni (UNC), A. Palmese (CMU), D. Gruen (LMU), L. Hu (CMU), T. Cabrera (CMU), K. Kunnumkai (CMU), A. Amsellem (CMU) report on behalf of a larger collaboration: 

Using Gemini/GMOS-N, starting at 2025-08-23 05:30:48 UTC, we observed the source AT 2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414) discovered during ZTF follow-up of the low-significance gravitational wave event S250818k (GCN 41437, 41440; https://gracedb.ligo.org/superevents/S250818k), which is a candidate binary neutron star merger. We took images in g and i bands.

We subtracted Gemini/GMOS-N observations taken on the previous night (starting at 2025-08-22 05:43:02) from these observations.  In the g-band, no source is detected in the difference image.   The i-band difference image reveals a positive source at the location of AT2025ulz/ZTF25abjmnps.  This positive detection corresponds to an increase in brightness of 0.63+/-0.09 microJanksy, which indicates rising emission in i-band at this phase.

Further observations are planned and additional follow-up is encouraged to further monitor the transient’s evolution.

We thank the staff of the Gemini Observatory, in particular Jen Andrews, Brian Lemaux, and Jen Miller, for their excellent support in rapidly scheduling these observations.

GCN Circular 41506

Subject
LIGO/Virgo/KAGRA S250818k: HST nIR detection of AT 2025ulz
Date
2025-08-23T14:34:22Z (2 months ago)
From
Eleonora <nora.gsfc@gmail.com>
Via
Web form
Eleonora Troja (U Rome), Brendan O’Connor (Carnegie Mellon U.), and Rosa L. Becerra (U Rome) report on behalf of a larger team:

We observed the field of AT2025ulz/ZTF25abjmnps, reported by Stein et al. (GCN 41414) and possibly associated with the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440) with the Hubble Space Telescope (PI: Troja) starting from 2025-08-22 20:04 UTC (~T_0+4.8 days).

The observations were carried out with the Wide-Field Camera 3 (WFC3) using the UVIS channel with the F336W filter and the IR channel with the F110W and F160W filters. Due to orbital constraints, we could acquire only short exposures of  ~120 s per filter.

At the location of AT2025ulz we clearly detect a source with preliminary AB magnitudes of:

F110W = 22.91 +/- 0.12
F160W = 22.8 +/- 0.3 (partial result based on 60 s exposure)
F336W = 23.9 +/ -0.4

The above values were derived from aperture photometry after subtracting a 2D GALFIT (Peng et al. 2004) model for the host galaxy, and are not corrected for Galactic extinction. 

The source is resolved in our nIR images and offset from the galactic center by ~0.9 arcsec, corresponding to a projected physical offset of ~1.5 kpc.  The measured nIR brightness matches well the kilonova AT2017gfo placed at the same distance of 388 Mpc. However, the blue color implied by the F336W observations is not consistent with a kilonova spectrum. 

At present we cannot determine whether the F336W flux is mostly dominated by the transient’s light, e.g. a rising supernova, or by host emission, e.g. underlying star-formation or a young star cluster. While the bulk of the galaxy is not detected in F336W, a star forming clump is still possible, and a clump is seen in the IR filters at the southern edge of the disk. Continued monitoring of the source is encouraged to assess its variability. 

We thank the HST staff, in particular Julia Roman-Duval, Patricia Royle, and William Januszweski for the rapid execution of these observations.

GCN Circular 41505

Subject
LIGO/Virgo/KAGRA S250818k: J-GEM follow-up observation for AT2025ulz
Date
2025-08-23T05:37:03Z (2 months ago)
From
Mahito Sasada at Tokyo Institute of Technology <sasada@hp.phys.titech.ac.jp>
Via
Web form
K. Taguchi (Kyoto U.), Y. Utsumi (NAOJ), M. Sasada (Science Tokyo), R. Itoh (Bisei Astronomical Observatory), T. Morokuma (Chiba Tech), K. Ohta (Kyoto U.), I. Takahashi (Science Tokyo), M. Tanaka (Tohoku), N. Tominaga (NAOJ), M. Yoshida (NAOJ), on behalf of J-GEM collaboration

We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al., GCN 41414), which has been reported by Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493; Bruni et al., GCN 41500; Santos et al., GCN 41501; Becerra et al., GCN 41502; An et al., GCN 41503, as a possible source to the Gravitational Wave Event S250818k (LVK collaboration GCN 41437; 41440), using TriCCS, a three-color simultaneous CMOS imager, on the 3.8m Seimei Telescope in Okayama, Japan.

We began the observation at 10:47:59 on Aug 20, 2025 (UT), 2.39 days after the merger.  Images in g, r, i, and z were integrated for 3899, 3899, 1919, and 1979 secs, respectively, resulting in coadded images with limiting magnitudes of 22.12, 21.87, 21.85, and 21.11 (AB, 5 sigma)

Differential image analysis against the PanSTARRS archival data (Chambers et al. 2016) using ZOGY (Zackay et al. 2016) has been attempted, but no obvious source has been revealed. 

A Sersic model of n=0.5 subtracted the host galaxy reasonably well, which revealed a possible remnant in g, r, i at the source location reported in GCN 41414; however, the forced aperture photometry at the reported location was consistent with the background sky fluctuation. 

Further analysis is underway.

GCN Circular 41504

Subject
LIGO/Virgo/KAGRA S250818k: PRIME nIR observations of AT2025ulz
Date
2025-08-22T23:39:31Z (2 months ago)
Edited On
2025-08-25T13:30:01Z (2 months ago)
From
N. Passaleva at Sapienza University of Rome <niccolo.passaleva@uniroma1.it>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of N. Passaleva at Sapienza University of Rome <niccolo.passaleva@uniroma1.it>
Via
Web form
N. Passaleva  (U Rome),  J. Durbak (UMD), O. Guiffreda (UMD), E. Troja (U Rome), R.Hamada (Osaka U), A. S. Kutyrev (NASA/GSFC), D. Suzuki (Osaka U), T. Sumi (Osaka U), A. Idei (Osaka U), D. Buckley (SAAO), S. B. Cenko (NASA/GSFC)

We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O'Connor et al. GCN 41452) possibly associated to the GW candidate S250818k (LVKC GCN 41437) in H filter and in J filter with PRIME ~87.62 hours and ~111.62 hours after the GW trigger.

Using  nearby stars from the Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006) for calibration we estimate a 3-sigma limiting magnitude of H>20 AB for the image.
Within the transient's host galaxy we see no obvious point source, although template subtraction is required to place a meaningful upper limit to its brightness.

PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024).

We thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations.

GCN Circular 41503

Subject
LIGO/Virgo/KAGRA S250818k: JinShan optical observations of AT2025ulz
Date
2025-08-22T18:13:18Z (2 months ago)
From
J. An <jiean0813@foxmail.com>
Via
Web form
J. An, X. Liu, Z.P. Zhu, S.Q. Jiang, L.B. He, D. Xu (NAOC), S.Y. Fu (HUST), J.Z. Liu (XAO) report on behalf of a large collaboration:

We observed the optical transient AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493; Bruni et al., GCN 41500; Santos et al., GCN 41501; Becerra et al., GCN 41502), possibly connected with the GW event S250818k (LVK Collaboration, GCNs 41437, 41440), using the 100C telescope of the JinShan project, located at Altay, Xinjiang, China. Our first epoch observation started on 2025 Aug 20.61 UT, i.e., 2.55 days after the GW trigger time, in the Sloan g-, r-, and i- filters with 1 hr exposure for each filter. 

By image subtraction with the Legacy Survey template, we obtained a provisional magnitude of r = 22.8 +/- 0.3 (AB) at 2025 August 20.63 UT for AT2025ulz.

We acknowledge the excellent support from T.Q. Chen and J.F. Zhang for enabling these observations.

GCN Circular 41502

Subject
LIGO/Virgo/KAGRA S250818k: GTC/OSIRIS Optical Detection of AT2025ulz
Date
2025-08-22T17:59:55Z (2 months ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (U Rome), Alan M. Watson (UNAM), and Eleonora Troja (U Rome) report on behalf of a larger team:

We observed the field of AT2025ulz/ZTF25abjmnps, reported by Stein et al. (GCN 41414) and possibly associated with the candidate gravitational wave signal S250818k (LVK Collaboration, GCN 41437, 41440), with the OSIRIS instrument mounted on the GTC telescope. Our observations began at 2025-08-21 21:40 UTC (3.9 days after the trigger) and were carried out in the r and z filters with an average airmass of ~1.3 and excellent seeing.

After image subtraction against Legacy Survey templates (Dey et al. 2019), we detect the source with a preliminary magnitude of r~23.4 AB, consistent with the near-contemporaneous observations of Malesani et al. (GCN 41492). 
We also confirm a very red color, in agreement with earlier observations (Hall et al., GCN 41433; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Perley et al., GCN 41480).

Further observations are planned.

We thank the staff at the GTC, especially Antonio García Rodríguez, Antonio Marante Barreto, and Antonio Cabrera for the rapid execution of these observations.

GCN Circular 41501

Subject
LIGO/Virgo/Kagra S250818k: STEP/T80N upper limits on 2025ulz
Date
2025-08-22T17:12:58Z (2 months ago)
From
André Santos at Centro Brasileiro de Pesquisas Físicas (CBPF) <andsouzasanttos@gmail.com>
Via
Web form
A. Santos (CBPF), C. R. Bom (CBPF), C. D. Kilpatrick (Northwestern), L. Santana-Silva (CBPF), P. Darc (CBPF), Gabriel Teixeira (CBPF), C. Mendes de Oliveira (IAG-USP) report on behalf of the STEP collaboration

We observed the optical transient AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480; P. D'Avanzo et al., GCN 41489; Malesani et al., GCN 41492; Smartt et al., GCN 41493), possibly associated with the GW superevent S250818k (LVK Collaboration, GCN 41437, GCN 41440), using the T80N 0.8-m robotic telescope using the 1.4 x 1.4 FoV T80N-Cam imager. 

The observations started on Aug 21, 21:46 UT (~3.85 days after trigger). We obtained 600s (2x300s)and 1200s (4x300s) stacked exposures in r and i bands respectively, with the T80-Cam centered at the position of the reported transient. Subtracting Pan-SSTARRS templates available at MAST archive using photpipe (Rest et al. 2005), we do not detect any sources in our difference images and derive 3-sigma limiting magnitudes of r > 22.9 and i > 22.8 mag for the transient.

We thank the OAJ Data Processing and Archiving Department (DPAD) for reducing and calibrating the OAJ data used in this work, as well as the distribution of the data products through a dedicated web portal.

GCN Circular 41500

Subject
LIGO/Virgo/KAGRA S250818k: 3 GHz MeerKAT observations of AT2025ulz
Date
2025-08-22T16:13:06Z (2 months ago)
From
Aishwarya L Thakur at INAF-IAPS, Rome <aishth@outlook.com>
Via
Web form
G. Bruni [1], L. Piro [1], G. Gianfagna [1] and A. L. Thakur [1] report:

We observed the field of AT2025ulz (announced by Stein et al., GCN 41414), which is proposed as a candidate counterpart for the subthreshold GW event S250818k (LVK Collaboration, GCNs 41437, 41440) with the MeerKAT radio telescope under the program SCI-20241101-GB-01 (PI: Bruni) at 3 GHz (S4 band) starting on Aug 21, 16:30 UT (at 3.6 days post-trigger) for a total of 2 hours (1.5 hours on source). J1939-6342 was used for flux scale calibration, and J1609+2641 for phase referencing. Data was processed with the SARAO Science Data Processor (SDP) continuum pipeline. The image RMS was 6.4 uJy/beam.

We detect radio emission at the position of the transient, and we measure an integrated flux density of 77 +/- 16 uJy (corresponding to a ~ 10-sigma detection). However, given the northern declination of the field, the angular resolution is insufficient to disentangle any potential contamination from the host with this epoch of observation alone.

Further MeerKAT observations are planned.

The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation.
—------------------------
[1] INAF-IAPS

GCN Circular 41493

Subject
LIGO/Virgo/KAGRA S250818k: Pan-STARRS coverage of the skymap and candidate counterparts
Date
2025-08-22T14:09:35Z (2 months ago)
Edited On
2025-09-04T17:15:43Z (a month ago)
From
James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>
Via
Web form
S. J. Smartt, J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Univ. Hawaii), K. W. Smith (Oxford/QUB), S. Srivastav, F. Stoppa, H. Stevance, J. Tweddle (Oxford), M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), T.-W. Chen (NCU, Taiwan), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G. Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs (Harvard).

We surveyed the skymap of the gravitational wave event S250818k (LVK Collaboration, GCNs 41437, 41440) using the Pan-STARRS twin telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560). Following on from the reported optical candidate AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Perley et al., GCN 41480, D’Avanzo et al. GCN 41489), we have been taking targeted grizy imaging of this transient (Gillanders et al., GCN 41454). At this specific region we have proprietary data to make significantly deeper reference stacks than those that are available through the STScI public website, and monitoring with Pan-STARRS is ongoing.

For the rest of the northern portion of the GW skymap, we surveyed the region in the i-band over three nights: 2025 Aug 20, 21, 22. We covered ~288 square degrees and ~33% of the bilby.fits skymap 90% area each night, with multiple 45s (Aug 20) and 120s (Aug 21, 22) exposures. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4). Transient candidates were identified and run through our standard filtering procedures, including rejection of artefacts with machine learning tools and cross-matching with galaxy, stellar and solar-system catalogs (e.g., Smith et al., 2020, PASP, 132, 1;  Smartt et al., 2024, MNRAS 528, 2299).

The depth of our images are typically i ~ 21 - 21.5 per exposure, with the first night having better seeing and transparency conditions. We detected 68 candidate extragalactic transients, of which 30 were already registered on the IAU Transient Name Server (TNS) as having been discovered and detected before S250818k merger time. We have registered the other 38 sources on the TNS. To down-select for optical counterpart candidates of S250818k, we applied the following filters:

    1. Spatially associated with a host galaxy that has a photometric or spectroscopic redshift within the 2-sigma limit from LVK of 0.025 < z < 0.08,
    2. Has some indication of lightcurve evolution over the three nights of observation,
    3. We rejected all faint transients spatially coincident with the nucleus of galaxies as being due to either AGN variability or possible subtraction artefacts.

We found no plausible candidate for the optical counterpart of S250818k other than AT2025ulz. Six sources with plausible kilonova-like absolute magnitudes and in hosts with consistent redshifts (within errors) with S250818k were found (AT2025uuw, AT2025uso, AT2025uuf, AT2025utr, AT2025usy, AT2025uxs), but their lightcurves are either rising or are flat and show no evolution over 48hrs, and they do not appear to be viable candidates at this point. Discovery details and magnitudes of the ATs can be found on the TNS.

Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and NNX14AM74G, issued through the SSO Near-Earth Object Observations Program. Data processing is enabled by Queen's University Belfast and the University of Oxford, enabled through STFC grants ST/Y001605/1, ST/T000198/1 and ST/X001253/1, the Royal Society, and the Hintze Centre for Astrophysical Surveys.

GCN Circular 41492

Subject
LIGO/Virgo/KAGRA S250818k: NOT optical observations of AT2025ulz
Date
2025-08-22T11:30:51Z (2 months ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
D. B. Malesani (DAWN/NBI and Radboud), A. Boye (DTU Space), L. Izzo (INAF/OAC and DARK/NBI), G. Leloudas (DTU Space), J. An (NAOC), X. Liu (NAOC), D. Xu (NAOC), M. Fraser (UCD), S. J. Brennan (MPE), J. Broe Bendsten (SDU), I. A. Koch (SDU), S. Lund Wagner (SDU), J. Magaard Knudsen (SDU), J. Hein Pedersen (SDU), J. P. U. Fynbo (DAWN/NBI), R. Holmberg Rasmussen (NOT and Aarhus), K. Valeckas (NOT and NBI), M. De Pasquale (U. Messina) report on behalf of a larger collaboration:

We observed the optical transient AT2025ulz (Stein et al., GCN 41414; Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454; Liu et al., GCN 41461; Banerjee et al., GCN 41476; Perley et al., GCN 41480), possibly connected with the GW event S250818k (LVK Collaboration, GCNs 41437, 41440), using the Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. Observations were started on 2025 Aug 20.875 UT (2.82 days after the GW trigger time). We adopted the filters r, i, and z (exposure times of 1200, 1200, and 1500 s, respectively).

Image subtraction using the Legacy sky survey as reference reveals a transient at coordinates (J2000):

RA = 15:51:54.19
Dec = +30:54:08.3

with an estimated error of < 0.5". These are consistent with the ZTF position (Stein et al., GCN 41414). The transient is located 1" southwest of the nucleus of its host galaxy, at a projected distance of 1.6 kpc (z = 0.0848; Karambelkar et al., GCN 41436; Banerjee et al., GCN 41476). Using nearby calibrators from the Legacy Survey, we obtained a provisional magnitude measurement of r = 23.2 ± 0.2 (AB), at a mean time of 2025 August 20.883 UT. We caution that we expect to refine our results after securing a deep template with the same instrument and filter.

GCN Circular 41489

Subject
LIGO/Virgo/KAGRA S250818k: GRAWITA TNG NIR observations of AT2025ulz
Date
2025-08-22T08:32:41Z (2 months ago)
From
Paolo D'Avanzo at INAF - OAB <paolo.davanzo@inaf.it>
Via
email
P. D'Avanzo, (INAF-OAB), M.T. Botticella (INAF - OACn),  E. Cappellaro (INAF-OAPd), L. Izzo (INAF - OACn), A. Rossi (INAF-OAS), V. D’Elia (ASI-SSDC), S. Piranomonte (INAF-OAR), G. Greco (INFN), M.G. Bernardini (INAF-OAB), S. Ronchini (GSSI), B. Patricelli (Univ. Pisa), O. S. Salafia, G. Ghirlanda (INAF-OAB), N. Elias-Rosa (INAF-OAPd), Fabio Ragosta (Università degli studi di Napoli “Federico II”), R. Salvaterra (INAF-IASF Milano), E. Brocato (INAF-OAR), V. Lorenzi, A. Garcia De Gurtubai Escudero (INAF-TNG), on behalf of GRAWITA report:

We observed the optical transient AT2025ulz (Stein et al., GCN Circ. 41414; Busmann et al., GCN Circ. 41421; Hall et al., GCN Circ. 41433; Karambelkar et al., GCN Circ. 41436; O’Connor et al., GCN Circ. 41452; Gillanders et al., GCN Circ. 41454; Liu et al., GCN Circ. 41461; Perley et al., GCN Circ. 41480) possibly connected to the GW candidate event S280818k (LVK collaboration, GCN Circ. 41437 & GCN Circ. 41440) with the Italian 3.6m TNG telescope, located in Canary Islands (Spain), equipped with the near-infrared camera NICS in imaging mode. 
A series of images were obtained with the J filter starting on 2025-08-20T20:27:00 UT (i.e. 2.80 days post the GW T0), with the H filter starting on 2025-08-20T21:42:37 UT (i.e. 2.85 days post the GW T0)  and with the K filter starting on 2025-08-20T23:00:15 UT (i.e. 2.90 days post the GW T0). The exposure time for each filter was 50 minutes.

The transient host galaxy is clearly detected in all our images, while no source is detected at the position of AT2025ulz down to the following 3sigma upper limits (AB magnitudes, calibrated against the 2MASS catalogue and obtained after subtracting a 2D galaxy model obtained with galfit, Peng et al.,  2010, AJ, 139, 2097):

J > 21.7 mag
H > 21.3 mag
K > 20.5 mag

GCN Circular 41480

Subject
LIGO/Virgo/KAGRA S250818k: Continued fading and reddening of AT2025ulz from Keck/LRIS imaging observations
Date
2025-08-21T23:06:59Z (2 months ago)
Edited On
2025-08-22T00:29:22Z (2 months ago)
From
Daniel Perley at Liverpool JMU <d.a.perley@ljmu.ac.uk>
Edited By
Leo P. Singer at NASA/GSFC <leo.p.singer@nasa.gov> on behalf of Daniel Perley at Liverpool JMU <d.a.perley@ljmu.ac.uk>
Via
Web form
Daniel A. Perley (LJMU), Mansi M. Kasliwal (Caltech), Viraj Karambelkar (Caltech), Michael Lundquist (WMKO), Xander Hall (CMU), Tomas Ahumada (Caltech), Kritti Sharma (Caltech), and Sam Rose (Caltech) report on behalf of the larger ZTF and GROWTH collaboration:
 
The location of AT2025ulz (ZTF25abjmnps; Stein et al., GCN 41414), a candidate counterpart to S250818k (LVK Collaboration, GCNs 41437, 41440), was imaged using the Low-Resolution Imaging Spectrometer (LRIS) on the Keck I 10m Telescope on the consecutive nights of 2025 August 20 and 21 (UT), simultaneously in g-band and i-band on each night. On the first night weather conditions were good, and two exposures of 30s each were acquired in each band starting at 2025-08-20 07:04 UT.  The weather was cloudy on the second night.  Three 300s exposures were taken, but due to highly variable atmospheric extinction only the highest-transmission image (starting at 2025-08-21 07:45 UT) was used for this analysis.  The 5-sigma limiting magnitudes of the images are g>24.8, i>23.9 (08-20) and g>23.9, i>23.2 (08-21).

While determining the magnitude of the transient is limited by the depth of available reference images, we performed difference imaging between the two nights in each band. Subtracting the 08-20 image from the 08-21 image in each filter, there is a clear detection of a negative point source at the reported location of the transient in the g-band difference image, but no significant residual flux in the i-band difference image.   The difference flux at the transient position from forced photometry is -1.60 +/- 0.20 microJy (g-band) and +0.31 +/- 0.38 microJy (i-band).  This suggests the transient is still rapidly fading in g-band, and likely reddening in g-i color.  This observation is consistent with the kilonova hypothesis and inconsistent with the re-brightening proposed by Liu et al. (GCN 41461).

GCN Circular 41476

Subject
LIGO/Virgo/KAGRA S250818k: ENGRAVE observations of AT 2025ulz
Date
2025-08-21T20:58:24Z (2 months ago)
From
Andrew Levan at Radboud University <a.levan@astro.ru.nl>
Via
Web form
Smaranika Banerjee (Stockholm University), Maria-Teresa Botticella (INAF - Capodimonte Obs.), Seán J. Brennan (MPE), Enrico Cappellaro (Padua Obs.), Ting-Wan Chen (NCU Taiwan), Paolo D'Avanzo (INAF - Brera Obs.), Max De Pasquale (Univ. Messina), Rob A. J. Eyles-Ferris (Leicester), Morgan Fraser (UCD), James H. Gillanders (Oxford), Ben Gompertz (Birmingham), Nusrin Habeeb (Leicester), Luca Izzo (INAF/OAC and DARK/NBI), Peter G. Jonker (Radboud), Andrew J. Levan (Radboud and Warwick), Daniele Bjørn Malesani  (DAWN/NBI and Radboud), Antonio Martin-Carrillo (UCD), Sam Oates (Lancaster U.), Matt Nicholl (QUB), Andrea Rossi (INAF - Bologna), Om Sharan Salafia (INAF - Brera Obs.), Nikhil Sarin (Cambridge), Steve Schulze (Northwestern), Stephen J. Smartt (Oxford), Danny Steeghs (Warwick), Nial R. Tanvir (Leicester), Aishwarya L. Thakur (INAF-IAPS) report for the ENGRAVE collaboration:

“We report preliminary analysis of ESO VLT observations obtained by the ENGRAVE collaboration of AT 2025ulz (Stein et al., GCN 41414), which was discovered within the 3D localization of the GW alert S250818k (LVK Collaboration, GCNs 41437, 41440).

Following the announcement of the discovery of AT 2025ulz, we triggered optical imaging and spectroscopy, as well as NIR imaging using FORS2, X-shooter and HAWK-I.

Using X-shooter, we obtained total exposure times of 3480, 3600, 3600 s in the UVB, VIS and NIR arms respectively, between 2025 Aug 20.99 and Aug 21.04 UT. As AT 2025ulz is faint, we acquired the target using a blind offset. We see no obvious broad features in our extracted spectrum that would be suggestive of either a KN or a SN. We do, however, see galaxy features, with prominent lines from the Balmer series (alpha, beta, gamma and delta), Paschen-alpha, [OII], [OIII], [OI], [NII], [SII] in emission, and Ca II in absorption, which securely place the redshift at z = 0.0848 (consistent with Karambelkar et al., GCN 41436).

With HAWK-I we obtained 690 s of Ks-band imaging between Aug 20.96 and Aug 20.98 UT. While the host galaxy of AT 2025ulz is well detected, we see no obvious point source corresponding to the transient itself. We note that, in the optical, template subtraction is required to detect AT 2025ulz, and, as we have no deep templates, we are currently unable to do this in the NIR. We estimate a limiting magnitude of K > 23 (AB mag) for the image, and K > 22 at the location of AT 2025ulz.

We obtained 3000 s of FORS2 long-slit spectroscopy using the 300I grism at the position of AT 2025ulz between Aug 21.01 and Aug 21.05 UT, but again see no obvious signal from the transient. We also examined the 3x60 s I-band FORS2 acquisition images, but see no clear point source.

With the data in hand, we cannot confirm a signal that is kilonova-like. Neither of the two VLT spectra show signatures of a transient source and there is no detectable K-band emission to around -16 (AB mag) absolute magnitude. The host galaxy contamination makes faint object observations challenging without deep templates.

If AT 2025ulz is associated with a KN, then preliminary modeling of the optical fluxes reported (Busman et al. GCN 41421, Hall et al. GCN 41433, O'Connor et al. GCN 41452, Hall et al. GCN 41453, Gillanders et al. GCN 41454, Mo et al. GCN 41456, Lui et al. GCN 45461) indicate that it must have a fairly large ejecta mass (of perhaps ~0.05 Msun) and the lack of K-band detection indicates it may be lanthanide-poor. If AT 2025ulz is a cooling tail following shock breakout in a core-collapse supernova (as seen for example in SN 1993J), then we should expect the lightcurve to begin to rise again approximately four days from now. 

Further ENGRAVE observations of AT 2025ulz will be carried out at the VLT.

We acknowledge Célia Desgrange, Lorena Faundez, Camila de Sa Freitas, Elisa Garro, Cecilia Bustos, Claudia Cid, Miguel Lopez, Francesca Lucertini and the observing staff in Paranal for their excellent support.“

GCN Circular 41474

Subject
LIGO/Virgo/KAGRA S250818k : Tautenburg observations, upper limits
Date
2025-08-21T19:06:59Z (2 months ago)
From
Sylvio Klose at TLS Tautenburg <klose@tls-tautenburg.de>
Via
email

S. Klose, B. Stecklum, A. Nicuesa Guelbenzu, and F. Ludwig (all
Tautenburg) report:

We observed the optical transient ZTF25abjmnps/AT2025ulz (Stein et
al., GCN 41414), possibly associated with the gravitational wave event
S250818k (GCN 41437), with the Tautenburg 1.34-m Schmidt telescope
using the Sloan filter set.

Observations were performed on 20-08-2025 between 20:06 and 20:39 UT
as well as between 21:57 and 22:30 UT. They reached a 3-sigma upper
limit in r and i of 22.3 and 22.4 mag (AB), respectively. Photometric
calibration was performed using Pan-STARRS DR2.

At the position of the optical transient (OT) discovered by Stein et
al. (GCN 41414; see also Busmann et al., GCN 41421; Hall et al., GCN
41433), no point source superimposed the underlying host galaxy was
detected / could be identified.

Given the closeness of the OT to the host galaxy's nucleus (Liu et
al., GCN 41461) we caution that the photometric limits are preliminary.

GCN Circular 41468

Subject
LIGO/Virgo/KAGRA S250818k: GOTO counterpart search and upper limits
Date
2025-08-21T16:29:36Z (2 months ago)
From
kendall.ackley@warwick.ac.uk
Via
Web form
K. Ackley, A. Kumar, D. O'Neill, M. J. Dyer, J. Lyman, K. Ulaczyk, S. Belkin, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, G. Ramsay, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, D. Pollacco, J. Casares Velazquez, T. Killestein report on behalf of the GOTO collaboration:

We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022; Dyer et al. 2024) in response to the LIGO/Virgo/KAGRA low-significance event S250818k (The LIGO-Virgo-KAGRA Collaboration, GCN 41437). 

As part of our strategy for LVK O4 follow-up, targeted observations of the event between 2025-08-18 01:22:29 UT (2.4 minutes post-trigger) and 2025-08-21 05:29:32 UT (76.2 hours post trigger) with GOTO-North and between 2025-08-18 08:34:23 UT (7.2 hours post-trigger) and 2025-08-19 09:29:09 UT (32.2 hours post-trigger) with GOTO-South.

We covered a total of 521.4 deg2 (46.0%) of the localisation probability of the Bilby skymap (The LIGO-Virgo-KAGRA Collaboration, GCN 41440). Exposures were taken in L-band (400-700 nm passband) and the average 5 sigma depth was L = 19.95 AB magnitudes with GOTO-1 (North), L = 19.74 AB with GOTO-2 (North), L = 19.05 AB with GOTO-3 (South), and L = 19.70 AB with GOTO-4 (South).

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using recent survey observations of the same pointings. Source candidates were initially filtered using a classifier (Killestein et al. 2021) and cross-matched against a variety of contextual and minor planet catalogs. Human vetting was carried out in real time on any candidates that passed the above checks. 

No significant counterpart sources that can be associated with S250818k have been identified. We do not detect emission of the candidate counterpart AT 2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O’Connor et al. GCN 41452) down to a 3-sigma limiting magnitude of L = 20.3 AB at 2025-08-18 20:53:41 UTC (~19.6 hours post-trigger).

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and not corrected for Galactic extinction.

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).

GCN Circular 41464

Subject
LIGO/Virgo/KAGRA S250818k: 10 GHz VLA observations of AT2025ulz
Date
2025-08-21T12:54:35Z (2 months ago)
From
Roberto Ricci at INAF-IRA <ricci@ira.inaf.it>
Via
Web form
Authors: R. Ricci, M. Yadav, E. Troja (U Rome) report:

We observed the optical transient AT2025ulz/ZTF25abjmnps (Stein et al. GCN 41414; Hall et al. GCN 41433; Karambelkar et al. GCN 41436; O’Connor et al. GCN 41452) possibly associated to the GW candidate S250818k (LVKC GCN 41437) with the Very Large Array in X-band at the centre frequency of 10 GHz with a bandwidth of 4 GHz on Aug 21st 2025, 3.1 days after the GW trigger. 

After reducing the data using the latest version of the CASA VLA pipeline (v6.6.1) and standard imaging procedures, we found no detection at the optical transient position down to a 3-sigma flux density upper limit of 30 microJy/beam. 

We thank the VLA staff for promptly executing the observations.

GCN Circular 41461

Subject
LIGO/Virgo/KAGRA S250818k: WFST pre-discovery limits and follow-up observations of AT 2025ulz
Date
2025-08-21T09:00:21Z (2 months ago)
From
Zhengyan Liu at USTC <ustclzy@mail.ustc.edu.cn>
Via
Web form
Z. Y. Liu, Z. L. Xu, D. Z. Meng, J.-A. Jiang, W. Zhao, M. X. Cai, Z. G. Dai, L. L. Fan, N. Jiang, X. Kong, T. G. Wang (USTC), Y. Z. Fan, J. J. Geng, Z. P. Jin, X. F. Wu (PMO) report on behalf of the WFST Collaboration:
 
We report the pre-discovery upper limits and follow-up photometry results of AT2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414), possibly associated with the sub-threshold gravitational wave (GW) event S250818k (GCN 41437), using the Wide Field Survey Telescope (WFST Collaboration; Wang et al., 2023).
 
After checking WFST archive data, we found u- and g-band 30s exposures taken at MJD 60901.66, about 3.4 days prior to the GW event trigger. The images were processed with the WFST pipeline, using stacked WFST multi-exposures taken in February 2025 as templates. We do not found any residuals in difference images in the two bands. We report the following pre-discovery 3-sigma upper limits and time since the S20250818k trigger (t-t0):
 
MJD t-t0 (days) AB mag Band
60901.64 -3.42 > 22.1 u
60901.67 -3.38 > 22.4 g
 
We conducted WFST follow-up observations of AT2025ulz with 120s exposures in g and r bands and a 180s exposure in u band beginning at 2025-08-20T14:24:03 UTC. After image subtraction, a clear source was detected in g band, and faint sources detected in u and r bands, which are close to the detection limit. The preliminary AB magnitudes of AT2025ulz are (all photometries are not corrected for Galactic extinction):
 
MJD t-t0 (days) AB mag Band
60907.60 2.54 22.71+/-0.10 g
60907.61 2.55 22.78+/-0.16 r
60907.65 2.59 23.24+/-0.24 u
 
The g-r and u-g color from WFST observations are approximately -0.1 mag and 0.5 mag, respectively, which became bluer compared with the previous reports (Hall et al., GCN 41433; O’Connor et al., GCN 41452; Gillanders et al., GCN 41454). Additionally, our results show a re-brightening trend for AT2025ulz, which may be related to the shock-cooling process mentioned in Gillanders et al. (GCN 41454). We also note that AT2025ulz is significantly more luminous than AT 2017gfo at the same phase, especially in g and u bands assuming a host redshift 0.0848 reported by Karambelkar et al. (GCN 41436). The angular separations between the detected source in g band and the host center are approximately 0.85 and 0.83 arcseconds using host coordinates in PS1 DR2 (Chambers et al., 2016) and SDSS DR16 (Ahumada et al., 2020), respectively. Given the close distance between the source and the bright host nucleus, and the peculiar photometric evolution in the past 24 hours, we suggest that the variability might be influenced by the host-nuclear activity. Further deep multiband imaging follow-ups are highly encouraged.

GCN Circular 41460

Subject
LIGO/Virgo/KAGRA S250818k: EP/FXT observation of AT 2025ulz
Date
2025-08-21T08:38:43Z (2 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
R.-Z. Li (YNAO, CAS), X.-P. Xu, H. Sun, D.-Y. Li, Y. Liu, W. Yuan (NAO ,CAS) and B. Zhang (HKU) on behalf of the Einstein Probe (EP) team:

We performed a ToO observation with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) for AT2025ulz, reported by Stein et al. (GCN 41414) as a possible counterpart to the LVK sub-threshold event S250818k (GCN 41437, 41440).

The observation was carried out from 2025-08-20 08:31:13 (UTC) to 2025-08-20 13:19:05 (UTC), with a total exposure time of about 9 ks. The midpoint time of this observation is 2025-08-20T10:55:09 (UTC), approximately 57.6 hours after the trigger (LVK Collaboration, GCN 41437).

The source was not detected with EP/FXT with an upper limit of 1.1e-3 cts/s at 0.5-10 keV. The estimated flux upper limit is 1.68e-14 erg/cm^2/s (90% C.L., at 0.5-10 keV). Given the redshift of z = 0.0848 (GCN 41436), this gives a rough luminosity upper limit of 3.2e41 erg/s.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

GCN Circular 41456

Subject
LIGO/Virgo/KAGRA S250818k: WINTER J-band observations of AT 2025ulz
Date
2025-08-20T18:41:15Z (2 months ago)
From
Geoffrey Mo at MIT <gmo@mit.edu>
Via
Web form
Geoffrey Mo (MIT), Robert Stein (JSI), Mansi Kasliwal (Caltech), Viraj Karambelkar (Caltech), Danielle Frostig (CfA), Nathan Lourie (MIT), and Robert Simcoe (MIT) report:

We observed AT2025ulz (Stein et al., GCN 41414), a possible counterpart to the LVK sub-threshold event S250818k (LVK, GCN 41437, 41440), with the 1.2 sq. degree near-IR WINTER camera on the Palomar 1-m telescope (Lourie et al. 2021, Frostig et al. 2024).

Our observations were conducted in J-band beginning at 2025-08-19 03:09:09 UTC, approximately 26 hours after the merger. Our observations consisted of 30 individual 120s dithers. The images were processed using the WINTER data reduction pipeline (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565) using images from the UKIRT Hemisphere survey (Dye et al. 2018) as references for image subtraction.

We do not detect any source at the position of AT2025ulz in our difference image. We place a J-band upper limit of m > 19.3 AB mag.

WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucile Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.

GCN Circular 41454

Subject
LIGO/Virgo/KAGRA S250818k: Pan-STARRS grizy-band imaging and photometry of AT2025ulz
Date
2025-08-20T18:11:46Z (2 months ago)
From
James Gillanders at University of Oxford <jhgillanders.astro@gmail.com>
Via
Web form
J. H. Gillanders (Oxford), M. E. Huber, K. C. Chambers (IfA, Univ. Hawaii), S. J. Smartt, K. W. Smith (Oxford/QUB), S. Srivastav, F. Stoppa, H. Stevance, J. Tweddle (Oxford), M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), T.-W. Chen (NCU, Taiwan), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), A. S. B. Schultz, T. de Boer, J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G. Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), A. Rest (STScI), C. Stubbs (Harvard).

We observed the optical transient AT2025ulz (Stein et al., GCN 41414), the transient plausibly associated with the (sub-threshold) gravitational wave event S250818k (LVK Collaboration, GCNs 41437, 41440) using the Pan-STARRS twin telescope system (Chambers et al., 2016, arXiv e-prints, 1612.05560). 

Our observation consisted of 6x150s exposures in each of the grizy filters in both telescopes. The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target stacked images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4).

From these difference images, we measure the following preliminary AB magnitudes, from averages of all forced photometry from the two GPC1 and GPC2 cameras:

MJD         t-t0 (days)    Filter    AB mag
60907.30    2.25           g         22.9 +/- 0.4
60907.31    2.26           r         22.2 +/- 0.2
60907.33    2.27           i         21.8 +/- 0.2
60907.34    2.28           z         21.4 +/- 0.2

Here, t0 corresponds to the GW trigger time (LVK Collaboration, GCN 41437).

Our difference imaging and photometric measurements indicate a consistent flux excess at the position of AT2025ulz (Stein et al., GCN 41414), although we note that these current magnitude estimates are derived from difference images generated with reference images shallower than the target images (900s vs. ~300s, respectively). We caution these are preliminary, given the differencing issues and the closeness of the source to the bright nucleus and body of the host. 

Comparing our current Pan-STARRS gr-band data to that previously reported by Busmann et al. (GCN 41421) and Hall et al. (GCN 41433), we find that AT2025ulz appears to have faded further, if both sets of photometry are accurate. 

We can certainly conclude that no re-brightening (which may be expected in SNe IIb or shock cooling) is yet taking place. 

Work is ongoing to generate a deeper reference image, and thus improved magnitude estimates. Further multi-band observations are scheduled.

GCN Circular 41453

Subject
LIGO/Virgo/KAGRA S250818k: Swift observations of AT 2025ulz
Date
2025-08-20T17:43:21Z (2 months ago)
From
xjh@andrew.cmu.edu
Via
Web form

Xander J. Hall (Carnegie Mellon U.), Robert Stein (UMD), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report:

We requested ToO observations of AT 2025ulz

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, reported by Stein et al. (GCN 41414) as a possible counterpart to the LVK sub-threshold event S250818k (GCN 41437, 41440).

The observation was taken with a midpoint time of 2025-08-19T11:48:55.755, approximately 30 hours after the trigger, with a duration of 3.5ks.

The source was not detected with XRT with an upper limit of 3.8 * 10^-3 cts/s at 0.3 - 10 keV. Given the redshift of z = 0.0848 (GCN 41436

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), this gives a rough luminosity upper limit of 3.3 * 10^42 erg/s.

We clearly detect a source at the position of AT2025ulz

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in UVOT observations, conducted in the UVM2 filter, with a magnitude of m=22 AB. However, this flux is consistent with archival detections of the host galaxy in GALEX. We therefore find no evidence of transient UV emission in UVOT data.

We thank the Swift team, Observatory Duty Scientists and the PI for promptly scheduling and making these observations possible.

GCN Circular 41452

Subject
LIGO/Virgo/KAGRA S250818k: Multi-band Gemini GMOS Detections
Date
2025-08-20T17:28:14Z (2 months ago)
From
Brendan O'Connor at Carnegie Mellon University <boconno2@andrew.cmu.edu>
Via
Web form
B. O’Connor (CMU), J. Freeburn (UNC), X. J. Hall (CMU), Malte Busmann (LMU), I. Andreoni (UNC), A. Palmese (CMU), D. Gruen (LMU), L. Hu (CMU), T. Cabrera (CMU), K. Kunnumkai (CMU), A. Amsellem (CMU) report on behalf of a larger collaboration: 

Using Gemini GMOS-N, we observed the source AT 2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414) discovered during ZTF follow-up of the low-significance gravitational wave event S250818k (GCN 41437, 41440; https://gracedb.ligo.org/superevents/S250818k), which is a candidate binary neutron star merger. 

Following the discovery of rapid fading and reddening of the source with the Fraunhofer Telescope at Wendelstein Observatory (FTW; see Busmann et al. GCN 41421, Hall et al. GCN 41433), we carried out observations in the grizY filters with GMOS-N under photometric observing conditions starting at 2025-08-20 05:29:56 UT. This corresponds to 2.17 days after the gravitational wave detection (GCN 41437). 

A visual inspection reveals that the transient is significantly detected on top of the host galaxy. The source is clearly off-nuclear, but lies on top of the galaxy’s disk. We performed difference imaging with respect to Legacy Survey templates that further reveal the source. The brightness of the transient is g~23.0 AB mag and r~22.6 AB mag. The photometry is not correct for Galactic extinction. 

Our data reveals continued rapid fading and reddening of the transient in the 9.4 hours following the FTW report (Hall et al. GCN 41433). This would imply a steepening of the fade rate in g-band. We caution that the galaxy appears over subtracted in our difference imaging, which may affect the preliminary photometry. Additional analysis is underway. 

Further observations are planned and additional follow-up is encouraged.

We thank the staff of the Gemini Observatory, in particular Jen Andrews, Brian Lemaux, and Jen Miller, for their excellent support in rapidly scheduling these observations.

GCN Circular 41451

Subject
LIGO/Virgo/KAGRA S250818k: ATLAS pre-discovery limits for AT2025ulz
Date
2025-08-20T15:30:45Z (2 months ago)
From
S. Srivastav at Oxford <shubhamsrivastav@gmail.com>
Via
Web form
S. Srivastav (Oxford), K. W. Smith, S. J. Smartt (Oxford/QUB), D. R. Young, M. Nicholl, M. D. Fulton, T. Moore, A. Aamer, C. R. Angus, M. McCollum, S. Sim, J. Weston, X. Sheng (QUB), L. Shingles (GSI/QUB), J. Sommer (LMU), J. Gillanders, H. Stevance, L. Rhodes, A. Andersson (Oxford), L. Denneau, J. Tonry, H. Weiland, A. Lawrence, R. Siverd (IfA, University of Hawaii), N. Erasmus, W. Koorts (South African Astronomical Observatory), J. Anderson (ESO), A. Jordan, V. Suc (UAI, Obstech) A. Rest (STScI), T.-W. Chen (NCU), C. Stubbs (Harvard):

We report pre-discovery upper limits from ATLAS observations of AT2025ulz/ZTF25abjmnps reported by Stein et al. (GCN 41414), possibly associated with the sub-threshold gravitational wave event S250818k (GCN 41437).

ATLAS observed the field most recently before the GW event on MJD 60903.32, 1.74 days prior to the trigger. The observations were made as part of regular survey operations in the o-band. During the August 2025 lunation, we have been experimenting with a different cadence and longer exposures. The data on MJD 60903.32 were taken as 4 x 110 sec exposures (compared to 30 sec exposures in normal operations). These longer exposures provide a 3-sigma upper limit of o > 21.6 mag (AB) after stacking the 4 individual measurements. The discovery r-band mag of r = 21.29 +/- 0.13 (Stein et al., GCN 41414) implies a significant 3-sigma rise in 1.74 days, or a new transient appearing on that timescale. 

The Asteroid Terrestrial-impact Last Alert System [ATLAS] project is primarily funded to search for Near-Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889,  STFC grants ST/Y001605/1, ST/X001253/1, the Royal Society and Schmidt Sciences. ATLAS-Teide is an IAC instrument included in the present “Strategic plan of the Canarian Observatories”, funded by the European Union  NextGenerationEU  EQC2021-007122-P and ICT2022-007828 projects. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy,  Queen's University Belfast, University of Oxford, the Space Telescope Science Institute, the South African Astronomical Observatory, The Millennium Institute of Astrophysics (MAS), Chile, and the Instituto de Astrofisica De Canarias.

GCN Circular 41443

Subject
LIGO/Virgo/KAGRA S250818k: AstroSat CZTI non-detection and upper limits
Date
2025-08-20T13:06:27Z (2 months ago)
From
Gaurav Waratkar at IIT Bombay <gauravwaratkar@iitb.ac.in>
Via
Web form

G. Waratkar (IITB), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (TIFR), S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

We have carried a search [1] for X-ray candidates in Astrosat CZTI data in a 100 sec window around the trigger time of the event S250818k (UTC 2025-08-18 01:20:06; LVK Collaboration, GCN Circ. 41437

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). We use the Bilby.offline0.multiorder.fits,0 map (https://gracedb.ligo.org/api/superevents/S250818k/files/Bilby.offline0.multiorder.fits,0) for our analysis.

CZTI is a coded aperture mask instrument that has considerable effective area for about 29% of the entire sky, but is also sensitive to brighter transients from the entire sky. At the time of the merger, Astrosat's nominal pointing is RA,DEC = 00:46:53.8, 42:11:36.4 (11.7242,42.1934), which is ~88 deg away from the maximum probability location, which severely reduces the effective area of CZTI. At the time of the merger event, the Earth-satellite-transient angle corresponding to maximum probability location is ~33 deg and hence is occulted by Earth in satellite's frame. In a time interval of 100 sec around the event, the region of the localisation map which is not occulted by Earth in the satellite's frame has a total probability of 0.59 (59%).

We note that the source AT 2025ulz

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(Stein et al., GCN Circ. 41414) was occulted behind Earth for AstroSat at the time of the merger event.

CZTI data were de-trended to remove orbit-wise background variation. We then searched data from three of the four independent, identical quadrants to look for coincident spikes in the count rates. Quadrant C being noisy was excluded from further analysis. Searches were undertaken by binning the data in 0.1s, 1s, and 10s respectively. Statistical fluctuations in background count rates were estimated by using data from 5 preceding orbits. We selected confidence levels such that the probability of a false trigger in a 100 sec window is 10^-3, after excluding one noisy quadrant. We do not find any evidence for any hard X-ray transient in this window, in the CZTI energy range of 20-200 keV.

We use a detailed mass model of the satellite to calculate the direction-dependent instrument response for points in the visible sky. We then assume the source is modelled as a power law with photon index alpha = -1, and convert our count rate upper limits to direction-dependent flux limits. We obtain the following upper limits for source flux in the 20-200 keV band by taking a probability weighted mean over the visible sky:

Bin (s)Flux Limit (ergs/cm^2/s)Fluence Limit (ergs/cm^2)
0.11.47e-061.47e-07
1.03.78e-073.78e-07
10.08.00e-088.00e-07

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI EMGW detections are reported regularly on the payload site at: http://astrosat.iucaa.in/czti/?q=emgw

[1] Waratkar et al. ApJ 976, 123 (2024) doi:10.3847/1538-4357/ad84e6

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GCN Circular 41441

Subject
LIGO/Virgo/KAGRA S250818k: Upper limits from Fermi-GBM Observations
Date
2025-08-20T11:04:15Z (2 months ago)
From
Cuán de Barra at UCD <cuan.debarra@ucdconnect.ie>
Via
Web form
For S250818k (GCN 41437, LVK Collaboration) and using the updated bayestar skymap, Fermi-GBM was observing 51.0% of the localization probability at event time.

There was no Fermi-GBM onboard trigger around the event time of the LIGO/Virgo/KAGRA (LVK) detection of GW trigger S250818k . An automated, blind search for short gamma-ray bursts below the onboard triggering threshold in Fermi-GBM also identified no counterpart candidates. The GBM targeted search, the most sensitive, coherent search for GRB-like signals, was run from +/-30 s around merger time, and also identified no counterpart candidates.

Part of the LVK localization region is behind the Earth for Fermi, located at an RA=66.9, Dec=-25.2 with a radius of 68.0 degrees. We therefore set upper limits on impulsive gamma-ray emission for the GW localization region visible to Fermi at merger time. Using the representative soft, normal, and hard GRB-like templates described in arXiv:1612.02395, we set the following 3 sigma flux upper limits over 10-1000 keV, weighted by GW localization probability (in units of 10^-7 erg/s/cm^2):

Timescale  Soft   Normal   Hard 
------------------------------------
0.128 s: 1.5 	2.5 	4.3
1.024 s: 0.66   0.87    1.5
8.192 s: 0.22   0.29    0.45

Assuming the median luminosity distance of 259.4 Mpc from the GW detection, we estimate the following intrinsic luminosity upper limits over the 1 keV-10 MeV energy range (in units of 10^50 erg/s):

Timescale  Soft     Normal   Hard
------------------------------------
0.128s: 0.018 0.029 0.081
1.024s: 0.008 0.010 0.028
8.192s: 0.003 0.003 0.008

GCN Circular 41440

Subject
LIGO/Virgo/KAGRA S250818k: Updated Sky localization and EM Bright Classification
Date
2025-08-20T10:12:16Z (2 months ago)
From
Soichiro Morisaki at U. of Tokyo <soichiro.morisaki@ligo.org>
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250818k (GCN Circular 41437). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S250818k

Based on posterior support from parameter estimation [1], under the assumption that the candidate S250818k is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass above one solar mass (HasNS) is 80%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is 80%. [2] HasRemnant is assumed to be zero when the heavier component mass is below 1 solar mass. Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 7%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is >99%.

The source chirp mass falls with highest probability in the bin (0.1, 0.87) solar masses after parameter estimation [1], assuming the candidate is astrophysical in origin.

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 949 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 237 +/- 62 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe

GCN Circular 41439

Subject
LIGO/Virgo/KAGRA S250818k: Pan-STARRS pre-detection limits for AT2025ulz
Date
2025-08-20T09:52:22Z (2 months ago)
From
Matt Nicholl at Queens University Belfast <matt.nicholl@qub.ac.uk>
Via
Web form
M. Nicholl, D. R. Young, A. Aamer, C. R. Angus, M. D. Fulton, D. Magill, M. McCollum, T. Moore, S. Sim, J. Weston, X. Sheng (QUB), S. J. Smartt, K.W. Smith, J. Gillanders, S. Srivastav, H. Stevance, F. Stoppa, J. Tweddle (Oxford), L. Shingles (GSI/QUB), P. Ramsden (Birmingham/QUB), K. C. Chambers, M. E. Huber, A. S. B. Schultz, T. de Boer,  J. Fairlamb, C. C. Lin, T. Lowe, E. Magnier, P. Minguez, G, Paek, I. A. Smith, R. J. Wainscoat (IfA, Univ. Hawaii), T.-W. Chen (NCU), A. Rest (STScI), C. Stubbs (Harvard):

We report pre-discovery limits on the source AT2025ulz, reported by Stein et al. (GCN 41414) and possibly associated with the sub-threshold gravitational wave candidate S20250818k (The LIGO-Virgo-Kagra Collaboration, GCN 41437), obtained with the Pan-STARRS telescope system (Chambers et al., 2016, ArXiv e-prints, 1612.05560). The Pan-STARRS system comprises of two 1.8m telescope units located at the summit of Haleakala on the Hawaiian island of Maui, employing an SDSS-like filter system denoted as grizy, and a broad w-filter, which is a composite of the gri-filters. In our primary NASA mission for Near-Earth Object discovery, we scan the visible night sky North of -50 degrees declination to magnitude depths m~22, weather and Moon permitting.

Examining recent data obtained during normal Pan-STARRS survey coverage (see Fulton et al. 2025, MNRAS, 542, 541) at the location of AT2025ulz, we report the following pre-discovery upper limits and time since the S20250818k trigger (t-t0) 

Date       t-t0 (days)  Magnitude   Band
60904.233  -0.82        <19.7       y
60895.314  -9.7         <21.0       i

We note that the y-band non-detection was obtained only ~0.8 days before the GW signal. This indicates that there was no bright, red transient at this position less than a day prior.

The images were processed with the Pan-STARRS pipeline. After astrometric and photometric calibration, reference images were subtracted from the target images (Magnier et al., 2020a, ApJS, 251, 3; Magnier et al., 2020b, ApJS, 251, 6; Waters et al., 2020, ApJS, 251, 4).

Further Pan-STARRS grizy imaging of this source, and coverage of the S20250818k skymap (northern banana) is underway in the i-band with PS1 and PS2 jointly. 

The discoveries from this program are a byproduct of the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) NEO survey observations. Operation of the Pan-STARRS1 and Pan-STARRS2 telescopes is primarily supported by the National Aeronautics and Space Administration under Grant No. NNX12AR65G and Grant No. NNX14AM74G issued through the SSO Near-Earth Object Observations Program. Data are processed at Queen's University Belfast enabled through the STFC grants ST/P000312/1 and ST/T000198/1.

GCN Circular 41437

Subject
LIGO/Virgo/KAGRA S250818k: Properties of the low-significance GW compact binary merger candidate potentially associated with AT 2025ulz
Date
2025-08-20T06:04:02Z (2 months ago)
From
gwangeon.seong@ligo.org
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration (LVK) report:

We identified the compact binary merger candidate S250818k during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-18 01:20:06.030 UTC (GPS time: 1439515224.030). The candidate was found by the GstLAL SSM [1] and PyCBC Live [2] analysis pipelines. Two LVK Preliminary GCN Notices were issued within 5 minutes after the candidate was identified.

Based on the analysis of gravitational-wave (GW) data alone, this candidate does not meet our criteria for a high-significance public alert as its false alarm rate is estimated by the online analysis to be 6.8e-08 Hz or about one in 5 months. However, we are issuing this Circular to confirm the properties of the GW candidate because of its potential association with the optical transient ZTF25abjmnps / AT 2025ulz (Stein et al, GCN 41414) which has been confirmed and reported to be fading and reddening rapidly (Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436). The GW candidate’s properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250818k

The classification of the GW candidate, in order of descending probability, is Terrestrial (71%), BNS (29%), NSBH (<1%), or BBH (<1%).
Initial data quality checks found that there is no evidence for presence of a glitch or other data quality issues.

Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects is consistent with a neutron star mass (HasNS) is >99%. [3] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is >99%. [3] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The source chirp mass falls with highest probability in the bin (0.1, 0.87) solar masses, assuming the candidate is astrophysical in origin.

Three sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 28 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 2 days after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 786 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 259 +/- 74 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Hanna et al. (2024) arXiv 2412.10951
 [2] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [3] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [4] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

GCN Circular 41436

Subject
LIGO/Virgo/KAGRA S250818k: Keck I LRIS spectroscopy of ZTF25abjmnps (AT2025ulz)
Date
2025-08-20T05:23:52Z (2 months ago)
From
Mansi Kasliwal at Caltech <mansikasliwal@gmail.com>
Via
Web form
Viraj Karambelkar (Caltech), Mansi M. Kasliwal (Caltech) and Xander J.  Hall (CMU) report on behalf of the larger ZTF and GROWTH collaborations

We observed ZTF25abjmnps (AT2025ulz; Stein et al. GCN 41414) starting UTC 2025-08-19 06:42 with the LRIS spectrograph on Keck I. We derive a redshift of 0.0848 based on host galaxy emission lines - this is consistent with the 3D GW localization for S250818k. Subtracting host galaxy light, we see a blue continuum excess from the transient, and no tell-tale supernova-like features. The data in-hand thus far appears consistent with a kilonova. However, continued monitoring (such as Busmann et al. GCN 41421, Hall et al. GCN 41433) is encouraged to rule out any future re-brightening from a faint and fast supernova Type IIb (e.g., Barna et al. 2025, arXiv:2506.15900). Panchromatic follow-up is ongoing and encouraged.

GCN Circular 41433

Subject
LIGO/Virgo/KAGRA S250818k: FTW Fast reddening of AT 2025ulz
Date
2025-08-20T01:24:57Z (2 months ago)
From
xjh@andrew.cmu.edu
Via
Web form
Xander J. Hall (Carnegie Mellon U.), Malte Busmann (LMU), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report:

We observed the source AT 2025ulz reported by Stein et al. (GCN 41414) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, r, i, z, and J bands starting at 2025-08-19T20:09:46 for 22 x 180 s. We took 11 observations in the giJ configuration and 11 in the rzJ configuration. In the difference imaging with templates from the Legacy Survey, we detect AT 2025ulz at

g = (22.08 +/- 0.09) AB mag,
r = (21.83 +/- 0.06) AB mag.

We note significant reddening and fading from our previous report (GCN 41421), with the g band dropping 0.83 AB mag in 24.6 hours and r band now being brighter than g.

The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction.

Further analysis is underway.

We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations.

GCN Circular 41421

Subject
LIGO/Virgo/KAGRA S250818k: FTW optical and NIR observations of AT 2025ulz
Date
2025-08-19T01:00:05Z (2 months ago)
From
Malte Busmann at Ludwig-Maximilians-Universität München <m.busmann@physik.lmu.de>
Via
Web form
Malte Busmann (LMU), Xander J. Hall (Carnegie Mellon U.), Daniel Gruen (LMU), Brendan O’Connor (Carnegie Mellon U.), and Antonella Palmese (Carnegie Mellon U.) report:

We observed the source AT 2025ulz reported by Stein et al. (GCN 41414) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the g, r, i, z, and J bands starting at 2025-08-18T19:35:11 for 40 x 180 s. We took 30 observations in the giJ configuration and 10 in the rzJ configuration. In the difference imaging with templates from the Legacy Survey, we detect AT 2025ulz at

g = (21.25 +/- 0.03) AB mag,
r = (21.43 +/- 0.06) AB mag.

The magnitudes are calibrated against the PS1 catalog and not corrected for Galactic extinction.

We thank Michael Schmidt from the Wendelstein Observatory staff for obtaining these observations.

GCN Circular 41414

Subject
LIGO/Virgo/KAGRA S250818k: Candidates from the Zwicky Transient Facility
Date
2025-08-18T17:01:30Z (2 months ago)
From
Robert David Stein at JSI <rdstein@umd.edu>
Via
Web form
Robert Stein (JSI/UMD), Tomás Ahumada (Caltech) Mansi Kasliwal (Caltech), Theophile du Laz (Caltech), Utkarsh Pathak (IITB), Vishwajeet Swain (IITB), Anirudh Salgundi (UNC), Varun Bhalerao (IITB), Xander J. Hall (CMU) report,

On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations: 

We observed the localization region of the LVK trigger S250818k with the Palomar 48-inch telescope, equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We started observations in the g- and r-band beginning at 2025-08-18 04:02 UTC, approximately 2.7 hours after merger. We covered 25.2% (168.3 sq deg) of the reported localization region. This estimate accounts for chip gaps. Each exposure was 300s with a typical depth of 22 mag. 
 
The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). AMPEL (Nordin et al. 2019, Stein et al. 2021) was used to search the alerts database for candidates. We reject stellar sources (Tachibana and Miller 2018) and moving objects, and apply machine learning algorithms (Mahabal et al. 2019) , and removing candidates with history of variability prior to the merger time. 

We are left with 58 transient candidates, all lying within the 95.0% localization of the skymap. We perform additional vetting of these candidates, and identify those which appear to be hosted in galaxies at plausible redshifts for S250818k.

We highlight one of these candidates, ZTF25abjmnps/AT2025ulz.

+--------------------------------------------------------------------------------+
| ZTF Name     | IAU Name  | RA (deg)    | DEC (deg)   | Filter | Mag   | MagErr |
+--------------------------------------------------------------------------------+
| ZTF25abjmnps | AT2025ulz | 237.9757129 | +30.9023146 | r      | 21.29 | 0.13   |
+--------------------------------------------------------------------------------+

ZTF25abjmnps is in an elliptical galaxy with a Legacy Survey photometric redshift of z = 0.091 +/- 0.016, is therefore consistent with the estimated distance of S250818k. The lower 95th percentile limit of the photoz is 0.057, and at this distance S250818k would have an absolute g-band peak magnitude of M=-16.1. However, the higher redshifts would be more consistent with a supernova luminosity of M=-17. 

Forced photometry of this source reveals several detections in our data. The source appears to possibly be fading in g-band, but given the low SNR and short baseline it is difficult to constrain this.

There are also several recent non-detections, but all upper limits are shallower than the magnitude of the transient in our images. We therefore cannot confirm whether the source is young, or if it predates the merger.

We encourage observations of ZTF25abjmnps, to determine the nature of this source. 

Analysis of the remaining ZTF candidates is ongoing.

Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO) and Caltech/IPAC. GROWTH acknowledges the generous support of the NSF under PIRE Grant No 1545949. Alert database searches are done by AMPEL (Nordin et al. 2019) and Kowalski (Duev et al. 2019). The GROWTH India Telescope (GIT, Kumar et al., 2022) is set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. Its operations are partially supported by funding from the IIT Bombay alumni batch of 1994. The Fritz and SkyPortal projects acknowledge the generous support of The Gordon and Betty Moore Foundation.

GCN Circular 41410

Subject
LIGO/Virgo/KAGRA S250818k: Coverage and upper limits from MAXI/GSC observations
Date
2025-08-18T09:38:35Z (2 months ago)
From
Satoshi Sugita at Aoyama Gakuin U. <sugita@phys.aoyama.ac.jp>
Via
Web form
M. Nakajima, H. Negoro, K. Takagi (Nihon U.),
N. Kawai, T. Mihara, (RIKEN),
S. Sugita, M. Serino, Y. Kawakubo, H. Hiramatsu, Y. Kondo (AGU)
report on behalf of the MAXI team:

We examined MAXI/GSC all-sky X-ray images (2-20 keV)
after compact binary merger candidate S250818k at 2025-08-18 01:20:06.030 UTC.

At the trigger time of S250818k, the high-voltage of MAXI/GSC was on.
The instantaneous field of view of GSC at the GW trigger time covered 1% of the 90% credible region
of the bayestar sky map, in which we found no significant new X-ray source.
The first one-orbit (92 min) scan observation with GSC after the event covered 78%
of the 90% credible region of the bayestar skymap from 01:20:06 to 02:51:15 UTC (T0+0 to T0+5469 sec).

No significant new source was found in the region in the one-orbit scan observation.
A typical 1-sigma averaged upper limit obtained in one scan observation
is 20 mCrab at 2-20 keV.

If you require information about X-ray flux by MAXI/GSC at specific coordinates,
please contact the submitter of this circular by email.
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