LIGO/Virgo/KAGRA S250223dk
GCN Circular 39443
Subject
LIGO/Virgo/KAGRA S250223dk and Swift/BAT-GUANO trigger with ID 762004910: Identification of a GW compact binary merger candidate possibly associated with a sub-threshold Swift/BAT-GUANO trigger
Date
2025-02-24T03:12:15Z (3 months ago)
Edited On
2025-02-24T18:53:59Z (3 months ago)
From
minghuidu1993@gmail.com
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Samuele Ronchini at PSU <sjs8171@psu.edu>
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration along with the Swift/BAT-GUANO team report:
We identified the compact binary merger candidate S250223dk during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at T0 = 2025-02-23 12:01:15.360 UTC (GPS time: 1424347293.360). The candidate was found by the GstLAL [1] analysis pipeline.
S250223dk is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.4e-05 Hz, or about one in 19 hours. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/S250223dk
The classification of the GW signal, in order of descending probability, is Terrestrial (92%), BBH (8%), NSBH (<1%), or BNS (<1%).
Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.
Five GW-only 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 [3], distributed via GCN notice about a minute after the candidate event time.
* bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], distributed via GCN notice about 5 minutes after the candidate event time.
* bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [3], distributed via GCN notice about 12 hours after the candidate event time.
* bayestar.multiorder.fits,3, an initial localization generated by BAYESTAR [3], distributed via GCN notice about 12 hours after the candidate event time.
* bayestar.multiorder.fits,4, an initial localization generated by BAYESTAR [3], distributed via GCN notice about 13 hours after the candidate event time.
The preferred sky map at this time is bayestar.multiorder.fits,4. For the bayestar.multiorder.fits,4 sky map, the 90% credible region is 3203 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 6033 +/- 1778 Mpc (a posteriori mean +/- standard deviation).
The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [4]).
Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-45,+45] seconds around the time of the GW alert. All the requested event mode data was delivered to the ground.
The BAT likelihood search, NITRATES [5], performed on the temporal window [T0-20 s, T0+20 s], detects a burst candidate with a sqrt(TS) of 8.0 in a 8.192 s analysis time bin, starting at T0 - 10.240 s
The 90% credible area is 2666 deg2 and the 50% credible area is <1 deg2.
The integrated probability inside the coded field of view is 83%.
The joint LVK-Swift/BAT localization probability map peaks at
RA = 85.341 deg,
Dec = -47.554 deg.
A circle with a radius of 10 arcmin around this position contains 74% of the integrated joint probability.
Swift has already initiated TOO followup of this position with XRT and UVOT. Results will be reported in future circulars. We encourage followup by other, more sensitive, facilities.
A plot of the Swift/BAT probability skymap can be viewed here:
[https://guano.swift.psu.edu/trigger_report?id=762004910/#:~:text=Probability%20Skymap](https://guano.swift.psu.edu/trigger_report?id=762004910/#:~:text=Probability%20Skymap)
The Swift/BAT probability skymap file can be downloaded from the link here:
[https://guano.swift.psu.edu/files/762004910/0_n_PROBMAP](https://guano.swift.psu.edu/files/762004910/0_n_PROBMAP)
Instructions on how to read and manipulate this map can be found here:
[https://guano.swift.psu.edu/documentation](https://guano.swift.psu.edu/documentation)
More details about this burst can be found on the trigger report page here:
[https://guano.swift.psu.edu/trigger_report?id=762004910](https://guano.swift.psu.edu/trigger_report?id=762004910)
A search performed by the RAVEN pipeline [6] found a temporal coincidence between S250223dk and a sub-threshold Swift/BAT trigger with ID 762004910. The GRB trigger time is 10.24 seconds before the GW candidate event. The estimated joint false alarm rate for the coincidence using just timing info is 1.9e-07 Hz, or about one in a month. The GRB candidate was found during a joint targeted search between the LIGO/Virgo/KAGRA collaboration and Swift/BAT-GUANO, and has a false alarm rate of 7.4e-05 Hz, or about one in 3 hours.Combined sky maps are also available:
* combined-ext.multiorder.fits,0, an initial localization, distributed via GCN notice about 12 hours after the candidate event time.
* combined-ext.multiorder.fits,1, an initial localization, distributed via GCN notice about 12 hours after the candidate event time.
* combined-ext.multiorder.fits,2, an initial localization, distributed via GCN notice about 13 hours after the candidate event time.
For the combined-ext.multiorder.fits,2 sky map, the 90% credible region is <1 deg2. The joint localization is dominated by the Swift/BAT-GUANO candidate. Considering the overlap of the individual sky maps, the estimated joint false alarm rate for the spatial and temporal coincidence is 5.1e-09 Hz, or about one in 6 years.
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] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. (2023) arXiv:2305.05625
[2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
[3] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013
[4] Tohuvavohu et al. ApJ, 900, 1 (2020)
[5] DeLaunay & Tohuvavohu, ApJ, 941, 169 (2022)
[6] Urban, A. L. 2016, Ph.D. Thesis https://dc.uwm.edu/etd/1218 and Piotrzkowski, B. J. 2022, Ph.D. Thesis https://dc.uwm.edu/etd/3060
GCN Circular 39444
Subject
LIGO/Virgo/KAGRA S250223dk: DDOTI Upper Limits on the Optical Counterpart
Date
2025-02-24T04:53:21Z (3 months ago)
From
Alan Watson at UNAM <alan@astro.unam.mx>
Via
Web form
Alan M. Watson (UNAM), William H. Lee (UNAM), Océlotl López (UNAM), Camila Angulo Valdez (UNAM), Rosa L. Becerra (U Roma), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), and Margarita Pereyra (UNAM), and Eleonora Troja (U Roma) report:
We imaged the field of the Swift/BAT-GUANO trigger possibly associated with LIGO/Virgo/KAGRA S250223dk (GCN Circ. 39443) with the DDOTI wide-field imager at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in Mexico (http://ddoti.astroscu.unam.mx).
We observed the whole BAT error region from 2025-02-24 03:56 UTC to 04:24 UTC (15.9 to 16.4 hours after the GW event) at extremely high airmass. We obtained a total exposure of 24 minutes.
Comparing our observations to the USNO-B1 and PanSTARRS PS1 DR2 catalogues, we detect no uncatalogued sources within the observed BAT error region to a 5-sigma AB limiting magnitude of
w > 20.0.
Our photometry is in the AB system and is not corrected for Galactic extinction.
We thank the staff of the Observatorio Astronómico Nacional in San Pedro Mártir.
GCN Circular 39461
Subject
LIGO/Virgo/KAGRA S250223dk and Swift/BAT-GUANO ID 762004910: Upper limits from Fermi-GBM Observations
Date
2025-02-24T19:45:19Z (3 months ago)
From
Lorenzo Scotton at UAH <lscottongcn@outlook.com>
Via
Web form
L. Scotton (UAH) reports on behalf of the Fermi-GBM Team:
For S250223dk and the Swift/BAT-GUANO ID 762004910 (The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration along with the Swift/BAT-GUANO team, GCN 39443) and using the combined skymap combined-ext.multiorder.fits,3, Fermi-GBM was observing 100% 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 S250223dk, neither for the Swift/BAT-GUANO trigger ID 762004910. 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 joint localization region is behind the Earth for Fermi, located at RA=267.1, Dec=-5.5 with a radius of 68.0 degrees. We therefore set upper limits on impulsive gamma-ray emission for the joint 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 the joint LVK-Swift/BAT localization probability (in units of 10^-7 erg/s/cm^2):
Timescale Soft Normal Hard
------------------------------------
0.128 s: 2.00 2.90 6.40
1.024 s: 0.69 0.95 1.90
8.192 s: 0.23 0.32 0.63
Assuming the median luminosity distance of 6033 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: 14.15 16.94 64.06
1.024s: 4.88 5.55 19.02
8.192s: 1.63 1.87 6.31
GCN Circular 39462
Subject
LIGO/Virgo/KAGRA S250223dk: DECam DESGW Candidates
Date
2025-02-24T22:36:16Z (3 months ago)
Edited On
2025-04-01T15:07:23Z (2 months ago)
From
Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Isaac McMahon at University of Zürich <isaac.mcmahon@ligo.org>
Via
Web form
I. McMahon (UZH), S. MacBride (UZH), H. T. Diehl (FNAL), S. Kaur (U. of Michigan/UZH), L. Joseph (Benedictine U.), N. Sherman (Boston U.), K. Herner (Fermilab), M. Soares-Santos (UZH), reporting on behalf of the Dark Energy Survey Gravitational Wave (DESGW) Team:
At 03:45:38 UTC, the Dark Energy Camera (DECam) began observing in response to the joint LVK/Swift-BAT RAVEN alert issued for the candidate gravitational-wave event S250223dk (GCN 39443). We observed two fields centered on the following ICRS coordinates
(85.80,-47.60)
(85.83,-47.63)
Both fields were observed in DECam g, r, i, and z filters with 360 second exposures. The limiting magnitude achieved is 23.36 in r-band.
Images were processed by our difference imaging pipeline (Herner et al. 2020) using DES and public DECam images as templates. We employ the autoscan machine learning code (Goldstein et al. 2015) to reject subtraction artifacts. Candidates were initially selected by requiring at least two high signal to noise detections, which were separated in time in order to reject moving objects. We also require an autoscan score of at least 0.7 on at least one of those detections.
After candidate selection, we report five high confidence candidates (listed below). After vetting and identification, four candidates were classified as nuclear candidates (likely AGNs), one candidate (2017239) has been labeled as a possible supernova. All candidates have a host galaxy match within 1 arcsecond. None of the candidate hosts are included in the ALLWISE and MILLIQUAS AGN catalogs (Secrest et. al 2015, Flesch 2023). No other candidates were found in the area. We encourage followup of the five candidates identified herein.
| id | AT name | ra | dec | discovery_date (UT) | mag_g | mag_g_err | mag_r | mag_r_err | mag_i | mag_i_err | mag_z | mag_z_err |
| --------------- | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ------ | ----- | ------ | ----- | ------ | ----- |
| 2014974 | AT2025cpl | 86.327994 | -47.827215 | 2025-02-24 03:45:38.271 | 22.487 | 0.043 | 22.151 | 0.031 | 21.744 | 0.053 | 21.894 | 0.106 |
| 2014991 | AT2025cpm | 86.516403 | -47.893872 | 2025-02-24 03:45:38.271 | 23.448 | 0.107 | 22.905 | 0.060 | 22.453 | 0.103 | 22.308 | 0.153 |
| 2015571 | AT2025cpo | 86.105565 | -47.689987 | 2025-02-24 03:45:38.271 | 22.972 | 0.053 | N/A | N/A | 22.423 | 0.084 | 22.765 | 0.203 |
| 2015600 | AT2025cpp | 86.176728 | -47.763957 | 2025-02-24 03:52:10.108 | 21.980 | 0.024 | N/A | N/A | 22.097 | 0.061 | 22.312 | 0.132 |
| 2017239 | AT2025cpq | 85.552523 | -47.577823 | 2025-02-24 03:58:42.138 | 24.503 | 0.268 | 23.980 | 0.161 | 23.972 | 0.320 | 24.265 | 0.851 |
The DECam Search & Discovery Program for Optical Signatures of Gravitational Wave Events (DESGW) is carried out by the Dark Energy Survey (DES) collaboration in partnership with wide ranging groups in the community. DESGW uses data obtained with the Dark Energy Camera (DECam), which was constructed by the DES collaboration with support from the Department of Energy and member institutions, and utilizes data as distributed by the Science Data Archive at NOIRLAB. NOIRLAB is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the Cerro Tololo observatory staff for their support in acquiring these observations.
GCN Circular 39465
Subject
LIGO/Virgo/KAGRA S250223dk and Swift/BAT-GUANO ID 762004910: Magellan upper limits.
Date
2025-02-25T02:44:55Z (3 months ago)
From
Harsh Kumar at Center for Astrophysics | Harvard & Smithsonian <harshkosli13@gmail.com>
Via
Web form
H. Kumar, E. Berger, A. Villar, D. Hiramatsu, P. K. Blanchard, K. D. Soto, S. K. Yadavalli, A. Gagliano, C. Ransome, Anya Nugent, Y. Dong (Harvard) report:
We obtained imaging with the IMACS imager on the 6.5m Magellan Baade telescope to search for an optical counterpart following the announcement of Swift/BAT-GUANO 762004910 (GCN #39443) event, coincident with gravitational wave event S250223dk. We obtained 4 x 120-second images in each of g- and r-band, starting about 16.6 hours after the GW trigger, targeting the Swift/BAT-GUANO localization of 5 arcmin around RA(J2000)= 85.341 deg, Dec(J2000)= -47.554, covering ~95% of the joint LVK-Swift/BAT localization 50% credible region.
We do not detect new transients above 5-sigma level up to ~23.7 mag in the g-band and ~23.5 mag in the r-band after performing image subtraction with Legacy Survey-DECam images as a reference.
We checked for the DECam DESGW candidates (GCN #39462) in our images. One source (AT2025cpq) falls in the area covered by Magellan. The source is marginally detected at <3 Sigma. We performed forced photometry and obtained the following magnitudes:
—------------------------------------------
Name | Filter | Magnitude +/- e_magnitude
—------------------------------------------
AT2025cpq | g | 24.04 +/- 0.48
AT2025cpq | r | 23.78 +/- 0.28
—-------------------------------------------
We thank Yuri Beletsky for the rapid execution of these observations.
GCN Circular 39466
Subject
LIGO/Virgo/KAGRA S250223dk and Swift/BAT-GUANO ID 762004910: AstroSat CZTI non-detection and upper limits
Date
2025-02-25T04:51:53Z (3 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 S250223dk (UTC 2025-02-23 12:01:15). We use the combined-ext.multiorder.fits map (https://gracedb.ligo.org/api/superevents/S250223dk/files/combined-ext.multiorder.fits,3) 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 = 15:49:17.1, 70:18:47.9 (237.32, 70.31), which is ~153 deg away from the maximum probability location. At the time of the merger event, the Earth-satellite-transient angle corresponding to the BAT location is ~126 deg and hence is not 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 1.0 (100%).
CZTI data were de-trended to remove orbit-wise background variation. We then searched data from the four independent, identical quadrants to look for coincident spikes in the count rates. 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^-4. 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) |
| ------------ | ------------ | ------------ |
| 0.1 | 7.01e-06 | 7.01e-07 |
| 1.0 | 1.73e-06 | 1.73e-06 |
| 10.0 | 3.01e-07 | 3.01e-06 |
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
GCN Circular 39468
Subject
LIGO/Virgo/KAGRA S250223dk and Swift/BAT-GUANO ID 762004910: DECam GW-MMADS candidates
Date
2025-02-25T06:46:50Z (3 months ago)
From
xjh@andrew.cmu.edu
Via
Web form
Lei Hu (CMU), Xander J. Hall (CMU), Tomás Cabrera (CMU), Antonella Palmese (CMU), Igor Andreoni (UNC), Brendan O’Connor (CMU), Keerthi Kunnumkai (CMU), on behalf of the GW-MMADS team
The high probability area of the joint LVK/Swift-GUANO alert for the gravitational wave candidate S250223dk (GCN 39443) was observed using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope by the Dark Energy Survey Gravitational Wave (DESGW) program (PI: Soares-Santos; GCN 39462). Observations started at 2025-02-24T03:45:38 on the night of the alert, and at 2025-02-25T00:50:13 on the following night. We find that the median 5sigma depths of the exposures are r\~24.7 mag on the first night and r\~24.9 mag on the second night.
As part of the Gravitational Wave Multi-Messenger Astronomy DECam Survey (GW-MMADS), we run the SFFT difference imaging (Hu et al. 2022) on the available images using templates from DES, filter out likely stars and moving objects, visually inspect the remaining transients. We posted on TNS new transients from this analysis and report here the most likely extragalactic transients we find within the joint LVK-GUANO skymap 95% credible interval area:
| id | AT name | ra | dec | discovery_date (UT) | mag_r | mag_r_err | mag_r-mag_i |
| ------------------------ | ---------- | ---------- | ---------- | ----------------------- | ------ | ----- | ----- |
| C202502240542297m473928 | AT 2025cqb | 85.623700 | -47.657680 | 2025-02-24T03:58:42 | 23.04 | 0.05 | 0.20
| C202502240549253m474247 | AT 2025cqf | 87.355530 | -47.713117 | 2025-02-24T03:58:42 | 24.44 | 0.20 | N/A
| A202502240541155m473906* | AT 2025cpv | 85.314435 | -47.651722 | 2025-02-24T03:45:38 | 23.55 | 0.08 | 0.21
| C202502240541459m473517 | AT 2025cql | 85.441205 | -47.588088 | 2025-02-24T04:24:50 | 24.77 | 0.25 | N/A
| C202502240545203m483410 | AT 2025cqm | 86.334777 | -48.569306 | 2025-02-24T03:58:42 | 24.00 | 0.13 | N/A
| T202502240543468m475332 | AT 2025cqc | 85.944938 | -47.892308 | 2025-02-24T03:45:38 | 23.00 | 0.05 | 0.21
| C202502240544410m483014 | AT 2025cqe | 86.170896 | -48.503829 | 2025-02-24T03:58:42 | 23.75 | 0.10 | 0.49
| A202502240546005m475901* | AT 2025cqd | 86.502090 | -47.983653 | 2025-02-24T03:45:38 | 22.48 | 0.03 | 0.23
* Possible stellar origin
Magnitudes reported are from the discovery date and not corrected from Milky Way extinction.
Further analysis is underway.
We thank the CTIO and NOIRLab staff for supporting observations and data calibration.
GCN Circular 39497
Subject
LIGO/Virgo/KAGRA S250223dk: DECam optical detections of Swift X-ray sources
Date
2025-02-26T17:49:58Z (3 months ago)
From
Antonella Palmese at Carnegie Mellon University <apalmese@andrew.cmu.edu>
Via
Web form
Antonella Palmese (CMU), Lei Hu (CMU), Xander J. Hall (CMU), Tomás Cabrera (CMU), Igor Andreoni (UNC), Brendan O’Connor (CMU), Keerthi Kunnumkai (CMU), report on behalf of the GW-MMADS team:
The high probability area of the joint LVK/Swift-GUANO alert for the gravitational wave candidate S250223dk (GCN 39443) was observed using the wide-field Dark Energy Camera (DECam) on the 4m Blanco telescope (GCN 39462), and we analyzed the data from observations starting 2025-02-24T03:45:38 and 2025-02-25T00:50:13 (GCN 39468).
We detect a variable source consistent with the location of the Swift XRT source S250223dk_X1 (GCN 39485) at the position of a Quaia (Storey-Fisher et al. 2024) likely quasar source (unWISE source id 0850m470o0042962, RA, dec=85.499,-47.358 [J2000]) with a photometric redshift of 1.4+-0.2, consistent with the distance of the GW alert. Stacked difference imaging with respect to templates from the Dark Energy Survey data reveals a clear source (AT 2025csi) in the griz bands. Our preliminary photometry on the difference imaging results in the following magnitudes:
| MJD | Band | mag |
| ---------- | ---- | -------------- |
| 60730.1567 | g | 20.90 +- 0.01 |
| 60730.1748 | i | 20.38 +- 0.01 |
| 60731.0349 | g | 20.82 +- 0.007 |
| 60731.0530 | i | 20.41 +- 0.009 |
We also inspected archival DECam data at this location and note that this possible quasar showed a steady ~1 mag increase in brightness in all bands between 2013 and 2018. Some archival detections are also present from ATLAS forced photometry over the past two years. The archival detections suggest that the source variability may be unrelated to the GW alert, although we encourage follow-up observations to establish the nature of this object and whether it is currently experiencing flaring activity.
We also detect a transient (AT 2025cpu) in the griz bands consistent with the location of S250223dk_X5 at position RA, dec= 85.51643, -47.70934 [J2000], with the following magnitudes:
| MJD | Band | mag |
| ---------- | ---- | ------------- |
| 60730.1569 | g | 22.71 +- 0.03 |
| 60730.1748 | i | 22.63 +- 0.07 |
| 60731.0349 | g | 22.55 +- 0.03 |
| 60731.0530 | i | 22.74 +- 0.05 |
We note that this source appears as a nuclear transient in a possible galaxy with photometric redshift of 0.797+-0.088 (from the DESI Legacy Survey photometric redshift catalog). The archival DECam observations for this source show variability of up to a magnitude, thus we cannot exclude that these detections are due to AGN variability.
Further analysis of these sources is planned and follow-up observations are encouraged.
We thank the CTIO and NOIRLab staff for supporting observations and data calibration.
GCN Circular 39499
Subject
LIGO/Virgo/KAGRA S250223dk and Swift/BAT-GUANO ID 762004910: Updated Sky localization and Coincidence with External Event
Date
2025-02-26T18:26:53Z (3 months ago)
From
ethan.payne@ligo.org
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration along with the Swift/BAT-GUANO team 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 S250223dk (GCN Circular 39443). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:
https://gracedb.ligo.org/superevents/S250223dk
For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 18323 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 9589 +/- 6347 Mpc (a posteriori mean +/- standard deviation).
A search performed by the RAVEN pipeline [2] found a temporal coincidence between S250223dk and a sub-threshold Swift/BAT trigger with ID 762004910 (GCN circular 39443). The GRB trigger time is 10.24 seconds before the GW candidate event. The estimated joint false alarm rate for the coincidence using just timing info is 1.9e-07 Hz, or about one in a month. The GRB candidate was found during a joint targeted search between the LIGO/Virgo/KAGRA collaboration and Swift/BAT-GUANO, and has a false alarm rate of 7.4e-05 Hz, or about one in 3 hours. RAVEN has also identified an additional detection from Fermi GBM.
Combined sky maps are also available:
* combined-ext.multiorder.fits,0, an initial localization, distributed via GCN notice about 12 hours after the candidate event time.
* combined-ext.multiorder.fits,1, an initial localization, distributed via GCN notice about 12 hours after the candidate event time.
* combined-ext.multiorder.fits,2, an initial localization, distributed via GCN notice about 13 hours after the candidate event time.
* combined-ext.multiorder.fits,3, an initial localization, distributed via GCN notice about 15 hours after the candidate event time.
* combined-ext.multiorder.fits,5, an updated localization, distributed via GCN notice about 3 days after the candidate event time.
The joint LVK-Swift/BAT localization probability map peaks at
RA = 85.341 deg,
Dec = -47.554 deg.
A circle with a radius of 10 arcmin around this position contains 52% of the integrated joint probability.
For the combined-ext.multiorder.fits,5 sky map, the 90% credible region is 1 deg2. Considering the overlap of the individual sky maps, the estimated joint false alarm rate for the spatial and temporal coincidence is 1.7e-08 Hz, or about one in 2 years. After considering trials factors, this means the joint FAR is larger than the ‘'high significance’' alert threshold and this joint event is no longer considered significant.
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. (2023) arXiv:2307.13380
[2] Urban, A. L. 2016, Ph.D. Thesis https://dc.uwm.edu/etd/1218 and Piotrzkowski, B. J. 2022, Ph.D. Thesis https://dc.uwm.edu/etd/3060
GCN Circular 39508
Subject
LIGO/Virgo/KAGRA S250223dk: Insight-HXMT/HE sub-threshold detection of a burst (HEB 250223501)
Date
2025-02-27T03:06:58Z (3 months ago)
From
yqzhang_cl@163.com
Via
Web form
Ce Cai (HEBNU), Shao-Lin Xiong, Xiao-Bo Li, Shu-Xu Yi, Shuang-Nan Zhang (IHEP) report on behalf of the Insight-HXMT team:
At the event time (T0 = 2025-02-23T12:01:15.360 UTC) of the LIGO/Virgo/KAGRA S250223dk event, which was reported to be possibly associated with a sub-threshold burst candidate by Swift/BAT (GCN 39443, 39499), Insight-HXMT was observing 99.97% of the localization probability region based on the combined skymap and there is no Insight-HXMT/HE triggers from the on-ground automated blind search around this event.
Thus, we implemented a targeted search [1] for sub-threshold burst signals within the window of T0+/-30 s using the central region of the joint LVK-Swift/BAT localization probability map (i.e. RA = 85.341 deg, Dec = -47.554 deg) and three types of GRB spectra. Our targeted search detected a burst candidate (named as HEB 250223501) with about 4 sigma on the 1-second timescale, starting at T0 - 9.50 s. We note that the time of this Insight-HXMT/HE burst candidate is well consistent with the start time of the Swift/BAT candidate (T0 - 10.240 s, GCN 39443), but the duration is much shorter, which could be explained as the detection energy band of Insight-HXMT/HE is higher than that of Swift/BAT in terms of GRB observation.
We note that these results are very preliminary. Refined analysis will be reported later.
Insight-HXMT is the first Chinese space X-ray telescope, which was funded jointly by the China National Space Administration (CNSA) and the Chinese Academy of Sciences (CAS). More information about it could be found at: http://hxmten.ihep.ac.cn/.
[1] Cai, C., Xiong, S. L., Li, C. K., et al. 2021, MNRAS, 508, 3910S
GCN Circular 39577
Subject
LIGO/Virgo/KAGRA S250223dk and Swift/BAT-GUANO ID 762004910: Swift/UVOT Upper Limits
Date
2025-03-03T19:55:29Z (3 months ago)
From
noelklin@umbc.edu
Via
Web form
Noel Klingler (NASA-GSFC / UMBC / CRESST II), Sam Shilling (Lancaster U.), P.A. Evans (U. Leicester), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), S.B. Cenko (NASA/GSFC), R.A.J. Eyles-Ferris (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL), S. Campana (INAF-OAB), J.J. Delaunay (PSU), M. De Pasquale (University of Messina), S. Dichiara (PSU), P. D'Avanzo (INAF-OAB), A. D'Aì (INAF-IASFPA) , V. D'Elia (ASI-SSDC & INAF-OAR), C. Gronwall (PSU), D. Hartmann (Clemson University), N. Klingler (NASA-GSFC / UMBC / CRESST II), N.P.M. Kuin (UCL-MSSL), S. Laha (NASA/GSFC), S.R. Oates (U. Birmingham), J.P. Osborne (U. Leicester), P. O'Brien (U. Leicester), M.J. Page (UCL-MSSL), S. Ronchini (PSU), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), M.H. Siegel (PSU), G. Tagliaferri (INAF-OAB), E. Troja (U Tor Vergata, INAF) report on behalf of the Swift team:
Swift/UVOT has carried out 1 observation of the combined LVK/Swift-BAT-GUANO error region for the GW trigger S250223dk (GCN 39443), centered on the most-probable location (RA,Dec = 85.4260, -47.5420 [J2000]; GCN 39485). UVOT collected 4.9 ks of data from 106 ks to 140 ks after the LVK trigger, using the White filter.
For the X-ray sources reported by Evans et al. (GCN 39485): X1, X5, and X6 are outside of the UVOT field of view. X2 is not detected, with a 3 sigma detection limit > 21.54 mag (AB). X3 is not detected, with a 3 sigma detection limit > 23.01 mag (AB). X7 is not detected, with a 3 sigma detection limit > 23.09 (AB). X4 is detected (20.14 +/- 0.03 mag, AB) but its position is coincident with a known source (classified as an AGN in the AllWISE catalog), WISEA J054219.27-473400.6.
No uncatalogued/transient sources are detected, with an average 3 sigma limit > 23.0 mag (AB) over the UVOT field of view.