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LIGO/Virgo/KAGRA S241125n

GCN Circular 38443

Subject
LIGO/Virgo/KAGRA S241125n: gamma-ray upper limits from joint observations by the LST-1 and MAGIC telescopes
Date
2024-12-05T15:33:48Z (9 months ago)
From
David Paneque at Max Planck Institute for Physics <dpaneque@mppmu.mpg.de>
Via
Web form
D. Paneque (MPP Munich), M. Teshima (MPP Munich), M. Seglar Arroyo (IFAE Barcelona), D. Miceli (INFN Padova), A. Stamerra (INAF Rome), J. Jimenez (IFAE Barcelona), S. Menon (University & INAF Rome), A. Simongini (University & INAF Rome) on behalf of the LST and MAGIC Collaborations report:

We observed the Swift/BAT-GUANO gamma-ray counterpart candidate (GRB 241125A, DeLaunay, GCNC 38308) presumably related to the GW S241125n (LVK Collaboration, GCNC 38305, 38315).  A total of 4h of pointed observations towards the gamma-ray counterpart candidate position were obtained, starting approximately on Nov 25, 20 UT (i.e. about 19h post trigger time). 

A preliminary offline analysis of the LST-1 and MAGIC dataset shows no excess of gamma-rays above 300 GeV at the position of the Swift/BAT-GUANO candidate. These results have been obtained using the LST analysis software, lstchain (https://zenodo.org/records/14227973 , v0.10.13), and the MAGIC analysis software MARS (Zanin et al. 2013). Observations were affected by the presence of clouds and by reduced atmospheric transparency. A more in-depth analysis of this data set is ongoing.

LST-1 is the first telescope of the Large-Sized Telescope (LST) for the Cherenkov Telescope Array Observatory. It is located on the Canary island of La Palma, Spain. The telescope design is optimized for observing gamma rays in the range from 20 GeV to 3 TeV.

The LST-1 contact persons for these observations are Masahiro Teshima (mteshima@mpp.mpg.de) and Monica Seglar-Arroyo (mseglar@ifae.es). The preliminary offline analysis has been performed by Sweta Menon (sweta.menon@inaf.it) and Juan Jimenez (juan.jimenez@ifae.es).

MAGIC is a system of two 17m-diameter Imaging Atmospheric Cherenkov Telescopes located on the Canary island of La Palma, Spain, and designed to perform gamma-ray astronomy in the energy range from 50 GeV to greater than 50 TeV.

The MAGIC contact persons for these observations are David Paneque (dpaneque@mpp.mpg.de), Antonio Stamerra (antonio.stamerra@inaf.it) and Davide Miceli (davide.miceli@pd.infn.it). The preliminary offline analysis has been performed by Andrea Simongini (andrea.simongini@inaf.it).

GCN Circular 38396

Subject
LIGO/Virgo/KAGRA S241125n: GRANDMA Optical Upper Limits
Date
2024-11-30T17:57:41Z (10 months ago)
From
Dalya Akl at American Uni. SHJ <dalyaakl.d@gmail.com>
Via
Web form
D. Akl (AUS), C. Andrade (UMN), E. de Bruin  (UMN), M. Tanasan (NARIT), N. Kochiashvili (AbAO), T. Hussenot-Desenonges (IJCLAB), M. Coughlin (UMN), M. Molham (NRIAG), S. Agayeva (Shamakhy Obs.), S. Antier (OCA), S. Karpov (FZU), D. Turpin (CEA-Saclay/Irfu), I. Tosta e Melo (UniCT-DFA), P. Hello (IJCLAB), P-A Duverne (APC), T. Pradier (Unistra/IPHC), N. Guessoum (AUS), M. Masek (FZU), K. Noysena (NARIT), M. Eldepsy (NRIAG), A. Shokry (NRIAG), E. Elhosseiny (NRIAG), A. Takey (NRIAG) on behalf of the GRANDMA collaboration:

We observed the field of the Swift/BAT-GUANO candidate counterpart (DeLaunay et al., GCN 38308) of the GW compact binary merger candidate S241125n (LIGO/Virgo/KAGRA Collaborations, GCN 38305) with the GRANDMA network. 

Our observations were conducted with the FRAM-CTA-N, TRT-SRO, and KAO telescopes starting ~0.8 days post T0 in the R, I, and i' bands, respectively. 

No clear candidate is identified within the 5 arcmin uncertainty region around the BAT position (RA, Dec = 58.079, 69.689 deg), with the following 5-sigma upperlimits: 

+---------------------+-------------+--------+----------+------------+
| T-mid(UTC)          | Exposure(s) | Filter | U.L.(AB) | Instrument |
+=====================+=============+========+==========+============+
| 2024-11-25T20:32:50 | 20x120      | R      | 16.38    | FRAM-CTA-N |
| 2024-11-25T23:38:47 | 23x150      | i'     | 21.87    | KAO        | 
| 2024-11-27T07:30:28 | 6x300       | I      | 19.83    | TRT-SRO    | 
| 2024-11-28T02:35:43 | 8x300       | I      | 20.22    | TRT-SRO    | 
+---------------------+-------------+--------+----------+------------+

This non-detection is consistent with Chen et al., GCN 38314, Watson et al., GCN 38317, Swain et al., GCN 38322, Mohan et al., GCN 38325, Jiang et al., GCN 38328, Becerra et al., GCN 38329.

Our FRAM-CTA-N observation includes the positions of all 5 Swift-XRT X-ray sources (Page et al., GCN 
38324). The KAO and TRT-SRO observations include only the position of the S241125n_X3 XRT source. We do not detect any clear candidates within the localization regions of these sources. This non-detection is consistent with Becerra et al., GCN 38329 and Akl et al., GCN 38334. 

Further, the FRAM-CTA-N observation includes all of the X-ray sources detected by EP-FXT (Wang et al., GCN 38345), while the KAO and TRT-SRO images include only the X-ray source detected by both modules of the EP-FXT with an averaged position at RA, Dec = 58.1097, 69.6392 deg and an uncertainty of 10 arcsec. We do not detect any of the EP-FXT sources across all of our images.  

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained in Johnson Cousin filters were calibrated using the Gaia DR3 Synphot catalog, while images taken with Sloan filters were calibrated using the Pan-STARRS DR1 catalog.

We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023).

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518).

GCN Circular 38356

Subject
LIGO/Virgo/KAGRA S241125n: Update on Coincidence False Alarm Method
Date
2024-11-28T02:26:46Z (10 months ago)
From
Brandon Piotrzkowski <brandon.piotrzkowski@ligo.org>
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration along with the Swift/BAT-GUANO Collaboration report:

A search performed by the RAVEN pipeline [1] (reported in GCN Circular 38309) found a spatio-temporal coincidence between S241125n and a sub-threshold Swift/BAT trigger with ID 754189311 (DeLaunay et al., GCN Circular 38308).

The estimated joint false alarm rate (FAR) for the spatial and temporal coincidence was reported to be 1.8e-11 Hz, or about one in 1e3 years (GCN 38315). See https://lscsoft.docs.ligo.org/raven/joint_far.html for a description of the Targeted Search joint FAR method. 

The difference between the joint FAR estimate in GCN 38315 and GCN 38308 can be accounted for by the updated gravitational-wave skymap, the use of the highest resolution information in the sky map combination, and by the fact that the GCN 38308 method included a correction for trials arising from multiple GW pipelines.

The Targeted Search joint FAR presented above is dominated by the low FAR of the confident GW signal, which is much lower than the detected rate of BBH mergers.  In this case of a confident GW signal, using a version of the Untargeted joint FAR method (see link above) is likely to more appropriately describe the background association rate. Using a conservative accounting of the BBH detection rate (1 per 3 days) along with the Untargeted joint FAR method increases the joint FAR to 1 per 6 years.

 [1] 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 38351

Subject
LIGO/Virgo/KAGRA S241125n: Preliminary flux estimate and spectral analysis of the Swift/BAT-GUANO candidate counterpart
Date
2024-11-27T19:23:19Z (10 months ago)
From
Jimmy DeLaunay at Penn State <delauj2@gmail.com>
Via
Web form
James DeLaunay (PSU), Aaron Tohuvavohu (Caltech), Samuele Ronchini (PSU), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:

We have performed a preliminary spectral study on the sub-threshold GRB candidate that was found to be spatially and temporally coincident with the GW event S241125n (GCN 38308). 

This candidate was identified in BAT-GUANO data by the NITRATES pipeline in an analysis time bin that starts at T_GW + 11.264 s and has a duration of 0.512 s. The NITRATES analysis uses Comptonized spectral templates that range from soft to hard GRB spectra. The maximum likelihood template found in the search had Epeak = 49 keV and spectral index = -1.2 (gamma on results webpage is -1 x spectral index). The best fit 15-350 keV flux for this spectral template at this position and analysis time bin is 1.1(-0.3, +0.2)E-7 erg/cm2/s with a 90% error bar. Note that the parameters of the template that maximized the likelihood in the low-latency search are not necessarily the true parameters of the signal.

Due to the low S/N of this event, error bars on Epeak and the spectral index are large and correlated. The spectral index is unconstrained in the range of -0.4 to -2.0, but the allowed values of Epeak vary with it. 
With a spectral index fixed at -0.6, the 90% confidence limits of Epeak are approximately 25 to 80 keV. With a spectral index fixed at -1.6, the 90% confidence limits of Epeak are approximately 20 to 300 keV.
The flux estimate varies with spectral index and Epeak. 

In the most recent joint localization map of the candidate GRB and S241125n, ~84% of the localization probability is within 5 arcminutes of the most likely position. The other 16% is spread out across the sky and is multimodal. Some of this remaining probability is also within BAT’s coded field of view and some is outside of it. The response changes greatly whether in or outside the coded field of view, which drives substantial range in the flux estimate depending on sky position. The Southern lobes of the joint localization are outside of the BAT’s coded field of view and thus have higher estimated flux values. This region is most likely excluded by external upper limits from Fermi-GBM (GCN 38316) and Konus-Wind (GCN 38321). The most likely position cannot be confidently excluded by these upper limits.

All values in this circular are preliminary.

GCN Circular 38350

Subject
LIGO/Virgo/KAGRA S241125n: Swift-UVOT Upper Limits
Date
2024-11-27T16:50:48Z (10 months ago)
From
N. Klingler at NASA-GSFC/UMBC/CRESST II <noelklingler@gmail.com>
Via
Web form
N.J. Klingler (NASA-GSFC / UMBC / CRESST II), S.R. Oates (Lancaster U.), A. Tohuvavohu (U. Toronto), A.A. Breeveld (UCL-MSSL), C. Gronwall (PSU), N.P.M. Kuin (UCL-MSSL), F.E. Marshall (NASA-GSFC), M. De Pasquale (University of Messina), M.J. Page (UCL-MSSL), S. Shilling (Lancaster U.), M.H. Siegel (PSU), A. D’Aì (INAF-IASFPA), P. D’Avanzo (INAF-OAB), A.P. Beardmore (U. Leicester), M.G. Bernardini (INAF-OAB), S. Campana (INAF-OAB), S.B. Cenko (NASA-GSFC), J.J. Delaunay (PSU), S. Dichiara (PSU), V. D’Elia (ASI-SSDC & INAF-OAR), R.A.J.Eyles-Ferris (U. Leicester), P.A. Evans (U. Leicester), R. Gayathri (PSU), D. Hartmann (Clemson University), J.A. Kennea (PSU), S. Laha (NASA/GSFC), H.A. Krimm (NSF), D. B. Malesani (DTU Space), P. O’Brien (U. Leicester), J.P. Osborne (U. Leicester), T. Partosan (NASA-GSFC), M. Perri (ASDC), K.L. Page (U. Leicester), D.M. Palmer (LANL), S. Ronchini (PSU), T. Sbarrato (INAF-OAB), B. Sbarufatti (INAF-OAB), G. Tagliaferri (INAF-OAB), and E. Troja (U. Tor Vergata, INAF) report on behalf of the Swift Team:

Swift has carried out observations of the error region of the BAT-GUANO candidate counterpart, which was detected 11.2641 s after T0 of S241125n (DeLaunay et al., GCN Circ. 38308).  Swift-UVOT observations were carried out using the White filter for a total of 4.8 ks, spanning from 55 ks to 74 ks after the LVC trigger.  

No new/uncatalogued sources were detected within the BAT-GUANO localization (RA, Dec = 58.079, +69.689 deg, r=5', 50% containment; GCN Circ. 38308), with an average 3sigma limit on the magnitude >23.2 (AB; White filter).

As reported by Evans et al. (GCN Circ. 38324), 5 X-ray sources were seen.  However, none of these sources were detected with UVOT.  S241125n_X1 and S241125n_X2 were located outside of UVOT's field of view.  S241125n_X3 was not detected, with a 3sigma limit on the magnitude (AB) >23.16.  S241125n_X4's position was heavily contaminated by image artifacts and coincidence loss from a bright nearby source.  S241125n_X5 was not detected, though its position was also affected by an image artifact from a bright nearby source, though to a lesser degree.  The precise upper limit at this location can not be accurately determined, but can be approximated as >23.

This circular is an official data product of the Swift-UVOT team.

GCN Circular 38347

Subject
LIGO/Virgo/KAGRA S241125n: EP-FXT follow-up observations (duplicate submission of 38345)
Date
2024-11-27T14:24:24Z (10 months ago)
Edited On
2024-11-27T15:15:55Z (10 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>
Via
Web form
This is a duplicate submission of GCN 38345.

GCN Circular 38346

Subject
LIGO/Virgo/KAGRA S241125n: EP-FXT follow-up observations (duplicate submission of 38345).
Date
2024-11-27T14:24:18Z (10 months ago)
Edited On
2024-11-27T15:15:18Z (10 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov>
Via
Web form
This is a duplicate submission of GCN 38345.

GCN Circular 38345

Subject
LIGO/Virgo/KAGRA S241125n: EP-FXT follow-up observations
Date
2024-11-27T14:24:16Z (10 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Y. L. Wang, S. X. Wen, W. X. Wang (NAO, CAS), B.-T. Wang (YNAO, CAS), T. C. Zheng (PMO, CAS), W. Yuan, D. Y. Li, Y. Liu (NAO, CAS), B. Zhang (UNLV) report on behalf of the Einstein Probe team:

Following on the trigger of the detection of the gravitational-wave (GW) event S241125n (LIGO/Virgo/KAGRA collaborations, GCN 38305) and the Swift/BAT-GUANO detection of a candidate counterpart (DeLaunay et al., GCN 38308), we performed a target-of-opportunity observation of the BAT position (RA, Dec = 58.079, +69.689 deg) with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP). The observation started at 2024-11-26 03:08:45 (UTC), about 26 hours after the GW event, with an exposure of ~ 11 ks.

Within the 5-arcmin error circle of Swift/BAT (50% containment; DeLaunay et al., GCN 38308), one X-ray source is detected by both modules of the EP-FXT, at an averaged position at R.A. = 58.1097 deg, DEC = 69.6392 deg (J2000), with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a photon index of 0.43 (+0.76/-0.74) (with a column density fixed at the Galactic value of 3.4 x 10^21 cm^-2). The derived average unabsorbed 0.5-10 keV flux is 1.17 (+1.18/-0.63) x 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. The location of this source is consistent with S241125n_X3, an uncatalogued X-ray source detected by Swift/XRT (Page et al., GCN 38324).

Outside the 5-arcmin error circle of the Swift/BAT source position, EP-FXT detected several other X-ray sources, some of which are consistent with the sources reported by Swift/XRT (Page et al., GCN 38324). We list the sources within the 10-arcmin radius in the followed two tables for FXT-A and FXT-B, respectively, with the seperations from the BAT source given. The observed 0.5-10 keV flux given in the tables is a rough estimate assuming the same spectral paramters as quoted before, and is not corrected for absorption. 

|    FXT-A source      | R.A. (deg, J2000) | DEC (deg, J2000) | Sep. (arcmin) | Flux (erg/s/cm^2) |
| EPF_J035226.4+693824*|    58.1096        |    69.6383       |    3.11       |  1.05 x 10^(-13)  | 
| EPF_J035109.4+693830*|    57.7917        |    69.6411       |    6.64       |  1.52 x 10^(-13)  |
| EPF_J035330.7+693812 |    58.3826        |    69.6391       |    7.00       |  1.64 x 10^(-13)  |
| EPF_J035104.0+693411 |    57.7667        |    69.5698       |    9.68       |  2.34 x 10^(-13)  |
| EPF_J035318.9+693257*|    58.3287        |    69.5492       |    9.88       |  1.28 x 10^(-13)  |
| EPF_J035113.2+694926*|    57.8023        |    69.8230       |    9.88       |  5.84 x 10^(-13)  |

|    FXT-A source      | R.A. (deg, J2000) | DEC (deg, J2000) | Sep. (arcmin) | Flux (erg/s/cm^2) |
| EPF_J035226.4+693824*|    58.1098        |    69.6401       |    3.00       |  1.87 x 10^(-13)  | 
| EPF_J035109.4+693830*|    57.7891        |    69.6418       |    6.68       |  2.45 x 10^(-13)  | 
| EPF_J035330.7+693812 |    58.3779        |    69.6368       |    6.98       |  1.52 x 10^(-13)  |
| EPF_J035044.0+694519 |    57.6832        |    69.7554       |    9.14       |  2.45 x 10^(-13)  |
| EPF_J035113.2+694926*|    57.8051        |    69.8238       |    9.89       |  7.71 x 10^(-13)  |
| EPF_J035028.2+694407 |    57.6174        |    69.7353       |    10.00      |  1.17 x 10^(-13)  |
* sources also detected by Swift/XRT (Page et al., GCN 38324)

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

GCN Circular 38334

Subject
LIGO/Virgo/KAGRA S241125n: COLIBRÍ Upper Limits on Optical Counterparts to Two of the Swift/XRT Sources
Date
2024-11-27T01:54:00Z (10 months ago)
From
Alan Watson at UNAM <alan@astro.unam.mx>
Via
email
D. Akl (AUS), S. Antier (OCA), J.-G. Ducoin (CPPM), Francesco Magnani
(CPPM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H.
Lee (UNAM), Jean-Luc Atteia (IRAP), Nathaniel R. Butler (ASU), Damien
Dornic (CPPM), Francis Fortin (IRAP), Simona Lombardo (LAM), and
Margarita Pereyra (UNAM) report:

We imaged the field of the Swift/BAT-GUANO candidate counterpart
(DeLaunay et al., GCN Circ. 38308) of the GW compact binary merger
candidate S241125n (LIGO/Virgo/KAGRA Collaborations, GCN Circ. 38305)
during the commissioning of the COLIBRÍ (SVOM/F-GFT) telescope at the
Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir in
Mexico.

We observed with the engineering test camera in a red filter that
approximates SDSS r from 2024-11-26 02:08 to 07:50 UTC (25.1 to 30.8
hours after the event) and obtained 255 minutes of exposure on a field
about 12 arcmin to a side centered on the BAT position given by
DeLaunay et al. (GCN Circ. 38308). Results from a subset of these
observations were reported by Watson et al. (GCN Circ. 38317).

Our image includes the positions of the S241125n_X3 and S241125n_X5
XRT sources (Page et al., GCN 38324). We do not detect any clear
candidates within the localization regions of these sources to a
3-sigma upper limit of

r > 23.4

Our upper limits are consistent with GCN. 38314, 38325, GCN 38328, GCN 38329.

We warmly thank the COLIBRÍ engineering team and the staff of the
Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir.
We warmly thank the GRANDMA IJCLAB team and S. Karpov for the access
of the STDWeb service for STDPipe.

GCN Circular 38333

Subject
LIGO/Virgo/KAGRA S241125n: SAGUARO follow-up observations with the MMT
Date
2024-11-27T00:47:02Z (10 months ago)
From
Manisha Shrestha at University of Arizona <mshrestha1@arizona.edu>
Via
Web form
Jillian C. Rastinejad (NU), Manisha Shrestha (UA), Griffin Hosseinzadeh (UCSD), David J. Sand (UA), Charles D. Kilpatrick (NU), Wen-fai Fong (NU), Bhagya Subrayan (UA), K. Azalee Bostroem (UA), Philip N. Daly (UA), Michael J. Lundquist (Keck), Kerry Paterson (MPIA) report on behalf of the SAGUARO collaboration:

We observed the field of the Swift/BAT trigger (DeLauney et al. GCN 38308) discovered within the localization region of the GW event S241125n (LIGO/Virgo/KAGRA GCN 38305) with the Binospec imager and spectrograph mounted on the MMT 6.5-meter telescope on Mount Hopkins, Arizona. We obtain 20x75 s imaging in the r-band at a mid-time of 2024-11-26 08:08:50 UT (1.30 days post-burst) and 30x60 s in the i-band at a mid-time of 2024-11-26 09:13:11 UT (1.34 days post-burst). Observations were taken at an average airmass of 1.3 and seeing of 1.2 - 1.8’’. Our observations covered 68.0 percent of the 5 arcmin BAT localization (DeLauney et al. GCN 38308) and the localization region of XRT source S241125n_X2 (Page et al. GCN 38324).  

Calibrated to PS1 (Flewelling et al. 2020), our imaging reached limiting magnitudes of r>25.5 mag and i>25.5 mag (3 sigma; AB system). Within the error circle of S241125n_X2, we report magnitudes for 5 sources, as shown in the table below. The bright source (S241125n_X2_O5) is in the PS1 catalog, and the rest are below the PS1 detection limit. Therefore we cannot determine whether these are transient sources.

name           RA (J2000)    Dec (J2000)   Comment      rmag       imag
S241125n_X2_O1 3:53:20.4510  +69:33:06.688 Point Src    22.3       21.6
S241125n_X2_O2 3:53:19.8100  +69:33:07.471 Extended     24.1       22.6
S241125n_X2_O3 3:53:19.6728  +69:33:13.275 Faint        25.4       26.3  
S241125n_X2_O4 3:53:19.0313  +69:33:10.008 Faint        25.4       24.6
S241125n_X2_O5 3:53:21.1088  +69:33:11.602 Point Src    18.9       18.4

The reported magnitudes are not corrected for the Milky Way galactic extinction value of E(B-V) = 0.4678 mag (Schlafly & Finkbeiner 2011). Further observations are planned. We thank Ryan Howie and Benjamin Weiner at the MMT for the rapid scheduling and execution of these observations.

*SAGUARO stands for Searches After Gravitational-waves Using ARizona's Observatories. It is a partnership between the University of Arizona and Northwestern University.

GCN Circular 38329

Subject
LIGO/Virgo/KAGRA S241125n: DDOTI Optical Upper Limits
Date
2024-11-26T17:19:16Z (10 months ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (Tor Vergata, Roma), Eleonora Troja (Tor Vergata, Roma), Camila Angulo Valdez (UNAM), Alan M. Watson (UNAM), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl López (UNAM), and Margarita Pereyra (UNAM) report:

We observed the field of LIGO/Virgo/KAGRA S241125n (LVKC, GCN 38305) with the DDOTI/OAN wide-field imager at the Observatorio Astronomico Nacional on Sierra San Pedro Martir (http://ddoti.astroscu.unam.mx) on the night of 2024-11-26 UTC.

We performed two pointings which include the position of the Swift/GUANO candidate counterpart (DeLaunay et al., GCN 38308) and the 5 X-ray sources reported by XRT (Page et al., GCN 38324). DDOTI observed from 02:01 UTC to 04:17 UTC (from T+25.0 to T+27.2 hours after the event) down to a 10-sigma limiting magnitude of w = 20.4.

Comparing our observations to the USNO-B1 and PanSTARRS PS1 DR2 catalogs we detect no uncatalogued sources within the observed field to our 10-sigma limit. 

We thank the staff of the Observatorio Astronómico Nacional in San Pedro
Mártir.

GCN Circular 38328

Subject
LIGO/Virgo/KAGRA S241125n: Gaoyazi/GOT optical observations
Date
2024-11-26T16:22:50Z (10 months ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
Web form
S.Q. Jiang, X. Liu, S.Y. Fu, J. An, Z.P. Zhu, D. Xu (NAOC) report on behalf of a large collaboration:

We observed the field of the Swift/BAT-GUANO candidate counterpart (DeLaunay et al., GCN 38308) of the LIGO/Virgo/KAGRA gravitational wave event S241125n (LIGO/Virgo/KAGRA, GCN 38305; GCN 38309; GCN 38312; GCN 38313) using the GOT-0.5m telescope located at Gaoyazi, Xinjiang, China. Observations started at 18:49:17 UT on 2024-11-25, i.e., 17.796 hr after the S241125n trigger, and we obtained 32 x 180 s frames in the Sloan r filter.

No new optical source is detected in our stacked image within the Swift/BAT-GUANO 5 arcmin 50% containment area via cross-match with the PanSTARRS catalog, down to the 5-sigma limiting magnitude of r > 20.6, calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction.

We acknowledge the excellent support from L.F. Huo and M.M. Yang for enabling these observations.

GCN Circular 38322

Subject
LIGO/Virgo/KAGRA S241125n: HCT upper limit
Date
2024-11-26T14:31:18Z (10 months ago)
From
V. Swain at IIT Bombay <vishwajeet.s@iitb.ac.in>
Via
Web form

V. Swain (IITB), R.S. Teja (IIA), T. Mohan (IITB), V. Bhalerao (IITB), G. C. Anupama (IIA), S. Barway (IIA), D.K. Sahu (IIA), Surya Prakash (IAO):

We observed field of LIGO/Virgo/KAGRA S241125n event (GCN Circ. 38305

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, 38309, 38312, 38313, 38315) with the 2.0m Himalayan Chandra Telescope (HCT) of the Indian Astronomical Observatory (IAO), FOV: 10 x 10 arcmin and scale: 0.296 arcsec/pixel. We obtained multiple exposures in the SDSS r’ filter and performed image subtraction on individual images with a limiting magnitude of ~22.5. No new transient was detected in the subtracted images at the coordinate reported by DeLaunay et al., GCN 38308. Furthermore, after stacking the images and conducting catalog matching, no new source was identified. The derived upper limit is as follows:

JD (mid)t-t0 (hours)FilterExposure (s)Upper limit (AB)
2460640.46615763922.16r'3 x 120023

Our result is consistent with (Watson et al., GCN 38317

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; Chen et al., GCN 38314).

The image subtraction was performed using the ZOGY algorithm, with the PanSTARRS-1 survey used as the reference image. The magnitudes are calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.

These observations were carried out under the ToO program HCT-2024-C3-P38. 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 38321

Subject
LIGO/Virgo/KAGRA S241125n: Upper limits from Konus-Wind observations
Date
2024-11-26T12:20:39Z (10 months ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A.Ridnaia, D. Frederiks, A.Lysenko, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

Konus-Wind (KW) was observing the whole sky at the time of the
LIGO/Virgo/KAGRA event S241125n (2024-11-25 01:01:16.780 UTC, hereafter T0;
LIGO/Virgo/KAGRA Collaboration GCN Circ. 38305, 38309, 38312, 38313, 38315).

No triggered or waiting-mode KW GRBs happened between ~2 days
before and ~15 hours after T0 (the current available data).
Using waiting-mode data within the interval T0 +/- 200 s,
we found no significant (> 5 sigma) excess over the background
in both KW detectors on temporal scales from 2.944 s to 100 s.

We estimate an upper limit (90% conf.) on the 20 - 1500 keV fluence
to 7.5x10^-7 erg/cm^2 for a burst lasting less than 2.944 s and having a
typical KW short GRB spectrum (an exponentially cut off power law (CPL) 
with alpha =-0.5 and Ep=500 keV) and to 4.7x10^-7 erg/cm^2 for a burst
having spectrum similar to that of GRB 170817A (a CPL with alpha =-0.62
and Ep=185 keV). For a typical long GRB spectrum (the Band function with
alpha=-1, beta=-2.5, and Ep=300 keV), the corresponding limiting peak flux 
is 2.2x10^-7 erg/cm^2/s (20 - 1500 keV, 2.944 s scale).

All the quoted values are preliminary.

GCN Circular 38317

Subject
LIGO/Virgo/KAGRA S241125n: COLIBRÍ Upper Limits on a Optical Counterpart at the BAT Position
Date
2024-11-26T06:20:40Z (10 months ago)
From
Alan Watson at UNAM <alan@astro.unam.mx>
Via
email
Alan M. Watson (UNAM), S. Antier (OCA), Stéphane Basa (UAR Pytheas),
William H. Lee (UNAM), D. Akl (AUS), Jean-Luc Atteia (IRAP), Nathaniel R.
Butler (ASU), Damien Dornic (CPPM), Francis Fortin (IRAP), J.-G. Ducoin
(CPPM), Simona Lombardo (LAM), Francesco Magnani (CPPM), and Margarita
Pereyra (UNAM) report:

We imaged the field of the Swift/BAT-GUANO candidate counterpart (DeLaunay
et al., GCN Circ. 38308) of the GW compact binary merger candidate S241125n
(LIGO/Virgo/KAGRA Collaborations, GCN Circ. 38305) during the commissioning
of the COLIBRÍ (SVOM/F-GFT) telescope at the Observatorio Astronómico
Nacional on the Sierra de San Pedro Mártir in Mexico.

We observed with the engineering test camera in a red filter that
approximates SDSS r from 2024-11-26 02:08 to 05:49:58 UTC (25.1 to 28.8
hours after the event) and obtained 188 minutes of exposure on a field
about 12 arcmin to a side centered on the BAT position given by DeLaunay et
al. (GCN Circ. 38308). The data were reduced using custom software and then
analysed and calibrated against the PS1 catalog using the STDWeb service.

No clear candidate is identified to a 5-sigma upper limit of

r > 23.0

This limit is below the catalog limit, so definite statements on the
absence of an optical counterpart will require a second epoch of similar
depth.

Further observations are planned.

We warmly thank the COLIBRÍ engineering team and the staff of the
Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir. We
warmly thank the GRANDMA IJCLAB team and S. Karpov for the access of the
STDWeb service for STDPipe.

GCN Circular 38316

Subject
LIGO/Virgo/KAGRA S241125n: Upper limits from Fermi-GBM Observations
Date
2024-11-25T23:53:23Z (10 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 LIGO/Virgo/Kagra (LVK) S241125n (GCN 38305) and the Swift/BAT-GUANO candidate counterpart (Tohuvavohu, A. et al. 2024, GCN 38308), we used the LVK/BAT combined skymap (combined-ext.multiorder.fits,2; LVK and Swift/BAT, GCN 38315). Fermi-GBM was observing 97.1% of the localization probability at event time.

There was no Fermi-GBM onboard trigger around the event time of the LVK detection of GW trigger S241125n. 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-BAT localization region is behind the Earth for Fermi, located at an RA=335.8, Dec=12.1 with a radius of 67.7 degrees. We therefore set upper limits on impulsive gamma-ray emission for the LVK-BAT 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:   1.7      2.9        5.5 
1.024 s:   0.6      1.0        1.7 
8.192 s:   0.2      0.3        0.6 

Assuming the median luminosity distance of 3213.8 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:    3.3      4.9      15.8
1.024s:    1.3      1.6       4.9
8.192s:    0.4      0.5       1.6

GCN Circular 38315

Subject
LIGO/Virgo/KAGRA S241125n: Updated Coincidence with External Event
Date
2024-11-25T22:54:22Z (10 months ago)
From
Brandon Piotrzkowski <brandon.piotrzkowski@ligo.org>
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration along with the Swift/BAT 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 S241125n (GCN Circular 38305).

A search performed by the RAVEN pipeline [1] previously (reported in GCN Circular 38309) found a temporal coincidence between S241125n and a sub-threshold Swift/BAT trigger with ID 754189311 (DeLaunay et al., GCN Circular 38308).

Combined sky maps are also available:
 * combined-ext.multiorder.fits,0, an updated localization, distributed via GCN notice about 17 hours after the candidate event time.
 * combined-ext.multiorder.fits,1, an updated localization, distributed via GCN notice about 20 hours after the candidate event time.
 * combined-ext.multiorder.fits,2, an updated localization, distributed via GCN notice about 21 hours after the candidate event time.

For the combined-ext.multiorder.fits,2 sky map, the 90% credible region is 76 deg2 while the 50% credible region is less than 1 deg2. Considering the overlap of the individual sky maps, the estimated joint false alarm rate for the spatial and temporal coincidence is 1.8e-11 Hz, or about one in 1e3 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] 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 38314

Subject
LIGO/Virgo/KAGRA S241125n: Kinder optical upper limit in the BAT error region
Date
2024-11-25T22:44:46Z (10 months ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
T.-W. Chen (NCU), S. J. Brennan (OKC), S. Ronchini (PSU), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. L. Thakur (INAF-IAPS), S. J. Smartt (Oxford/QUB), C.-S. Lin, C.-C. Ngeow, A. Aryan, C.-H. Lai, H.-Y. Miao (all NCU),  I. Mandel (Monash), A. K. H. Kong (NTHU),  S. Yang (HNAS), Morgan Fraser (UCD), S. Srivastav (Oxford), M. Nicholl, M. Fulton, T. Moore, D. R. Young, and K. W. Smith (QUB) report:

We observed the field of the Swift/BAT-GUANO candidate counterpart (DeLaunay et al., GCN 38308) for the LIGO/Virgo/KAGRA S241125n (LIGO/Virgo/KAGRA, GCN 38305; GCN 38309; GCN 38312) using the 1m LOT and 40cm SLT at the Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024 arXiv:2406.09270). The first SLT epoch of observations started at 18:55 UTC on 25th November 2024 (MJD 60639.788), 17.90 hr after the S241125n trigger. 

We utilized the astroalign (Beroiz et al., 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al., 2022, ApJ, 935, 167) packages to align and stack the individual frames. We do not find any new and uncataloged optical source in the stacked frames within the 5 arcmin 50% error circle of the Swift/BAT-GUANO localization, in comparison to the Pan-STARRS1 3Pi archive images (Chambers et al., 2016 arXiv:1612.05560). The LOT has a field of view of 13 x 13 arcmin and SLT's is 12.8 x 12.8 arcmin. 

Moreover, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform PSF photometry on our stacked frames, after subtracting Pan-STARRS1 references images using SFFT (Lei Hu et al., 2022, ApJ, 936, 157). The details of the observations and measured 3-sigma upper limit (in the AB system) are as follows:
----------------------------------------------------------------------------------------------------
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60639.786 | 17.85 | 300 * 6 | > 22.6 |0".929  | 1.75
SLT | i | 60639.788 | 17.90 | 300 * 10 | > 21.05 | 1".42  | 1.75

The presented magnitudes are calibrated using Pan-STARRS1 field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 1.24 mag and A_i = 0.93 mag in the direction of the transient (Schlafly & Finkbeiner 2011) taken by NED.

GCN Circular 38313

Subject
LIGO/Virgo/KAGRA S241125n: Updated Sky localization
Date
2024-11-25T22:09:34Z (10 months ago)
From
Aaron Zimmerman at U. of Texas at Austin <aaron.zimmerman@utexas.edu>
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further offline 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 S241125n (GCN Circular 38305). 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/S241125n

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 2196 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 4174 +/- 1591 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

GCN Circular 38312

Subject
LIGO/Virgo/KAGRA S241125n: Updated Coincidence with External Event
Date
2024-11-25T21:06:54Z (10 months ago)
From
Brandon Piotrzkowski <brandon.piotrzkowski@ligo.org>
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration along with the Swift/BAT 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 S241125n (GCN Circular 38305).

A search performed by the RAVEN pipeline [1] previously (reported in GCN Circular 38309) found a temporal coincidence between S241125n and a sub-threshold Swift/BAT trigger with ID 754189311 (DeLaunay et al., GCN Circular 38308). At the current time, the GRB trigger time has been updated to be 11.5 seconds after the GW candidate event. The estimated joint false alarm rate for the coincidence using just timing info is 1.5e-10 Hz, or about one in 1e2 years.

Combined sky maps are also available:
 * combined-ext.multiorder.fits,0, an updated localization, distributed via GCN notice about 17 hours after the candidate event time.
 * combined-ext.multiorder.fits,1, an updated localization, distributed via GCN notice about 20 hours after the candidate event time.

For the combined-ext.multiorder.fits,1 sky map, the 90% credible region is 174 deg2 while the 50% credible region is less than 1 deg2. Considering the overlap of the individual sky maps, the estimated joint false alarm rate for the spatial and temporal coincidence is 1.2e-10 Hz, or about one in 1e2 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] 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 38311

Subject
LIGO/Virgo/KAGRA S241125n: No counterpart candidates in INTEGRAL SPI-ACS prompt observation, comparisong with Swift/BAT counterpart
Date
2024-11-25T21:06:32Z (10 months ago)
From
Volodymyr Savchenko at UNIGE, EPFL <volodymyr.savchenko@epfl.ch>
Via
Web form

V. Savchenko, C. Ferrigno (ISDC/UniGE, Switzerland) J. Rodi (IAPS-Roma, Italy) A. Coleiro (APC, France) S. Mereghetti (INAF IASF-Milano, Italy)

on behalf of the INTEGRAL multi-messenger collaboration: https://www.astro.unige.ch/cdci/integral-multimessenger-collaboration

Using INTEGRAL/SPI-ACS realtime data (following [1]) we have performed a search for a prompt gamma-ray counterpart of S241125n (GCN 38305

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).

At the time of the event (2024-11-25 01:01:16.78 UTC, hereafter T0), INTEGRAL was operating in nominal mode. The peak of the event localization probability was at an angle of 104 deg with respect to the spacecraft pointing axis. This orientation implies strongly suppressed (6.4% of optimal) response of ISGRI, strongly suppressed (29% of optimal) response of IBIS/Veto, and near-optimal (80% of optimal) response of SPI-ACS.

Position of counterpart reported by Swift/BAT (GCN 38308

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) at the time of the event was at 25 deg with respect to the spacecraft pointing axis. This orientation implies suppressed response of all INTEGRAL all-sky instruments.

The background within +/-300 seconds around the event was rather stable (excess variance 1.2).

We have performed a search for any impulsive events in INTEGRAL SPI-ACS (as described in [2]) data.

We do not detect any significant counterparts and estimate a 3-sigma upper limit on the 75-2000 keV fluence of 1.6e-07 erg/cm^2 (within the 50% probability containment region of the source localization) for a burst lasting less than 1 s with a characteristic short GRB spectrum (an exponentially cut off power law with alpha=-0.5 and Ep=600 keV) occurring at any time in the interval within 300 s around T0. For a typical long GRB spectrum (Band function with alpha=-1, beta=-2.5, and Ep=300 keV), the derived peak flux upper limit is ~1.4e-07 (3.4e-08) erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.

We report for completeness and in order of FAP, all excesses identified in the search region. We find: 8 likely background excesses:

T-T0scaleS/Nflux ( x 1e-06 erg/cm2/s)FAP
7.160.3530.475 +/- 0.188 +/- 0.3770.44
-48.71.853.12.03 +/- 0.814 +/- 1.610.453
12523.52.06 +/- 0.783 +/- 1.630.454
12.10.13.81.1 +/- 0.358 +/- 0.8710.539
-18523.52.05 +/- 0.783 +/- 1.620.634
-320.73.20.347 +/- 0.133 +/- 0.2750.712
1040.056.62.84 +/- 0.528 +/- 2.250.739
38.30.054.41.85 +/- 0.516 +/- 1.460.868

Note that FAP estimates (especially at timescales above 2s) may be possibly further affected by enhanced non-stationary local background noise. This list excludes any excesses for which FAP is close to unity.

One of the listed excesses is 0.8 s from Swift/BAT counterpart of the LVK event. It also features comparable detection timescale (0.1s). Tentatively, FAP of associating this event to the Swift/BAT event is at moderately significantly low level of 0.036. However, 0.8s delay can not be explained by light travel time between INTEGRAL and Swift. If these events are indeed related, the difference in timing could potentially be explained by spectral evolution.

We note that no independent IBAS alerts happened in the vicinity.

SPI-ACS data can be retrieved in MMODA with this link.

INTEGRAL follow-up alert was NOT distributed to SCIMMA through HERMES few minutes after the trigger GCN since the event, not expected to be EM-bright, did not pass the cuts.

All results quoted are preliminary.

This circular is an official product of the INTEGRAL Multi-Messenger team.

Note that we send GCNs Circulars only when one of the following conditions is met: merger contains at least one neutron star, a singificant counterpart is reported.

[1] Savchenko et al. 2017, A&A 603, A46 [2] Savchenko et al. 2012, A&A 541A, 122S

GCN Circular 38309

Subject
LIGO/Virgo/KAGRA S241125n: Updated Sky localization and Coincidence with External Event
Date
2024-11-25T18:38:57Z (10 months ago)
From
Brandon Piotrzkowski <brandon.piotrzkowski@ligo.org>
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration along with the Swift/BAT 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 S241125n (GCN Circular 38305). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S241125n
Note that this skymap may be affected by the noise transient (glitch) mentioned in the original circular. 

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 1957 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 4474 +/- 1675 Mpc (a posteriori mean +/- standard deviation).

A search performed by the RAVEN pipeline [2] found a temporal coincidence between S241125n and a sub-threshold Swift/BAT trigger with ID 754189311 (DeLaunay et al., GCN Circular 38308). The GRB trigger time is 0.2 seconds after the GW candidate event. The estimated joint false alarm rate for the coincidence using just timing info is 1.5e-10 Hz, or about one in 1e2 years. 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 0.00037 Hz, or about one in 44 minutes.

A combined sky map is also available:
 * combined-ext.multiorder.fits,0, an updated localization, distributed via GCN notice about 17 hours after the candidate event time.

For the combined-ext.multiorder.fits,0 sky map, the 90% credible region is 873 deg2. The joint localization is dominated by the Swift/BAT candidate, which was identified with right ascension, declination of 58.079, +69.689 deg. Considering the overlap of the individual sky maps, the estimated joint false alarm rate for the spatial and temporal coincidence is 1.2e-10 Hz, or about one in 1e2 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] 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] 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 38308

Subject
LIGO/Virgo/KAGRA S241125n: Swift/BAT-GUANO candidate counterpart
Date
2024-11-25T18:29:13Z (10 months ago)
From
Aaron Tohuvavohu at Caltech <aaron.tohu@gmail.com>
Via
Web form

James DeLaunay (PSU), Aaron Tohuvavohu (Caltech), Samuele Ronchini (PSU), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:

The LIGO/Virgo/KAGRA detection of GW candidate S241125n (GCN 38305

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), reported in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).

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 [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

Using the NITRATES analysis (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find a candidate counterpart with sqrt(TS) of 7.41 in a 0.512 analysis time bin, starting at T0+11.2641 s.

The GRB candidate False Alarm Rate (FAR) is 3.74E-4 Hz. The joint GW-GRB FAR, combining the spatial and temporal information of both signals and correcting for trials, is 2.581E-9 Hz, or 1 every 12 years. This candidate has been submitted to the RAVEN pipeline (Urban 2016, Piotrzkowski 2022) for assessment, the joint FAR calculation above is computed by RAVEN.

Using the NITRATES analysis, parameter estimation was performed to obtain the localization of this burst in the form of a HEALPIX Multi-Order Coverage (MOC) skymap. This localization accounts for both statistical and systematic errors. More details in the creation and calibration of these maps will soon be published (DeLaunay et al. 2024. in prep)

The 90% credible area is 7694 deg2 and the 50% credible area is 0.02 deg2. The integrated probability inside BAT the coded field of view is ~63%. Over half of the credible region is contained within a single ~arcminute position.

The BAT position is RA, Dec = 58.079, +69.689 deg which is RA(J2000) = 03h 52m 18.96s Dec(J2000) = +69d 41’ 20.4″ with an estimated uncertainty of 5 arcmin (50% containment).

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.

The remainder of the localization posterior is spread over the sky.

A plot of the probability skymap can be viewed here: skymap plot

The probability skymap file can be downloaded from the link here: skymap fits file

Instructions on how to read and manipulate this map can be found here: https://guano.swift.psu.edu/documentation

A combined GW+GRB skymap can be found here: https://doi.org/10.5281/zenodo.14218311

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More details about this burst can be found on the trigger report page here https://guano.swift.psu.edu/trigger_report?id=754189311

GCN Circular 38305

Subject
LIGO/Virgo/KAGRA S241125n: Identification of a GW compact binary merger candidate
Date
2024-11-25T04:30:36Z (10 months ago)
From
Chia-Hsuan Hsiung <sw56540@gmail.com>
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We identified the compact binary merger candidate S241125n during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2024-11-25 01:01:16.780 UTC (GPS time: 1416531694.780). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], PyCBC Live [5], and SPIIR [6] analysis pipelines.

S241125n is an event of interest because its false alarm rate, as estimated by the online analysis, is 9.5e-10 Hz, or about one in 33 years. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S241125n

The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH (<1%).

Noise transients (glitches) are present in the LIGO Hanford detector which may affect the parameters or the significance of the candidate.

Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [7] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [7] 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%.

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 [8], distributed via GCN notice about 24 seconds after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [8], distributed via GCN notice about 33 seconds after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [8], distributed via GCN notice about 5 minutes 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 2367 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 6423 +/- 1740 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] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. (2023) arXiv:2305.05625
 [4] Aubin et al. CQG 38, 095004 (2021) doi:10.1088/1361-6382/abe913
 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [6] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023
 [7] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [8] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013
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