GRB 250814A, LIGO/Virgo/KAGRA S250814bg
GCN Circular 41465
Subject
GRB 250814A: CMO SAI and SAO RAS izJ observations.
Date
2025-08-21T13:09:03Z (4 months ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
Web form
A. Volnova (IKI), A. Moskvitin (SAO RAS), A. Pozanenko (IKI), N. Pankov (HSE, IKI), P. Minaev (IKI), S. Kotov (SAO RAS), A.M. Tatartnikov (SAI MSU), V.K. Postnikova (SAI MSU) report on behalf of IKI-GRB-FuN:
We observed the field of GRB 250814A detected by Swift/BAT and Fermi/GBM (Caputo et al. GCN Circ. 41358; Fermi team, GCN Circ. 41357) and possibly associated with the sub-threshold GW trigger S250814bg (LVK, GCN Circ. 41364) with 2.5-meter SAI-25 telescope of the Caucasian Mountain Observatory of Sternberg Astronomical Institute (SAI CMO), equipped with the IR camera ASTRONIRCAM. The observations in the J-filter were carried out on 2025-08-16 starting UT 23:20:51. We also observed this field with the BTA 6-m telescope of SAO RAS, equipped with the Scorpio-2 multi-mode focal reducer (Afanasiev & Moiseev, 2011, BaltA, 20, 363), and obtained series of images in SDSS i,z bands on 2025-08-17 starting 20:47:31 UT.
The two NIR sources reported previously (Karambelkar et al. GCN Circ. 41377; Castro-Tirado et al., GCN Circ. 41381; Ahumada et al. GCN Circ. 41382; Pankov et al. GRB Circ. 41389; D'Avanzo et al. GCB Circ. 41391; Kumar et al. GCN Circ. 41392; D'Avanzo et al. GCN Circ. 41408) are clearly detected. The preliminary photometry of the sources Source 1 and Source 2 according to D'Avanzo et al. (GCB Circ. 41391) is:
date |UT start|t-T0,d |Filter|Exp,s|Source 1 |Source 2 |UL3sigma|FWHM"|Telescope/Site
=========================================================================================
25-08-16|23:20:51|2.68958|J |4224 |21.72(0.07)|21.96(0.08)|22.9 |1.2 |SAI-25/CMO
25-08-17|20:47:31|3.56255|z |1200 |22.39(0.04)|22.46(0.04)|24.2 |1.4 |BTA/SAO
25-08-17|21:25:21|3.58647|i |1200 |23.39(0.05)|23.31(0.05)|24.6 |1.4 |BTA/SAO
All magnitudes are in AB system and not corrected for the Galactic extinction. The photometry is calibrated using Pan-STARRS i,z magnitudes and 2MASS J magnitudes for the following stars:
RA Dec i_PS1 z_PS1 J_2MASS*
20:25:12.01 +48:24:08.4 16.924(0.004) 16.703(0.008) 15.750
20:25:17.82 +48:24:04.6 17.234(0.003) 16.952(0.005) 15.627
* - J magnitudes of the reference stars are in Vega system.
(We are grateful to the management of the observatories for observational time and the staff for excellent observations.)
GCN Circular 41450
Subject
GRB 250814A: Swift/UVOT Upper Limits
Date
2025-08-20T15:20:29Z (4 months ago)
From
Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>
Via
Web form
M. H. Siegel (PSU) and R. Caputo (GSFC) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 250814A 179 s after the BAT trigger (Caputo et al., GCN Circ. 41358). No optical afterglow consistent with the XRT position (Salvaggio et al., GCN Circ. 41379) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
u_FC 179 429 246 >20.3
white 460 1537 264 >21.8
v 510 1412 97 >20.2
b 436 1511 117 >20.2
u 179 1486 343 >20.5
w1 560 1462 97 >19.5
m2 708 1263 58 >18.6
w2 486 1556 130 >20.0
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 1.066 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 41408
Subject
GRB 250814A: Further GRAWITA TNG NIR observations
Date
2025-08-18T06:30:03Z (5 months ago)
From
Paolo D'Avanzo at INAF - OAB <paolo.davanzo@inaf.it>
Via
email
P. D'Avanzo, (INAF-OAB), M.T. Botticella, L. Izzo (INAF - OACn), A. Rossi (INAF-OAS), V. D’Elia (ASI-SSDC), G. Greco (INFN), M.G. Bernardini (INAF-OAB), B. Patricelli (Univ. Pisa), E. Cappellaro (INAF-OAPd), E. Brocato (INAF-OAR), M. Pedani, G Mainella (INAF-TNG), on behalf of GRAWITA report:
We observed the field of GRB 250814A detected by Swift/BAT and Fermi/GBM (Caputo et al. GCN Circ. 41358; Fermi team, GCN Circ. 41357) and possibly associated with the sub-threshold GW trigger S250814bg (LVK, GCN Circ. 41364) 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 H filter starting on 2025-08-17T00:15:57 UT (i.e. 2.1 days post T0), centered at the position of the X-ray afterglow detected by Swift/XRT (Caputo et al. GCN Circ. 41358; Salvaggio et al., GCN Circ. 41379).
The two NIR sources reported by D'Avanzo et al. (GCN Circ. 41391) are clearly detected. From preliminary photometry we find no evidence for significant variability for both sources with respect to our previous epoch of observation (D'Avanzo et al. GCN Circ. 41391). We therefore conclude that these sources are likely unrelated with GRB 250814A.
GCN Circular 41396
Subject
GRB 250814A: Swift-BAT refined analysis
Date
2025-08-15T21:11:04Z (5 months ago)
From
Amy <yarleen@gmail.com>
Via
email
H. A. Krimm (NSF), S. D. Barthelmy (GSFC), R. Caputo (GSFC), R. Gupta
(GSFC), S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa), C. B. Markwardt (GSFC),
M. J. Moss (GSFC), D. M. Palmer (LANL), T. Parsotan (GSFC), D. Sadaula
(GSFC/UMBC), T. Sakamoto (AGU) (i.e. the Swift-BAT team):
Using the data set from T-239 to T+963 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 250814A (trigger #1342439)
(Caputo et al., GCN Circ. 41358). The BAT ground-calculated position is
RA, Dec = 306.313, 48.406 deg which is
RA(J2000) = 20h 25m 15.2s
Dec(J2000) = +48d 24' 23.1"
with an uncertainty of 1.9 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 54%.
The mask-weighted light curve shows a single-pulse structure that starts at
~T0 and peaks at ~T+10 s. The main pulse ends at ~T+25 s, but there is some
weak emission that lasts till ~T+100 s. T90 (15-350 keV) is 110.11 +- 47.90
sec (estimated error including systematics).
The time-averaged spectrum from T+1.06 to T+125.36 sec is best fit by a
simple power-law model. The power law index of the time-averaged spectrum
is 1.91 +- 0.21. The fluence in the 15-150 keV band is 1.4 +- 0.2 x 10^-06
erg/cm2. The 1-sec peak photon flux measured from T+10.22 sec in the 15-150
keV band is 0.8 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90%
confidence level.
The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1342439
GCN Circular 41393
Subject
GRB 250814A: Further COLIBRÍ optical upper limit
Date
2025-08-15T14:48:30Z (5 months ago)
From
Alan Watson at UNAM <alan@astro.unam.mx>
Via
Web form
Enrique Moreno Méndez (UNAM), Francesco Magnani (CPPM), Jean-Grégoire Ducoin (CPPM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Fredd Sánchez Álvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Jean-Luc Atteia (IRAP), Rosa L. Becerra (U Roma), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM) report:
We imaged the field of the Fermi and Swift GRB 250814A (Fermi team, GCN Circ. 41357, Caputo et al. GCN Circ. 41358) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-08-15 03:20 to 04:48 UTC (from 19.8 to 21.2 hours after the trigger) and obtained 64 minutes of exposure in the r filter.
The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked image, we do not detect any new source at the XRT source position (Caputo et al. GCN Circ. 41358) or possible optical and infrared counterpart position (Karambelkar et al. GCN Circ. 41377; Castro-Tirado et al., GCN Circ. 41381; Ahumada et al. GCN Circ. 41382; D'Avanzo et al. GCB Circ. 41391; Kumar et al. GCN Circ. 41392) down to the following 3-sigma limit:
r > 23.6
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 41392
Subject
GRB 250814A: detection of the afterglow candidate with Liverpool Telescope
Date
2025-08-15T13:59:44Z (5 months ago)
From
Amit Kumar at Royal Holloway - UoL/ U of Warwick, UK <amitkundu515@gmail.com>
Via
Web form
A. Kumar, J. R. Maund (RHUL), N. C. Sun (UCAS), W. X. Li, Y. N. Wang (NAOC), and K. Wiersema (Herts) report:
We observed the field of the Fermi/GBM and Swift/BAT GRB 250814A (Fermi GBM Team, GCN 41357; Caputo et al., GCN 41358), which is spatially coincident with the sub-threshold GW trigger S250814bg (LVK, GCN 41364), using the IO:O imager on the 2.0m Liverpool Telescope. Multi-band (B, g, V, r, i, and z bands) imaging was obtained between 2025-08-14 23:34:48.3 UT and 2025-08-15 01:29:02.2 UT (corresponding to ~15.99 and 17.90 hours post-trigger, respectively).
Based on our preliminary analysis, we detect the counterpart candidate in our stacked z-band image obtained ~16.86 hours post-trigger at RA = 20:25:13.8 and Dec = +48:23:56.5 (J2000), within the Swift-XRT localisation region (Salvaggio et al., GCN 41379); see also Karambelkar et al. (GCN 41377), Castro-Tirado et al. (GCN 41381), Ahumada et al. (GCN 41382) and Source 1 of D'Avanzo et al. (GCN 41391). No source is detected at this position in the B, g, V, r, and i bands. A summary of our observations and preliminary photometric results is provided below.
T_obs (start time) T_obs - T0 Exp Filter Mag
--------------------------------------------------------------------------------------------
2025-08-15T00:26:48.9 16.86 2x600 s z 21.3 ± 0.3
2025-08-14T23:34:48.3 15.99 600 s g >21.9
2025-08-14T23:45:18.2 16.16 2x600 s r >22.1
2025-08-15T00:06:03.3 16.51 2x600 s i >21.8
2025-08-15T00:47:35.3 17.21 2x600 s V >22.2
2025-08-15T01:08:24.4 17.55 3x600 s B >20.5
Photometric calibration was performed using stars from the Pan-STARRS (for g, r, i and z bands), USNO-B1 (for B band), and APASS (for V band) catalogues. The quoted magnitudes are not corrected for Galactic extinction in the direction of the GRB.
This circular may be cited.
GCN Circular 41391
Subject
GRB 250814A: GRAWITA TNG NIR observations
Date
2025-08-15T12:07:02Z (5 months ago)
From
Paolo D'Avanzo at INAF - OAB <paolo.davanzo@inaf.it>
Via
email
P. D'Avanzo, (INAF-OAB), M.T. Botticella, L. Izzo (INAF - OACn), A. Rossi (INAF-OAS), V. D’Elia (ASI-SSDC), G. Greco (INFN), M.G. Bernardini (INAF-OAB), B. Patricelli (Univ. Pisa), E. Cappellaro (INAF-OAPd), E. Brocato (INAF-OAR), V. Lorenzi, G Mainella (INAF-TNG), on behalf of GRAWITA report:
We observed the field of GRB 250814A detected by Swift/BAT and Fermi/GBM (Caputo et al. GCN Circ. 41358; Fermi team, GCN Circ. 41357) and possibly associated with the sub-threshold GW trigger S250814bg (LVK, GCN Circ. 41364) 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 H filter starting on 2025-08-14T20:33:39 UT (i.e. 13.0 hours post T0) and with the K filter starting on 2025-08-14T22:06:25 UT (i.e. 14.5 hours post T0), centered at the position of the X-ray afterglow detected by Swift/XRT (Caputo et al. GCN Circ. 41358; Salvaggio et al., GCN Circ. 41379).
Within the Swift/XRT error circle (Salvaggio et al., GCN Circ. 41379) we detect two sources. Below we report the source positions and AB magnitudes derived from preliminary photometry (calibrated against the 2MASS catalogue).
Source 1
RA (J2000) = 20:25:13.86
Dec (J2000) = +48:23:56.3
+/- 0.3"
H = 21.3 +/- 0.2 mag
K = 21.4 +/- 0.3 mag
Source 2
RA (J2000) = 20:25:14.06
Dec (J2000) = +48:23:54.8
+/- 0.3"
H = 21.8 +/- 0.2 mag
K = 21.2 +/- 0.2 mag
We note that Source 1 is the same reported by several authors in both the NIR and optical bands (Karambelkar et al. GCN Circ. 41337; Castro-Tirado et al. GCN Circ. 41381; Ahumada et al. GCN Circ. 41382). Compared with the results reported so far, our preliminary photometry suggests that this source has not varied, in agreement with the findings of Ahumada et al. (GCN Circ. 41382).
Further NIR observations are encouraged to check for any brightness evolution of Source 2.
GCN Circular 41389
Subject
GRB 250814A: Mondy optical observations of the candidate counterpart to LVK S250814bg
Date
2025-08-15T10:55:07Z (5 months ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), A. Pozanenko (IKI), E. Klunko (ISTP) report on behalf of IKI-GRB-FuN:
We observed the field of GRB 250814A detected by Fermi/GBM (The Fermi GBM team, GCN 41357), Swift/BAT (Caputo et. al, GCN 41358), which was temporally and spatially coincident with a GW event S250814bg detected by LVK (GCN 41364). The observations were carried out with the 1.5m AZT-33IK telescope of the Solar Sayan Observatory (Mondy). The series of 30x120 s images were captured in the R-filter starting on 2025-08-14 at 17:49 UT, i.e. ~0.44 days after the GBM trigger. In the co-add image of 28x120 s we do not detect any optical source at the position of the IR candidate (Karambelkar et. al, GCN 41377; Castro-Tirado et. al, GCN 41381; Ahumada et. al, GCN 41382). The preliminary upper limit on a candidate optical counterpart in the co-add image is presented below:
Date UT start t-T0 Exp. Filter OT Err UL
(mid, days) (n x s) (3sigma)
2025-08-14 17:51:08 0.44721 28x120 R n/d n/d 22.7
The photometry was calibrated using nearby stars from the PS1 DR2 catalog (R mags were obtained via Lupton 2005 transformations) and not corrected for the Galactic extinction. The candidate counterpart position was also observed in the optical and IR bands by (Becerra et. al, GCN 41362; de Ugarte Postigo et. al, GCN 41367; Karambelkar et.al, GCN 41372; Salazar Manzano et. al, GCN 41375; Jiang et. al, GCN 41387), which resulted in upper limits.
GCN Circular 41387
Subject
GRB250814A: TRT optical upper limits
Date
2025-08-15T09:39:01Z (5 months ago)
From
sqjiang at NAOC <sqjiang@bao.ac.cn>
Via
Web form
S.Q. Jiang (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), L.B. He, J. An, X. Liu, Z.P. Zhu, D. Xu (NAOC), S. Y. Fu (HUST) report on behalf of a large collaboration:
We observed the field of GRB 250814A detected by Swift (Caputo et al., GCN 41358) and Fermi (Fermi GBM team, GCN 41357), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Sierra Remote Observatories in California. We obtained several 180 s frames in the R and I filters. The observation started at 07:44:15.35 UTC on 2025-08-14, i.e., 9.27 mins after the Swift/BAT trigger time.
No new optical source is detected within the Swift/XRT enhanced error circle (Caputo et al., GCN 41358) down to R > 20.7 and I > 19.7 at about 17.3 mins post-burst, calibrated with nearby Pan-STARRS DR2 stars and not corrected for Galactic extinction.
GCN Circular 41382
Subject
GRB 250814A: P200/WIRC observations of the afterglow candidate
Date
2025-08-15T06:11:05Z (5 months ago)
From
Tomas Ahumada Mena at Caltech <tahumada@caltech.edu>
Via
Web form
Tomás Ahumada (Caltech), Viraj Karambelkar (Caltech), Mansi Kasliwal (Caltech), Iver Warburton (Yale), Alessandro Peca (Yale), and Vishwajeet Swain (IITB) report:
We observed the field of the Fermi and Swift GRB 250814A (Fermi team, GCN Circ. 41357; Caputo et al., GCN Circ. 41358, Salvaggio et al. GCN Circ. 41379), which is spatially coincident with the sub-threshold GW trigger S250814bg (GCN Circ. 41364), in the near-infrared J and Ks bands using the Wide-field Infrared Camera (WIRC; Wilson et al. 2003) on the Palomar 200-inch telescope. Observations began on 2025-08-15 at 03:20 UT and consisted of 1350 s in the Ks band, followed by J-band imaging starting at 04:00 UT with a total exposure time of 1200 s.
We clearly detect the source announced in Karambelkar et al. (GCN Circ. 41337) and observed in i-band by Castro-Tirado et al. (GCN Circ. 41381) at RA = 20:25:13.9, Dec = +48:23:55.9 (J2000), in both bands. This position is consistent with the enhanced X-ray position reported by Salvaggio et al. (GCN Circ. 41379). Aperture photometry yields:
Ks = 21.3 ± 0.2 mag (AB)
J = 21.72 ± 0.15 mag (AB)
These values are consistent, within the uncertainties, with those reported in Karambelkar et al. (GCN Circ. 41337) in both bands. We therefore conclude that this source shows no significant variability and is likely unrelated to the GRB.
GCN Circular 41381
Subject
GRB 250814A: possible optical afterglow candidate at Spanish observatories
Date
2025-08-15T00:40:01Z (5 months ago)
From
I. Perez-Garcia at Institute of Astrophysics of Andalusia <ipg@iaa.es>
Via
Web form
A.J. Castro-Tirado, I. Perez-Garcia, F. J. Aceituno, M. D. Caballero-Garcia, S. Guziy, R. Sanchez-Ramirez, E. Fernandez-Garcia, S.-Y. Wu (IAA-CSIC), Y.-D. Hu (GXU), J. Becerra (IAC), M. Gritsevich (Univ. of Helsinki), A. Sintes (Univ. Illes Balears), J. A. Font (Univ. de Valencia), S. B. Pandey (ARIES), B.-B. Zhang (NJU), J. Flores (CAHA), D. Garcia (GTC, IAC) and A. Romero (GTC), on behalf of a larger collaboration, report:
We initially observed the field of the GRB 250814A detected by both Fermi (Fermi GBM team, GCNC 41357) and Swift (Caputo et al. GCNC 41358) using the 1.5m OSN telescope at Observatorio de Sierra Nevada in Granada (clear filter, starting on Aug 14, 20:30 UT). In addition to this facility, we also used both the 2.2m telescope at the Calar Alto Observatory (z-band, starting at 21:55 UT) and the 10.4 GTC telescope (griz-band, starting on Aug 15, 00:00 UT).
This event was proposed to be temporally and spatially coincident with the subthreshold GW trigger S250814bg (the LVK Collaboration, GCNC 41364), which occurred 3.7 s prior to the Fermi GBM trigger (de Barra et al., GCNC 41378).
At the position of the X-ray afterglow detected by Swift/XRT (Salvaggio et al. GCNC 41379), we identify an optical source in our data (in particular on the GTC i-band image) at coordinates (J2000): RA = 20:25:13.87 Dec = +48:23:56.5 (+/- 0.5"), for which we measure i = 23.3 +/- 0.3, which could be the optical counterpart to GRB250814A. This object is consistent with the proposed NIR candidate (Karambelkar et al. GCNC 41377).
Further optical observations would be needed in order to confirm whether this object is the electromagnetic counterpart to S250814bg.
GCN Circular 41380
Subject
GRB 250814A: KAIT optical upper limit
Date
2025-08-14T22:30:47Z (5 months ago)
From
Weikang Zheng at UC Berkeley <weikang@berkeley.edu>
Via
email
WeiKang Zheng (UCB) and Alexei V. Filippenko (UCB) report on
behalf of the KAIT GRB team:
The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, observed the field of GRB 250814A (Fermi GBM
team, GCN 41357; Caputo et al., GCN 41358) starting at 07:50:38,
Aug. 14 UT, 939 seconds after the burst. A set of 120x60s clear
(roughly R) filter images were obtained. Preliminary analysis
do not reveal any new optical counterpart candidate within the
Swift/XRT error circles (Caputo et al., GCN 41358), neither in
single image, nor in the co-add images. Also no object was found
at the afterglow candidate position reported by Karambelkar et al.
(GCN 41377). Our upper limit is about 21.5 (Vega) mag at a mid
time of 1.27 hours after the burst. Our result is consistent with
the upper limit reported by other groups (Becerra et al., GCN 41362;
Postigo et al., GCN 41367; Karambelkar et al., GCN 41372; Salazar
et al., GCN 41375).
GCN Circular 41379
Subject
GRB 250814A: Swift-XRT refined Analysis
Date
2025-08-14T21:31:37Z (5 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), S. Dichiara (PSU), J.A.
Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester), R. Brivio (INAF-OAB) and P.A. Evans report on
behalf of the Swift-XRT team:
We have analysed 6.7 ks of XRT data for GRB 250814A, from 163 s to 40.1
ks after the trigger. The data comprise 184 s in Windowed Timing (WT)
mode (the first 10 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. The best available XRT position
(using the promptly downlinked event data, the XRT-UVOT alignment and
matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec =
306.30792, 48.39903 which is equivalent to:
RA (J2000): 20h 25m 13.90s
Dec(J2000): +48d 23' 56.5"
with an uncertainty of 2.2 arcsec (radius, 90% confidence).
The light curve can be modelled with an initial power-law decay with an
index of alpha=1.49 (+0.27, -0.28), followed by a break at T+338 s to
an alpha of 3.15 (+0.26, -0.20).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.33 (+0.24, -0.22). The
best-fitting absorption column is 1.2 (+0.4, -0.3) x 10^22 cm^-2, in
excess of the Galactic value of 6.5 x 10^21 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.9 (+/-0.4) and a
best-fitting absorption column of 1.0 (+0.4, -0.3) x 10^22 cm^-2. The
counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 5.5 x 10^-11 (9.3 x 10^-11) erg cm^-2
count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 1.0 (+0.4, -0.3) x 10^22 cm^-2
Galactic foreground: 6.5 x 10^21 cm^-2
Excess significance: 1.8 sigma
Photon index: 1.9 (+/-0.4)
If the light curve continues to decay with a power-law decay index of
3.15, the count rate at T+24 hours will be 1.3 x 10^-7 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 7.0 x
10^-18 (1.2 x 10^-17) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01342439.
This circular is an official product of the Swift-XRT team.
GCN Circular 41377
Subject
GRB 250814A: Possible afterglow candidate from P200/WIRC
Date
2025-08-14T19:35:32Z (5 months ago)
From
Viraj Karambelkar at Indian Inst of Tech,Bombay <karambelkarvraj21197@gmail.com>
Via
email
Viraj Karambelkar (Caltech), Mansi Kasliwal (Caltech), Steven Giacalone
(Caltech), Emily Gilbert (IPAC), Matt Lastovka (UMD), Vishwajeet Swain
(IITB), Gaurav Waratkar (IITB), Tomas Ahumada (Caltech) report:
We observed the field of the Fermi and Swift GRB 250814A (Fermi team, GCN
Circ. 41357, Caputo et al. GCN Circ. 41358) coincident with the
sub-threshold GW trigger S250814bg (GCN Circ. 41364) in the near-infrared J
and Ks bands with the Wide-field infrared Camera (WIRC, Wilson et al. 2003)
on the Palomar 200-inch telescope. Observations started on 2025-08-14 at
11:31:29 UT and lasted for 1665 seconds in the J-band, and started at
12:09:12 UT and lasted for 810 seconds in the Ks-band.
We marginally detect a faint source in the Ks-band image at RA=20:25:13.9,
Dec=48:23:55.9 (J2000), ~0.6 arcsec from the enhanced position of the X-ray
counterpart reported by Caputo et al (GCN 41358). We also detect a marginal
source at this location in the more sensitive J-band image, however, it is
blended with several faint sources and is near the detection limit of the
image. We performed aperture photometry at the location of this source and
measured the following magnitudes
Ks ~ 21.2 +/- 0.35 mag (AB),
J ~ 21.6 mag +/- 0.4 mag (AB)
We encourage deep NIR observations to trace the brightness evolution and
confirm the nature of this source.
GCN Circular 41375
Subject
GRB 250814A: Optical observations with MDM Hiltner 2.4m telescope
Date
2025-08-14T18:24:24Z (5 months ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
L. E. Salazar Manzano (Umich), H.-W. Lin (UMich), A. Aryan, T.-W. Chen (both NCU), and S. Yang (HNAS) report:
We observed the field of the GRB 250814A (Fermi GBM team, GCN41357) using the 2.4m Hiltner telescope (HT) at the Michigan-Dartmouth-MIT (MDM) Observatory in Arizona, United States. The Swift-XRT detection of GRB 250814A was proposed to be temporally and spatially coincident with the subthreshold GW trigger S250814bg, which occurred 6 s after the BAT trigger (Caputo et al., GCN41358). The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration, with the Fermi GBM team and Swift team, reported the identification of a GW candidate possibly associated with this GRB (GCN41364).
The first epoch of observations started at 10:22 UTC on the 14th of August 2025 (MJD = 60901.432), 2.79 hr after the Swift trigger.
We do not find any credible evidence of a new and uncataloged source in the difference images (subtraction using the Pan-STARRS1 template images) within the 2.2 arcseconds uncertainty circle of Swift-XRT localization. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction using the 'hotpants' (Becker A., 2015, ascl.soft. ascl:1504.004) algorithm and perform the PSF photometry. The details of the observations and the measured 3-sigma upper limits (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing
2.4m HT | g | 60901.432 | 2.79 | 300 * 5 | > 22.03 | 1".32
2.4m HT | r | 60901.436 | 2.88 | 300 * 7 | > 22.98 | 0".99
2.4m HT | i | 60901.462 | 3.51 | 300 * 4 | > 21.96 | 1".21
Our (detection) limits are consistent with those reported by Becerra et al. (GCN41362), Postigo et al. (GCN41367), and Karambelkar et al. (GCN41372).
The presented magnitudes are calibrated using the field stars from the PanStarrs1 catalog and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_g = 3.5 mag, A_r = 2.4 mag, and A_i = 1.8 mag in the direction of the transient (Schlafly & Finkbeiner 2011).
GCN Circular 41372
Subject
GRB 250814A: J-band upper limit from WINTER
Date
2025-08-14T15:52:22Z (5 months ago)
From
Viraj Karambelkar at Indian Inst of Tech,Bombay <karambelkarvraj21197@gmail.com>
Via
email
Viraj Karambelkar (Caltech), Tomas Ahumada (Caltech), Benjamin Schneider
(LAM), Robert Stein (UMD), Geoffrey Mo (MIT), Danielle Frostig (CfA),
Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech)
report:
We observed the field of the Fermi and Swift GRB 250814A (Fermi team, GCN
Circ. 41357, Caputo et al. GCN Circ. 41358) coincident with the
sub-threshold GW trigger S250814bg (GCN Circ. 41364) in the near-infrared
with the Palomar 1-m telescope, equipped with the 1-square degree WINTER
camera (Lourie et al. 2020, Frostig et al. 2024). Observations started on
2025-08-14 at 07:49:52 UT (15 min after the trigger) and consisted of 15
exposures of 120 s in the J-band.
In the stacked image, we do not detect any new source at the X-ray
afterglow position reported by Caputo et al, GCN 41358 down to the
following 5-sigma AB magnitude:
J > 19.3
The images were processed using the WINTER data reduction pipeline
implemented with mirar (https://github.com/winter-telescope/mirar,
https://doi.org/10.5281/zenodo.13352565). The photometric calibration was
performed using nearby stars from the 2MASS catalog and the magnitude is
not corrected for Galactic extinction.
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 41367
Subject
GRB 250814A: COLIBRÍ optical upper limit
Date
2025-08-14T11:04:35Z (5 months ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
Antonio de Ugarte Postigo (LAM), Ramandeep Gill (UNAM), Jean-Grégoire Ducoin (CPPM), Rosa L. Becerra (U Roma), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Fredd Alvarez (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (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):
We imaged the field of the Fermi and Swift GRB 250814A (Fermi team, GCN Circ. 41357, Caputo et al. GCN Circ. 41358) using the DDRAGO wide-field imager on the COLIBRÍ telescope. We observed from 2025-08-14 07:48 to 08:07 UTC (from 13.9 to 33.0 minutes after the trigger) and obtained 16 minutes of exposure in the i filter.
The data were reduced and coadded with the COLIBRÍ pipeline and analysed with STDWeb/STDPipe (Karpov 2025). The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
In the stacked image, we do not detect any new source at the XRT source position (Caputo et al. GCN Circ. 41358) down to the following 3-sigma limit:
i > 22.9
This upper limit is consistent with the one reported by Becerra et al. (GCN Circ. 41362).
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 41364
Subject
LIGO/Virgo/KAGRA S250814bg: Identification of a GW compact binary merger candidate possibly associated with GRB 250814A detected by Fermi GBM and Swift BAT
Date
2025-08-14T10:31:50Z (5 months ago)
From
Sreeta Roy at University of Warsaw <sreeta.roy@ligo.org>
Via
Web form
The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration with the Fermi GBM team and Swift team report:
We identified the compact binary merger candidate S250814bg during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-14 07:35:05.587 UTC (GPS time: 1439192123.587). The candidate was found by the GstLAL [1] analysis pipeline.
Based on the analysis of gravitational-wave 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 0.00012 Hz or about one in 2 hours. However, a search performed by the RAVEN pipeline found a significant coincidence between this candidate and Fermi GBM trigger with ID 776849714, which is associated with GRB 250814A. The event’s properties can be found at this URL:
https://gracedb.ligo.org/superevents/S250814bg
The classification of the GW signal, in order of descending probability, is Terrestrial (>99%), BBH (<1%), NSBH (<1%), or BNS (<1%).
Excess noise was present in the L1 detector at the time of this candidate, which may affect the parameters or the significance of the candidate.
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 <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 for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 3%.
The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin.
Two 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 and SCiMMA notices about 2 minutes after the candidate event time.
* bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], distributed via GCN and SCiMMA notices about 6 minutes after the candidate event time.
The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 7867 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 13504 +/- 6537 Mpc (a posteriori mean +/- standard deviation).
A search performed by the RAVEN pipeline [4] found a temporal coincidence between S250814bg and a Fermi GBM trigger with ID 776849714, which is associated with GRB 250814A (Fermi GBM Team, GCN Circular 41357). The GRB trigger time is 3.7 seconds after the GW candidate event. The estimated joint false alarm rate for the coincidence using just timing info before trials are applied is 7.5e-09 Hz, or about one in 4 years.
Combined sky maps are also available:
* combined-ext.multiorder.fits,0, an initial localization, distributed via GCN and SCiMMA notices about 2 minutes after the candidate event time.
* combined-ext.multiorder.fits,1, an initial localization, distributed via GCN and SCiMMA notices about 6 minutes after the candidate event time.
* combined-ext.multiorder.fits,2, an initial localization, distributed via GCN and SCiMMA notices about an hour after the candidate event time.
For the combined-ext.multiorder.fits,2 sky map, the 90% credible region is 471 deg2. Considering the overlap of the individual sky maps, the estimated joint false alarm rate for the spatial and temporal coincidence before trials are applied is 1.5e-08 Hz, or about one in 2 years. After including the trials factor of 30, the joint false alarm rate becomes 4.5e-07 Hz, or about 14 per year.
Moreover, the GW candidate is 6 seconds after the Swift-BAT trigger 1342439, associated with the same GRB 250814A (Swift Team, GCN Circular 41358). The Swift-BAT GRB is spatio-temporal coincident with the Fermi GBM one, therefore they can be reliably considered the same astrophysical event. Considering the Swift-BAT localization, the estimated joint false alarm rate between the Swift-BAT and the GW candidate for the spatial and temporal coincidence before trials are applied is 1.0e-08 Hz, or about one in 3 years. After including the trials factor of 30, the joint false alarm rate becomes 3.0e-07 Hz, or about 9 per year.
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. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
[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] 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 41362
Subject
GRB 250814A: DDOTI Optical Upper Limit
Date
2025-08-14T10:26:15Z (5 months ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (U Roma), Alan M. Watson (UNAM), Sahil Atri (U Roma), Camila Angulo Valdez (UNAM), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl López (UNAM), Margarita Pereyra (UNAM) and Eleonora Troja (U Roma) report:
We observed the field of GRB 250814A detected by Swift/BAT (Caputo et al., GCN Circ. 41358) with the DDOTI/OAN wide-field imager at the Observatorio Astronómico Nacional on Sierra San Pedro Mártir (http://ddoti.astroscu.unam.mx) on the night of 2025-08-14 UTC.
DDOTI observed the XRT position (Caputo et al., GCN Circ. 41358) from 08:50 UTC to 09:18 UTC (from T+ 1.3 h to T+ 1.7 h after the trigger) and obtained a total exposure of 24 minutes.
Comparing our observations to the USNO-B1 and Pan-STARRS PS1 DR2 catalogues, we
detect no uncatalogued fading sources within the observed field down to a 3-sigma limiting AB magnitude of:
w > 19.6
This value is not corrected for the Galactic extinction.
GCN Circular 41358
Subject
GRB 250814A: Swift detection of a burst coincident with a subthreshold GW trigger
Date
2025-08-14T07:58:28Z (5 months ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
R. Caputo (GSFC), A. P. Beardmore (U Leicester),
P. A. Evans (U Leicester), R. A. J. Eyles-Ferris (U Leicester),
C. Gronwall (PSU), A. Y. Lien (U Tampa) and K. L. Page (U Leicester)
report on behalf of the Neil Gehrels Swift Observatory Team:
At 07:34:59 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250814A (trigger=1342439). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 306.329, +48.391 which is
RA(J2000) = 20h 25m 19s
Dec(J2000) = +48d 23' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a complex structure
structure with a duration of about 20 sec. The peak at ~T+26 s in
the 15-25 keV band raw light curve is likely due to detector noise,
though further ground data is required to confirm this.
The peak count rate was ~500 counts/sec (15-350 keV), at ~10 sec after
the trigger.
The XRT began observing the field at 07:37:54.1 UT, 174.3 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source with an enhanced position: RA, Dec
306.30802, 48.39903 which is equivalent to:
RA(J2000) = 20h 25m 13.92s
Dec(J2000) = +48d 23' 56.5"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 57 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 6.51
x 10^21 cm^-2 (Willingale et al. 2013).
The initial flux in the 2.5 s image was 7.47e-10 erg cm^-2 s^-1 (0.2-10
keV).
UVOT took a finding chart exposure of 250 seconds with the U filter starting
179 seconds after the BAT trigger. No credible afterglow candidate has been
found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the
XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The
8'x8' region for the list of sources generated on-board covers 100% of the XRT
error circle. The list of sources is typically complete to about 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
1.066.
This GRB is temporally and spatially coincident with the subthreshold
GW trigger S250814bg which occurred 6 s after the BAT trigger. The XRT
position is towards the edge of the GW localisation. Given the low
significance of the GW event (a false alarm rate of ~10/day) this is
probably a chance coincidence; however, follow up of this GRB is
advised.
Burst Advocate for this burst is R. Caputo (regina.caputo AT nasa.gov).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)
GCN Circular 41357
Subject
GRB 250814A: Fermi GBM Final Real-time Localization
Date
2025-08-14T07:45:48Z (5 months ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB
At 07:35:09 UT on 14 Aug 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250814A (trigger 776849714.277078 / 250814316).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 298.0, Dec = 47.5 (J2000 degrees, equivalent to J2000 19h 52m, 47d 30'), with a statistical uncertainty of 8.6 degrees.
The angle from the Fermi LAT boresight is 30.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250814316/quicklook/glg_skymap_all_bn250814316.png
The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250814316/quicklook/glg_healpix_all_bn250814316.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250814316/quicklook/glg_lc_medres34_bn250814316.gif