GRB 250920B
GCN Circular 41977
Subject
GRB 250920B: AstroSat CZTI detection
Date
2025-09-25T04:20:37Z (4 hours ago)
From
Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in>
Via
Web form
S. Salunke (IUCAA), Harsha K. H. (IUCAA), M. Tembhurnikar (IUCAA), A. Goyal (IITB), A. Arya (IITB), G. Waratkar (Caltech/IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:
Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long-duration GRB 250920B which was also detected by Fermi/GBM (Fermi GBM Team, GCN Circ. 41897), Swift/BAT (Gupta et. al., GCN Circ. 41898), Konus-Wind (Svinkin et. al., GCN Circ. 41942), and Glowbug (Cheung et al., GCN Circ. 41937).
The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-09-20 08:46:36.25 UTC. The measured peak count rate associated with the burst is 104 (+43, -20) counts/s above the background in the combined data of three quadrants (out of four), with a total of 255 (+116, -121) counts. The local mean background count rate was 214 (+3, -4) counts/s. Using cumulative rates, we measure a T90 of 7.5 (+1.3, -5.2) s.
The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed multiple peaks of emission with the strongest peak at 2025-09-20 08:46:36.25 UTC. The measured peak count rate associated with the burst is 234 (+66, -52) counts/s above the background in the combined data of all quadrants, with a total of 1063 (+321, -341) counts. The local mean background count rate was 1304 (+7, -8) counts/s. We measure a T90 of 10 (+2, -3) s from the cumulative Veto light curve.
CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.
CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=grb
GCN Circular 41958
Subject
GRB 250920B: AbAO optical upper limits
Date
2025-09-23T21:32:13Z (a day ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), R. Ya. Inasaridze (AbAO) report on behalf of GRB-IKI-FuN:
We observed the field of a long GRB 250920B detected by Fermi/GBM (The Fermi GBM team GCN 41897), Swift (Gupta et. al, GCN 41898), with the AS-32 0.7m telescope of AbAO. We obtained two series of images in the R band on epochs 2025-09-22 and 2025-09-23 (~2.3 and ~3.3 days since trigger, respectively). The optical afterglow initially detected by Swift/UVOT (Gupta et. al, GCN 41898) and further observed by (WU et. al, GCN 41908; Hall et. al, GCN 41914; Pankov et. al, GCN 41931; Reguitti et. al, GCN 41932; Moskvitin et. al, GCN 41948; Pankov et. al, GCN 41953) is not detected in the stacked images of both epochs. Preliminary upper limits are as follows:
Date UTstart Exptime t-T0 Filter OT Err. UL
(nxs) (mid, days) (3sigma)
2025-09-22 16:09:15 20x2+97x30 2.32458 R n/d n/d 20.3
2025-09-23 16:04:34 153*30 3.33076 R n/d n/d 20.3
The photometry was calibrated against nearby PS1 DR2 stars (R-mags obtained via Lupton 2005 transformations) and not corrected for the Galactic extinction.
Ref. stars
PS1 DR2 (Lupton 2005)
RA Dec R R_err
187.24325 47.82521 15.134 0.007
187.41433 47.78151 14.974 0.008
GCN Circular 41953
Subject
GRB 250920B: CrAO ZTSH optical observations
Date
2025-09-23T12:38:45Z (2 days ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Moskvitin (SAO RAS), V. Rumyantsev (CrAO) report on behalf of GRB-IKI-FuN:
We observed the field of a long GRB 250920B detected by Fermi/GBM (The Fermi GBM team GCN 41897), Swift (Gupta et. al, GCN 41898), with the ZTSH telescope of CrAO. We obtained series of images in R,V bands starting on 2025-09-21 at 16:28:47.74 UT, i.e. 1.321 days since Fermi/GBM trigger. The optical afterglow initially detected by Swift/UVOT (Gupta et. al, GCN 41898) and further observed by (WU et. al, GCN 41908; Hall et. al, GCN 41914; Pankov et. al, GCN 41931; Reguitti et. al, GCN 41932; Moskvitin et. al, GCN 41948) is clearly visible in individual 120 s exposures taken in the R-band. Preliminary photometry of the OT in the R-band is as follows:
Date UTstart Exptime t-T0 Filter OT Err. UL
(s) (mid, days) (3sigma)
2025-09-21 17:21:58 120 1.35870 R 20.7 0.1 22
The photometry was calibrated against nearby USNO-B1.0 stars (R2 mags) and not corrected for the Galactic extinction.
GCN Circular 41948
Subject
GRB 250920B: SAO RAS optical observations
Date
2025-09-22T22:46:39Z (2 days ago)
From
Alexander Moskvitin at SAO RAS <mosk@sao.ru>
Via
Web form
A. Moskvitin, O. Spiridonova (SAO), N. Pankov (HSE), A. Pozanenko
(IKI) report on behalf of GRB follow-up collaboration and IKI-GRB-FuN.
We observed the field of the GRB 250920B (Fermi GBM Team, GCN 41897;
Gupta et al., GCN 41898; Page et al., GCN 41909; Evans et al.,
GCN 41910; Sonawane and Meegan, GCN 41919; Sakamoto et al., GCN 41923;
Cheung et al., GCN 41937; Svinkin et al., GCN 41942) with the 1-m
SAO RAS telescope Zeiss-1000 equipped with CCD-photometer.
We obtained 10 x 300 sec. images Rc band on September 22,
16:22:43--17:17:34 (t_mid - T0 = 2.33586 days).
The optical transient (Gupta et al., GCN 41898; Wu et al., GCN 41908;
Hall et al., GCN 41914; Kuin and Gupta, GCN 41915; Reguitti et al.,
GCN 41932) is clearly detected in the stacked frame
with the brightness of R = 21.7 +/- 0.1.
Date UT start t-T0 Exp. Filter OT Err. UL (3sigma)
(mid, d) (s)
2025.09.22 16:22:43 2.33586 10 x 300 Rc 21.7 +/- 0.1 23.2
Preliminary photometry is based on the nearby stars from USNO-B1
(R2 magnitudes) and not corrected for the Galactic extinction.
GCN Circular 41946
Subject
GRB 250920B: NIR observations with WINTER
Date
2025-09-22T22:39:42Z (2 days ago)
From
Geoffrey Mo at Caltech / Carnegie Observatories <gmo@mit.edu>
Via
Web form
Geoffrey Mo (Caltech/Carnegie), Tomas Ahumada (Caltech), Viraj Karambelkar (Columbia), Benjamin Schneider (LAM), Robert Stein (UMD), Danielle Frostig (CfA), Nathan Lourie (MIT), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of GRB 250920B (Fermi GBM team, GCN 41897; Gupta et al., GCN 41898; Page et al., GCN 41909; Evans et al., GCN 41910; Sonawane et al., GCN 41919; Sakamoto et al., GCN 41923; Cheung et al., GCN 41937) in the near-infrared J band with the Palomar 1-m telescope, equipped with the 1.2-square degree WINTER camera (Lourie et al. 2020, Frostig et al. 2024).
Observations began at 2025-09-21T02:30:43.439 UTC in the J band (~17.7 hours after the GRB trigger), consisting of 15 x 120 s exposures. 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).
We do not detect a source at the optical candidate location (Lipunov et al., GCN 41901; Wu et al., GCN 41908; Hall et al., GCN 41914; Kuin et al., GCN 41915; Pankov et al., GCN 41931; Reguitti et al., GCN 41932). We obtain the following 5-sigma upper limit: J ~ 18.9 mag (AB).
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 41942
Subject
Konus-Wind detection of GRB 250920B
Date
2025-09-22T18:37:52Z (3 days ago)
From
Dmitry Svinkin at Ioffe Institute <svinkin@mail.ioffe.ru>
Via
Web form
D. Svinkin, D. Frederiks, A. Lysenko, A. Ridnaia,
A. Tsvetkova, M. Ulanov, and T. Cline
on behalf of the Konus-Wind team, report:
The long-duration GRB 250920B
(Fermi-GBM detection: The Fermi GBM team, GCN 41897;
Sonawane and Meegan, GCN 41919;
Swift-BAT detection: Gupta et al., GCN 41898;
Sakamoto et al., GCN 41923;
Glowbug detection: Cheung et al., GCN 41937)
triggered Konus-Wind at T0=31587.304 s UT (08:46:27.304).
The burst light curve shows a multipeaked structure
which starts at ~T0-3.0 s and has a total duration of ~141 s.
The emission is seen up to ~10 MeV.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250920_T31587/
As observed by Konus-Wind, the burst
had the total fluence of 3.80(-0.78,+0.93)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0+6.368 s,
of 6.72(-1.17,+1.16)x10^-6 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).
The time-averaged spectrum of the burst
(measured from T0 to T0+131.328 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.41(-0.10,+0.11),
the high energy photon index beta = -2.41(-7.59,+0.35),
the peak energy Ep = 310(-69,+94) keV
(chi2 = 94/97 dof).
The spectrum near the maximum count rate
(measured from T0+0.256 to T0+8.448 s)
is best fit in the 20 keV - 10 MeV range
by the GRB (Band) model with the following parameters:
the low-energy photon index alpha = -1.06(-0.10,+0.10),
the high energy photon index beta = -2.33(-0.38,+0.18),
the peak energy Ep = 296(-38,+52) keV
(chi2 = 87/81 dof).
All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.
GCN Circular 41937
Subject
GRB 250920B: Glowbug gamma-ray detection
Date
2025-09-22T14:59:17Z (3 days ago)
From
C.C. Cheung at Naval Research Lab <Teddy.Cheung@nrl.navy.mil>
Via
Web form
C.C. Cheung, R. Woolf, M. Kerr, J.E. Grove (NRL), A. Goldstein (USRA), C.A. Wilson-Hodge, D. Kocevski (MSFC), and M.S. Briggs (UAH) report:
The Glowbug gamma-ray telescope [1,2,3], operating on the International Space Station, reports the detection of GRB 250920B, which was also detected by Fermi/GBM (GCN 41897, 41919) and Swift/BAT (GCN 41898, 41923).
Using an adaptive window with a resolution of 32-ms, the burst onset is determined to be 2025-09-20 08:46:28.344 with a duration of 129.5 s and a total significance of over 30 sigma. The observed light curve comprises an initial multi-peaked structure from ~T0 to T0+7.5s, followed by peaks at ~T0+76s, ~T0+107s, and ~T0+128s.
The analysis results presented here are preliminary and use a response function that lacks a detailed characterization of the surrounding passive structure of the ISS.
Glowbug is a NASA-funded technology demonstrator for sensitive, low-cost gamma-ray transient telescopes developed, built, and operated by the U.S. Naval Research Laboratory (NRL) with support from the University of Alabama in Huntsville, USRA, and NASA MSFC. It was launched on 2023 March 15 aboard the Department of Defense Space Test Program’s STP-H9 to the ISS, and operated until 2024 April when it was put in safe storage on orbit. Glowbug was recently removed from storage and resumed operation on 2025 September 12.
[1] Grove, J.E. et al. 2020, Proc. Yamada Conf. LXXI, arXiv:2009.11959
[2] Woolf, R.S. et al. 2022, Proc. SPIE, 12181, id. 121811O
[3] Woolf, R.S. et al. 2024, Proc. SPIE, 13151, id. 1315108
Distribution Statement A: Approved for public release. Distribution is unlimited.
GCN Circular 41932
Subject
GRB 250920B: Asiago optical observations
Date
2025-09-21T17:57:22Z (4 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
email
A. Reguitti (INAF/OAPd), L. Tomasella (INAF/OAPd), D. B. Malesani
(DAWN/NBI and Radboud) report on behalf of a larger collaboration:
We observed the field of GRB 250920B (Gupta et al., GCN 41898; Sonawane
& Meegan, GCN 41919) using the 67/92 Schmidt Telescope located at the
Asiago observatory (Italy). Observations were carried out using a clear
filter, which covers roughly the r and i bands.
The optical counterpart (Gupta et al., GCN 41898; Wu et al., GCN 41908;
Hall et al., GCN 41914; Kuin & Gupta, GCN 41915) is well detected in our
frames. At a mean epoch 2025 Sep 20.771 UT (9.74 hr after the GRB), we
measure an AB magnitude r = 18.62 +- 0.06 (calibrated directly against
r-band magnitudes from the Pan-STARRS catalog).
Considering the filter difference, our measurement is consistent with
the nearly simultaneous observation by Hall et al. (GCN 41914).
GCN Circular 41931
Subject
GRB 250920B: Kitab-ISON optical upper limit
Date
2025-09-21T17:38:23Z (4 days ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), A. Pozanenko (IKI), A. Novichonok (Petrozavodsk State University, KIAM) report on behalf of GRB-IKI-FuN:
We observed the field of a long GRB 250920B detected by Fermi/GBM (The Fermi GBM team GCN 41897), Swift (Gupta et. al, GCN 41898), with the RC-36 telescope of Kitab-ISON. The series of unfiltered 325x15 sec exposures was obtained starting on 2025-09-20 at 14:31 UT, i.e. ~0.26 days since Fermi/GBM trigger. The optical afterglow initially detected by Swift/UVOT (Gupta et. al, GCN 41898) and further observed by (WU et. al, GCN 41908; Hall et. al, GCN 41914) is not visible in our stacked image of 298x15 sec. Preliminary results of our observations provided below:
Date UTstart Exptime t-T0 Filter OT Err. UL
(nxs) (mid, days) (3sigma)
2025-09-20 14:31:30 298x15 0.26550 Clear n/d n/d 15.4
The photometry was calibrated against nearby USNO-B1.0 stars (R2 mags) and not corrected for the Galactic extinction.
Ref. stars
USNO-B1.0
RA Dec R2
187.73536 47.71878 13.75
187.52490 47.74643 13.74
GCN Circular 41923
Subject
GRB 250920B: Swift-BAT refined analysis
Date
2025-09-21T02:14:23Z (4 days ago)
From
Rahul Gupta at NASA GSFC <rahul.gupta@nasa.gov>
Via
Web form
T. Sakamoto (AGU), S. D. Barthelmy (GSFC),
R. Gupta (GSFC), H. A. Krimm (NSF),
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), (i.e. the Swift-BAT team):
Using the data set from T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 250920B (trigger #1351008)
(R. Gupta, et al., GCN Circ. 41898). The BAT ground-calculated position is
RA, Dec = 187.316, 47.843 deg which is
RA(J2000) = 12h 29m 15.8s
Dec(J2000) = +47d 50' 33.6"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 57%.
The BAT mask-weighted light curve shows multiple emission episodes
extending over ~150 sec. T90 (15-350 keV) is 128.44 +- 1.00 sec
(estimated error including systematics).
The time-averaged spectrum from T-4.83 to T+149.43 sec is best fit by a power law
with an exponential cutoff. This fit gives a photon index 1.31 +- 0.15,
and Epeak of 182.2 +- 153.0 keV (chi squared 36.64 for 56 d.o.f.). For this
model the total fluence in the 15-150 keV band is 1.1 +- 0.0 x 10^-05 erg/cm2
and the 1-sec peak flux measured from T+0.29 sec in the 15-150 keV band is
5.0 +- 0.3 ph/cm2/sec. A fit to a simple power law gives a photon index
of 1.52 +- 0.04 (chi squared 42.79 for 57 d.o.f.). 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/1351008
GCN Circular 41919
Subject
GRB 250920B: Fermi GBM Observation
Date
2025-09-20T21:03:38Z (4 days ago)
From
Rushikesh Digambar Sonawane PHD231014 at IISER, TVM <rushikesh23@iisertvm.ac.in>
Via
Web form
R. Sonawane (IISER, TVM) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 08:46:26.16 UT on 20 September 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250920B (trigger 780050791/250920366).
which was also detected by Swift BAT (R. Gupta et al. 2025, GCN 41898),
SVOM C-GFT (WU et al. 2025, GCN 41908), and Swift UVOT (Kuin et al. 2025, GCN 41915) .
The Fermi GBM Final Real-time localization (GCN 41897) is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 56 degrees.
The GBM light curve consists of multiple emission episodes with a duration (T90)
of about 127.7 s (50-300 keV). The time-averaged spectrum
from T0-3.6 to T0+141.8 s is best fit by
a Band function with Epeak = 118 +/- 7 keV,
alpha = -0.85 +/- 0.05, and beta = -2.09 +/- 0.05.
The event fluence (10-1000 keV) in this time interval is
(3.33 +/- 0.07)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+9.4 s in the 10-1000 keV band is 8.8 +/- 0.3 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog:
https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html
For Fermi GBM data and info, please visit the official Fermi GBM Support Page:
https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"
GCN Circular 41915
Subject
GRB 250920B: Swift/UVOT Detection
Date
2025-09-20T19:22:10Z (5 days ago)
From
Paul Kuin at MSSL <npkuin@gmail.com>
Via
email
Paul Kuin (MSSL) and Rahul Gupta (NASA GSFC) report on behalf of the
Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 250920B 99 s
after the BAT trigger (Gupta et al., GCN Circ. 41898). A source consistent
with the XRT position (Evans et al. GCN. Circ. 41910) is detected in the
initial UVOT exposures.
The preliminary UVOT position is:
RA (J2000) = 12:29:16.07 = 187.31697 (deg.)
Dec (J2000) = +47:50:14.4 = 47.83732 (deg.)
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).
Preliminary detections and 3-sigma upper limits using the UVOT photometric
system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures
are:
Filter T_start(s) T_stop(s) Exp(s) Mag
white 99 249 147 13.89 +/- 0.02
v 4433 4633 197 16.53 +/- 0.05
b 3818 4018 197 16.62 +/- 0.04
u 311 452 139 13.26 +/- 0.02
w1 4843 5043 197 17.02 +/- 0.08
m2 4638 4838 197 >19.2
w2 4228 4428 197 19.21 +/- 0.25
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.011 in the direction of the burst
(Schlegel et al. 1998).
The observation also included a 49s long UV grism exposure. Inspection of
the noisy grism spectrum suggests a redshift of 2.2 based on identifying an
broad absorption with the the Ly-alpha line.
GCN Circular 41914
Subject
GRB 250920B: FTW optical and NIR observations of the counterpart
Date
2025-09-20T19:17:41Z (5 days ago)
From
Xander J Hall at Carnegie Mellon University <xjh@andrew.cmu.edu>
Via
Web form
Xander J Hall (Carnegie Mellon U.), Malte Busmann (LMU), Brendan O’Connor (Carnegie Mellon U.), Daniel Gruen (LMU), and Antonella Palmese (Carnegie Mellon U.) report:
We observed the counterpart of GRB 250920B (Fermi GBM Team, GCN 41897; Gupta et al., GCN 41898; Wu et al., GCN 41908) with the Three Channel Imager (3KK) at the Fraunhofer Telescope at Wendelstein Observatory (FTW) in the r, i and J band simultaneously for 10 x 60s starting at 2025-09-20 18:19:54 UT (0.4 days after the trigger). We detect the counterpart clearly in all filters and report the following magnitude:
r = (18.81 +/- 0.01) mag.
The magnitude is calibrated against the PS1 catalog. All magnitudes are provided in the AB system and are not corrected for Galactic extinction.
We thank Christoph Ries and Ralf Bender from the Wendelstein Observatory for obtaining these observations.
GCN Circular 41910
Subject
GRB 250920B: Enhanced Swift-XRT position
Date
2025-09-20T17:03:19Z (5 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
P.A. Evans, M.R. Goad, J.P. Osborne and A.P. Beardmore (U. Leicester)
report on behalf of the Swift-XRT team.
Using 1850 s of XRT Photon Counting mode data and 5 UVOT
images for GRB 250920B, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 187.31688, +47.83750 which is equivalent
to:
RA (J2000): 12h 29m 16.05s
Dec (J2000): +47d 50' 15.0"
with an uncertainty of 1.9 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).
This circular was automatically generated, and is an official product of the
Swift-XRT team.
GCN Circular 41909
Subject
GRB 250920B: Swift-XRT refined Analysis
Date
2025-09-20T16:55:32Z (5 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
K.L. Page (U. Leicester), R. Brivio (INAF-OAB), A. D'Ai (INAF-IASFPA),
S. Campana (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU), D.N.
Burrows (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans report on
behalf of the Swift-XRT team:
We have analysed 6.4 ks of XRT data for GRB 250920B, from 87 s to 22.3
ks after the trigger. The data comprise 395 s in Windowed Timing (WT)
mode (the first 2 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. The refined XRT position is RA,
Dec = 187.3175, +47.8376 which is equivalent to:
RA (J2000): 12h 29m 16.21s
Dec(J2000): +47d 50' 15.2"
with an uncertainty of 3.5 arcsec (radius, 90% confidence).
The late-time light curve (from T0+3.8 ks) can be modelled with a
power-law decay with a decay index of alpha=1.20 (+/-0.05).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.490 (+0.035, -0.017). The
best-fitting absorption column is consistent with the Galactic value
of 1.1 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.89 (+/-0.07) and a best-fitting absorption column
of 3.3 (+1.5, -1.4) x 10^20 cm^-2. The counts to observed (unabsorbed)
0.3-10 keV flux conversion factor deduced from this spectrum is 3.5 x
10^-11 (3.7 x 10^-11) erg cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 3.3 (+1.5, -1.4) x 10^20 cm^-2
Galactic foreground: 1.1 x 10^20 cm^-2
Excess significance: 2.5 sigma
Photon index: 1.89 (+/-0.07)
If the light curve continues to decay with a power-law decay index of
1.20, the count rate at T+24 hours will be 0.10 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.5 x
10^-12 (3.8 x 10^-12) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01351008.
This circular is an official product of the Swift-XRT team.
GCN Circular 41908
Subject
GRB 250920B: SVOM/C-GFT optical observations
Date
2025-09-20T16:38:42Z (5 days ago)
From
SVOM_group <svomgroup@bao.ac.cn>
Via
Web form
Chao WU (NAOC), Zhe Kang (CHO), Liping Xin(NAOC), Xuhui Han(NAOC), Pinpin Zhang (NAOC), Xiaomeng Lu (NAOC), Zhenwei Li (CHO), You Lv (CHO), Ruosong Zhang (NAOC), Yujie Xiao(NAOC), Yulei Qiu (NAOC), Jing Wang (NAOC), Jinsong Deng(NAOC), Lei Huang(NAOC), Jianyan Wei (NAOC), report on behalf of the SVOM/C-GFT team:
We observed the field of GRB 250920B detected by Fermi/GBM (Fermi Team, GCN 41897) and Swift/BAT (Gupta et al., GCN 41898) with LATIOS on SVOM/C-GFT. Observations started at 2025-09-20 11:22:32.815 UTC, ~2.60 hr after the trigger.
The optical counterpart reported by Gupta et al.(GCN 41898) was clearly detected in our stacked images of band g, r, and i.
The magnitudes are:
| mid-time (T-T₀) / hr | exposure time / s | band | mag (AB) | mag err |
|---|---|---|---|---|
| 2.76 | 10×30 | g | 17.15 | ±0.07 |
| 2.64 | 10×30 | r | 17.05 | ±0.07 |
| 2.85 | 9×30 | i | 17.01 | ±0.05 |
The photometry was calibrated with nearby Pan-STARRS1 stars. Magnitudes were not corrected for dust extinction.
We thank the observation assistants Shuai Liu and Chunlei Guo at Jilin observatory for their excellent support.
The Chinese Ground Follow-up Telescope (C-GFT) for the SVOM mission is located at Jilin Station, Changchun Observatory, National Astronomical Observatories, CAS. It features two instruments: (1) CATCH at the Cassegrain focus with a 21 arcsec x 21 arcsec FOV for simultaneous g/r/i-band imaging, and (2) LATIOS, a 4k x 4k CMOS camera at the prime focus with a 1.28 deg x 1.28 deg FOV that images in g, r, and i bands via filter switching.
GCN Circular 41901
Subject
Swift GRB 250920B: Global MASTER-Net observations report
Date
2025-09-20T13:28:21Z (5 days ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko,
G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
MASTER-Tunka robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the Swift GRB 250920B ( R. Gupta et al., GCN 41898) errorbox 16004 sec after notice time and 16025 sec after trigger time at 2025-09-20 13:13:36 UT, with upper limit up to 19.9 mag. The observations began at zenith distance = 66 deg. The sun altitude is -19.4 deg.
The galactic latitude b = 69 deg., longitude l = 132 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2995928
We obtain a following upper limits.
Tmid-T0 | Site |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________
16056 | MASTER-Tunka | C | 60 | 19.9 |
16056 | MASTER- | C | 60 | 19.6 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 41898
Subject
GRB 250920B: Swift detection of a burst with a bright optical counterpart.
Date
2025-09-20T09:01:06Z (5 days ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
R. Gupta (NASA GSFC), C. Gronwall (PSU), N. P. M. Kuin (UCL-MSSL) and
K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift
Observatory Team:
At 08:46:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250920B (trigger=1351008). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 187.337, +47.844 which is
RA(J2000) = 12h 29m 21s
Dec(J2000) = +47d 50' 39"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed multiple peaks, with a
complex structure with a duration of about 150 sec. The peak count rate
was ~8200 counts/sec (15-350 keV), at ~5 sec after the trigger.
The XRT began observing the field at 08:48:01.1 UT, 90.6 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 187.3179, 47.8372 which is equivalent to:
RA(J2000) = 12h 29m 16.30s
Dec(J2000) = +47d 50' 13.9"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 52 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy.
The initial flux in the 2.5 s image was 8.20e-09 erg cm^-2 s^-1 (0.2-10
keV).
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 98 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
RA(J2000) = 12:29:16.07 = 187.31697
DEC(J2000) = +47:50:14.4 = 47.83734
with a 90%-confidence error radius of about 0.74 arc sec. This position is 2.3
arc sec. from the center of the XRT error circle. The estimated magnitude is
13.75 with a 1-sigma error of about 0.14. No correction has been made for
extinction.
Burst Advocate for this burst is R. Gupta (rahulbhu.c157 AT gmail.com).
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 41897
Subject
GRB 250920B: Fermi GBM Final Real-time Localization
Date
2025-09-20T08:56:46Z (5 days 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 08:46:26 UT on 20 Sep 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250920B (trigger 780050791.161292 / 250920366).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 190.8, Dec = 47.4 (J2000 degrees, equivalent to J2000 12h 43m, 47d 23'), with a statistical uncertainty of 2.4 degrees.
The angle from the Fermi LAT boresight is 56.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920366/quicklook/glg_skymap_all_bn250920366.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/bn250920366/quicklook/glg_healpix_all_bn250920366.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920366/quicklook/glg_lc_medres34_bn250920366.gif