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GRB 250920C

GCN Circular 42009

Subject
GRB 250920C: ULL-ASTRO-MASTER early detection of the optical afterglow with LCO 40-cm telescope at Siding Spring Observatory
Date
2025-09-26T19:40:45Z (11 days ago)
From
Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>
Via
Web form
V. Ghiraldo, J. Basurto Merino, P.G. Berdayes, A. Caballero-Almagro, A. Cerón, M. Contreras, F. Díaz-Segado, T. Ferrer-Laviña, B. Gandolfi, J. Hernández Fung, L. Juliá-Maroto, E. Lekaroz-Urriza, M. Manzano García, E. Mejía-Martínez, J. Prieto Polo, M. Pulido-Torres, M. Quintana-Ansaldo, A. Schenone-Zanuzzi, A. Selezneva, T. Tundidor Rodríguez, E. Urquijo-Rodríguez (all ULL), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL). 

We report on a Las Cumbres Observatory (LCO) 40-cm telescope early observation of the long-duration Gamma-Ray Burst GRB 250920C, detected  by Fermi (Fermi GBM team, GCN circ. 41903; and Mailyan et al., GCN circ. 41917), Swift (Gupta et al., GCN circ. 41904; Kuin et al., GCN circ. 41920; Goad et al., GCN circ. 41922; Gupta et al., GCN circ. 41924; and D'Ai et al. GCN circ. 41935), SVOM (Wang et al., GCN circ. 41929), and Konus-Wind (Panteleeva et al., GCN circ. 41957).

We observed the field of GRB 250920C with one of the two LCO 40-cm telescopes (Planewave Delta Rho 350 telescopes equipped with CMOS QHY600 cameras) located at the LCO node at Siding Spring Observatory near Coonabarabran, New South Wales, Australia. The observation, a single exposure of 300 sec in the SDSS r filter, started on 2025-09-20 at 17:21:51 UT, about 1.94 hours after the Fermi and Swift trigger. The optical and near-infrared counterpart reported first by Wortley et al. using GOTO (GCN circ. 41907) and with observations by several other groups (GCN circ. 41911, GCN circ. 41913, GCN circ. 41916, GCN circ. 41920, GCN circ. 41925, GCN circ. 41928, GCN circ. 41943, GCN circ. 41947, GCN circ. 41951, GCN circ. 41952, GCN circ. 41955, GCN circ. 41956, GCN circ. 41965, and GCN circ. 41967) is clearly detected in our image, with a magnitude of r = 18.77 +/- 0.10, calibrated against PanSTARRS-1 DR2 and not corrected for galactic extinction, that is consistent with another reported r-band detection at a similar epoch.

A redshift of z = 1.399 - 1.40 has been measured by Sánchez-Ramírez et al. (GCN circ. 41928), de Wet et al. (GCN circ. 41952), and Izzo et al. (GCN circ. 41955).

Based on observations made with the Las Cumbres Observatory’s education network telescopes that were upgraded through generous support from the Gordon and Betty Moore Foundation (program IAC2025B-010). These observations are part of a course in Astrophysical Techniques of the Master in Astrophysics of the Astrophysics Department of the University of La Laguna in collaboration with the Instituto de Astrofísica de Canarias (Tenerife, Canary Islands, Spain).

GCN Circular 41967

Subject
GRB 250920C: Kilonova-Catcher optical afterglow detection
Date
2025-09-24T14:37:22Z (13 days ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
M. Freeberg (KNC), C. Andrade(UMN), D. Turpin (CEA-Saclay/Irfu), S. Antier (OCA/IJCLab), M. Coughlin (UMN),S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:

We observed the field of GRB 250920C detected by Fermi/GBM (Fermi GBM team, GCN 41903), Swift/BAT (Gupta et al., GCN 41904), SVOM/GRM (Wang et al., GCN 41929) and Konus-WIND (Panteleeva et al., GCN 41957) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the iTelescope T30 telescope operated by M. Freeberg. Our observations started at TGRB+3.3hr. 

In our stacked frames, subtracted from the PanSTARRS DR2 template image, we detect the optical/IR counterpart reported by GOTO (Wortley et al., GCN 41907) and further confirmed by many other teams (Strausbaugh et al.,GCN 41911; Ghosh et al., GCN 41913; Kuin et al., GCN 41920; Mohan et al., GCN 41925; Sanchez-Ramirez et al., GCN 41928; Guiffreda et al., GCN 41943; Mo et al., GCN 41947; de Ugarte Postigo et al., GCN 41951; de Wet et al., GCN 41952; Izzo et al., GCN 41955; Zheng et al., GCN 41956; Zhu et al., GCN 41965)

We report some of our first image follow-up results in the table below 
+---------------+----------+-----------+----------------+--------------+
| Tmid-TGRB (hr)| Exp (s)  | Filter    | Magnitude      | Instrument   |
+===============+==========+===========+================+==============+
| 3.49          | 3 x 300s | Rc (Vega) | 18.93 +/- 0.07 | iT30         | 
+---------------+----------+-----------+----------------+--------------+

All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the Johnson-cousins filters were calibrated using the GAIA DR3 synphot 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). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).

GCN Circular 41965

Subject
GRB 250920C: TRT optical observation
Date
2025-09-24T13:37:17Z (13 days ago)
From
Zipei Zhu at NAOC <zpzhu@nao.cas.cn>
Via
Web form

Z.P. Zhu (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Y. Fu (HUST), L.B. He, X. Liu, J. An, S.Q. Jiang, D. Xu (NAOC) report on behalf of a large collaboration:

We observed the field of GRB 250920C (Fermi GBM team, GCN 41903

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; Gupta et al., GCN 41904; Mailyan & Meegan, GCN 41917; Gupta et al., GCN 41924; Wang et al., GCN 41929; Panteleeva et al., GCN 41957 ), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). Observations started at 18:21:14 UTC on 2025-09-20, i.e., ~ 2.9 hours after the Swift/BAT trigger and a series of frames in the R band were obtained.

The optical afterglow (Wortley et al., GCN 41907

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; Strausbaugh et al., GCN 41911; Ghosh et al., GCN 41913; Kuin et al., GCN 41920; Mohan et al., GCN 41925; Sanchez-Ramirez et al., GCN 41928; Guiffreda et al., GCN 41943; de Ugarte Postigo et al., GCN 41951; de Wet et al., GCN 41952; Izzo et al., GCN 41955; Zheng & Filippenko, GCN 41956; ) was clearly detected in our stacked R-band frame with a brightness of 18.87 +/- 0.02, calibrated with Pan-STARRS DR2 catalog and not corrected for Galactic extinction.

GCN Circular 41957

Subject
Konus-Wind detection of GRB 250920C
Date
2025-09-23T19:12:38Z (14 days ago)
From
Valentina Panteleeva at Ioffe Institute <panteleevav228@gmail.com>
Via
Web form
V. Panteleeva, D. Frederiks, A. Lysenko, A. Ridnaia, D. Svinkin,
A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB 250920C (Fermi-GBM detection:
The Fermi GBM team, GCN 41903; Mailyan & Meegan, GCN 41917;
Swift-BAT detection: Gupta et al., GCN 41904; Gupta et al., GCN 41924; 
SVOM-GRM observaion: Wang et al., GCN 41929)
triggered Konus-Wind at T0=55519.312 s UT (15:25:19.312).

The burst light curve shows a multipeaked structure
which starts at ~T0-0.4 s and has a total duration of ~41 s.
The emission is seen up to ~2 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB250920_T55519/

As observed by Konus-Wind, the burst
had the total fluence of 2.91(-0.21,+0.29)x10^-5 erg/cm2,
and the 64-ms peak flux, measured from T0+25.904 s,
of 8.27(-0.78,+0.78)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+41.216 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 = -1.22(-0.07,+0.09),
the high energy photon index beta = -3.19(-6.81,+0.51),
the peak energy Ep = 160(-12,+9) keV
(chi2 = 104/97 dof).

The spectrum near the maximum count rate
(measured from T0+24.832 to T0+33.024 s)
is best fit in the 20 keV - 10 MeV range
by a power law with exponential cutoff model:
dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep)
with alpha = -1.14(-0.07,+0.07)
and Ep = 220(-14,+16) keV (chi2 = 96/81 dof).
Fitting by a GRB (Band) model yields the same alpha and Ep,
and an upper limit on the high energy photon index: beta < -3.15
(chi2 = 97/80 dof).

Assuming the redshift z=1.399 (Sanchez-Ramirez et al., GCN 41928;
Izzo et al., GCN 41955)
and a standard cosmology with H_0 = 67.3 km/s/Mpc, Omega_M = 0.315,
and Omega_Lambda = 0.685 (Planck Collaboration, 2014),
we estimate the following rest-frame parameters:
the isotropic energy release E_iso is 1.54(-0.11,+0.16)x10^53 erg,
the peak luminosity L_iso is 1.05(-0.10,+0.10)x10^53 erg/s,
the rest-frame peak energy of the time-averaged spectrum
Ep,i,z is 383(-30,+22) keV and the spectrum near the maximum count rate
Ep,p,z is 528(-33,+38) keV.

With the obtained estimates, GRB 250920C is inside 68% prediction bands for
both the 'Amati' and the 'Yonetoku' relations derived for the sample of >300 long
KW GRBs with known redshifts (Tsvetkova et al., 2017; Tsvetkova et al., 2021),
see http://www.ioffe.ru/LEA/GRBs/GRB250920_T55519/GRB250920C_rest_frame.pdf

All the quoted errors are at the 68% confidence level.
All the quoted values are preliminary.

GCN Circular 41956

Subject
GRB 250920C: KAIT optical observations
Date
2025-09-23T18:23:18Z (14 days 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 250920C (Fermi GBM

team, GCN 41903; Gupta et al., GCN 41904; Mailyan et al., GCN 41917;

Gupta et al., GCN 41924; SVOM Team, GCN 41929) around mid time of

~0.778d after the burst. A set of 90x60s clear (roughly R) filter

images were obtained. We marginally detected the optical afterglow

(Wortley et al., GCN 41907; Strausbaugh et al., GCN 41911; Ghosh

et al., GCN 41913; Lipunov et al., GCN 41916; Kuin et al., GCN 41920;

Mohan et al., GCN 41925; Sanchez-Ramirez et al., GCN 41928;

Guiffreda et al., GCN 41943; de Ugarte Postigo et al., GCN 41951).

We estimate the afterglow to be 21.0 +/- 0.3 mag (Vega).

GCN Circular 41955

Subject
GRB 250920C: VLT redshift confirmation z = 1.399
Date
2025-09-23T16:00:11Z (14 days ago)
From
Valentina Abril-Melgarejo at Obs. Paris/LUX <valentina.abril-melgarejo@obspm.fr>
Via
Web form
L. Izzo (INAF/OACn and DARK/NBI), D. Xu (NAOC), V. Abril-Melgarejo (LUX-Paris Obs.), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), A. Martin-Carrillo (UCD), N. R. Tanvir (Leicester), V. D’Elia (ASI/SSDC), report on behalf of the Stargate collaboration:

We observed the optical afterglow (Wortley et al., GCN 41907) of GRB 250920C (Gupta et al., GCN 41904), using the ESO VLT UT4 (Yepun) equipped with the MUSE spectrograph. The mid time of our observation was Jan 21.209 UT (10.41 hr after the GRB trigger), and consisted of 4 exposures of 19 min each.

We extracted a 1D spectrum using a circular aperture of 0.4 arcsec radius, centered on the afterglow emission. In the reduced spectrum, which covers the wavelength range 4750 - 9330 AA, we detect clear continuum and a number of absorption features, which we identify, among others, as Mg II, Mg I, Fe II, Fe II*, Ni II*, Cr II, Zn II, Mn II, all at a common redshift z = 1.399. The detection of Fe II fine-structure lines cements the association of the GRB with this absorption system. Furthermore, an emission line consistent with [O II] at the same redshift is also detected, likely from the underlying host galaxy (Wortley et al., GCN 41907).

Our result is thus fully consistent, and strengthens, the previous reports by GTC (Sanchez-Ramirez et al., GCN 41928) and SALT (De Wet et al.. GCN 41952).

We acknowledge expert support from the observing staff in Paranal, in particular Boris Haeussler, Enrico Congiu, Ana Jimenez Gallardo, and Marcelo Lopez.

GCN Circular 41952

Subject
GRB 250920C: SALT redshift confirmation
Date
2025-09-23T12:27:05Z (14 days ago)
From
Simon de Wet at DTU Space <simdewet@gmail.com>
Via
Web form
S. de Wet (DTU Space), D. A. H. Buckley (SAAO), D. B. Malesani (DAWN/NBI and Radboud), P. J. Groot (Radboud, UCT, and SAAO) report:

We observed the optical afterglow (Wortley et al., GCN 41907) of GRB 250920C (Gupta et al., GCN 41904), using the Southern African Large Telescope (SALT) equipped with the Robert Stobie Spectrograph (RSS). Observations started at 23:11:13 UT on 2025 Sep 20 (7.8 hr after the GRB), and consisted of 2x1200 s of spectroscopy using the PG700 grating (which covers the wavelength range 3600-7500 AA).

In the coadded spectrum, continuum is detected across the whole wavelength range. A few narrow absorption features are visible which we interpret as due to C IV 1548,1550 (very strong), Fe II 2344 and 2374, Mg II 2796,2803 and Mg I 2852, all at a redshift of 1.399. Our result is thus consistent with, and confirms, the earlier measurement reported by Sanchez-Ramirez et al. (GCN 41928) using the GTC.

GCN Circular 41951

Subject
GRB 250920C: COLIBRÍ optical observations
Date
2025-09-23T11:33:46Z (14 days ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
Antonio de Ugarte Postigo (LAM), Diego López-Cámara (UNAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Fredd Alvarez (UNAM), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (OCA), 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), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Benjamin Schneider (LAM):

We imaged the field of the Fermi/GBM GRB 250920C (Fermi GBM team, GCN Circ. 41903; Mailyan
et al. GCN 41917), also detected by Swift/BAT (Gupta et al. GCN 41904; Gupta et al. GCN 41924) and SVOM/GRM (Wang et al. GCN 41929) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-09-23 08:01:47 to 10:19:49 UTC (from 2.69 to 2.75 days after the trigger) and obtained 64 minutes of simultaneous exposures in the r and z filters.

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.

We detected the optical counterpart reported by Wortley et al. (GCN Circ. 41907), at preliminary magnitudes of:

r = 22.70 +/- 0.14
z = 22.54 +/- 0.25

We note that the source is still significantly brighter than the candidate host galaxy identified by Wortley et al. (GCN Circ. 41907) which appears in the Legacy Survey with r ~ 25.1 mag, so our observations are still afterglow dominated.

Further observations and analysis are ongoing.

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 41947

Subject
GRB 250920C: NIR observations with WINTER
Date
2025-09-22T22:40:56Z (15 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 250920C (Fermi GBM Team, GCN 41903

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; Gupta et al., GCN 41904; Mailyan et al., GCN 41917; Goad et al., GCN 41922; Gupta et al., GCN 41924; SVOM Team, GCN 41929; Evans et al., GCN 41935) 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 2025-09-21T08:59:42.591 UTC in the J band (~17.5 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

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).

We do not detect a source at the optical counterpart location (Wortley et al., GCN 41907

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; Strausbaugh et al., GCN 41911; Ghosh et al., GCN 41913; Lipunov et al., GCN 41916; Kuin et al., GCN 41920; Mohan et al., GCN 41925; Sanchez-Ramirez et al., GCN 41928; Guiffreda et al., GCN 41943). We obtain the following 5-sigma upper limit: J ~ 19.0 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 41943

Subject
GRB 250920C: PRIME near-infrared detection
Date
2025-09-22T20:28:38Z (15 days ago)
Edited On
2025-09-26T20:17:19Z (11 days ago)
From
O. Guiffreda at UMD <oriogui@umd.edu>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of O. Guiffreda at UMD <oriogui@umd.edu>
Via
Web form

O. Guiffreda (UMD), J. Durbak (UMD), A. S. Kutyrev (NASA/GSFC), N. Passaleva (U Rome), M. El Kabir (U Rome), E. Troja (U Rome), S. B. Cenko (NASA/GSFC)

The following correction is primarily the result of an error in Vega to AB magnitude conversion in J and H filters.

Following the Fermi GBM (GCN 41903

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) and Swift BAT (GCN 41904), we observed the transient field in Z, Y, J and H filters with PRIME ~7 hours after the initial Fermi and Swift detection.

At the position of the optical counterpart reported by GOTO (GCN 41907

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), we detect an uncatalogued source in all aforementioned bands. Using nearby 2MASS and PanSTARRS stars for preliminary calibration we derive the following magnitudes and limits, not corrected for Galactic extinction:

FilterMag(AB)
Z19.8 +/- 0.10
Y19.7 +/- 0.07
J19.8 +/- 0.10
H19.4 +/- 0.07

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

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

GCN Circular 41935

Subject
GRB 250920C: Swift-XRT refined Analysis
Date
2025-09-22T10:15:36Z (16 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
A. D'Ai (INAF-IASFPA), S. Campana (INAF-OAB), A. Melandri (INAF-OAR),
J.A. Kennea (PSU), D.N. Burrows (PSU), S. Lanava (PSU), A.P. Beardmore
(U. Leicester), J.P. Osborne (U. Leicester) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 5.1 ks of XRT data for GRB 250920C, from 110 s to
131.0 ks after the   trigger. The data comprise 64 s in Windowed Timing
(WT) mode (the first 9 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode. 

The late-time light curve (from T0+7.1 ks) can be modelled with a
power-law decay with a decay index of alpha=1.53 (+/-0.09).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.94 (+/-0.10). The
best-fitting absorption column is  4.8 (+2.1, -2.0) x 10^20 cm^-2, in
excess of the Galactic value of 1.4 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.89 (+0.16, -0.15)
and a best-fitting absorption column of 6.7 (+4.0, -3.5) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum  is 3.6 x 10^-11 (4.1 x 10^-11) erg cm^-2
count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.7 (+4.0, -3.5) x 10^20 cm^-2
Galactic foreground: 1.4 x 10^20 cm^-2
Excess significance: 2.5 sigma
Photon index:	     1.89 (+0.16, -0.15)

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01351039.

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

GCN Circular 41929

Subject
GRB 250920C: SVOM/GRM observation
Date
2025-09-21T12:22:56Z (16 days ago)
From
Chenwei Wang at IHEP <cwwang@ihep.ac.cn>
Via
Web form
SVOM/GRM team: Chen-Wei Wang. Wen-Jun Tan, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Jean-Luc Atteia (IRAP), Stéphane Schanne (CEA)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by a burst GRB 250920C (SVOM trigger reference: sb25092008) at 2025-09-20T15:25:17.400 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN#41895) and Swift/BAT (R. Gupta, GCN#41904). 

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of multi-pulses with a T90 of 39.5 +0.5/-1.0 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn//admin/static/svgrb250920C.png

In addition, the position of this burst, as determined by Swift/BAT (RA= 51.459, DEC= -29.646, GCN#41904), is located at about 81 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view, but this burst is still detected by ECLAIRs through the shield. 

With this localization, the time-averaged spectrum from T0-2 to T0+41 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -1.08 +0.08/-0.08 and the cutoff energy, parameterized as Epeak, is 166 +19/-15 keV. The event fluence (10-1000 keV) in this time interval is (3.36 +0.17/-0.16)E-05 erg/cm^2. 

With the redshift of z=1.40 (R. Sanchez-Ramirez et al., GCN#41928), the localization of GRB 250920C in the 'Amati' relation diagram is consistent with type II GRBs, as shown at: 
https://www.bursthub.cn//admin/static/svgrb250920C_amati.png

The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.

The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang@ihep.ac.cn)

GCN Circular 41928

Subject
GRB 250920C: GTC/OSIRIS+ spectroscopic redshift z = 1.40
Date
2025-09-21T09:47:37Z (17 days ago)
From
Ruben Sanchez-Ramirez at IAA-CSIC <ruben@iaa.es>
Via
Web form
R. Sanchez-Ramirez, A. J. Castro-Tirado, S. Guziy, M.D. Caballero-Garcia, S.-Y. Wu and I. Perez-Garcia (IAA-CSIC), A. M. Garcia-Rodríguez (GTC, IAC), A. Tejero-Caro (GTC), S. B. Pandey (ARIES), M. Gritsevich (Univ. of Helsinki), Y.-D. Hu (GXI), A. Nicuesa Guelbenzu (Tautenburg Obs.), L. Piro (INAF/IAPS) and B.-B. Zhang (NJU), on behalf of a larger collaboration, report:

Following the detection of GRB 250920C by Fermi (Fermi Team, GCN 41903; Mailyan et al. GCN 41907), Swift (Gupta et al. GCN 41904, 41924), we observed the optical afterglow (Wortley et al. GCN 41907, Strausbaugh et al. GCN 41911, Ghosh et al. GCN 41913, Kuin et al. GCN 41920, Mohan et al. GCN 41925) with the 10.4m GTC telescope, at the Spanish Roque de los Muchachos Observatory, on the island of La Palma, equipped with the OSIRIS+ instrument.

The spectroscopic observations were performed at a high airmass and consisted of 3x900s exposures using grism R1000B, with a spectral coverage between 3,600 and 7,700 A and a resolving power of  ~600. The observations started on Sep 21, 04:29 UT (i.e. 13.07 h after the burst trigger).

From a preliminary reduction, we find a low SNR continuum over all the wavelength range but the bluest extreme, most probably due to atmospheric extinction. The extracted spectrum shows several absorption features that we interpret as AlII, AlIII, ZnII, FeII, MgII, and MgI at z=1.40, which we propose as the redshift of GRB 250920C.

GCN Circular 41925

Subject
GRB 250920C: GROWTH-India Telescope optical observations
Date
2025-09-21T05:57:11Z (17 days ago)
From
Varun Bhalerao at IIT Bombay <varunb@iitb.ac.in>
Via
Web form

GRB 250920C: GROWTH-India Telescope optical observations

T. Mohan, V. Swain, A.P. Saikia, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and K. Angail (IAO) report on behalf of the GIT team:

We observed the field of Swift GRB 250920C (Gupta et al., 41904), also detected by Fermi/GBM (GBM Team, GCN 41903

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) with 0.7m GROWTH-India Telescope (GIT). We started the observation at 2025-09-20 20:17:41 UT, i.e., 4.87 hours after the Fermi trigger. We obtained multiple exposures in g', r', and i' filters. We detect the optical afterglow in our stacked image at position reported by Wortley et al., GCN 41907. The photometry result follows as:

MJD (mid)tmid - t0 (hours)FilterExposure (s)Magnitude (AB)
60938.855555.11r'3*36019.52+-0.1
60938.858365.18i'2*36019.22+-0.1
60938.914806.53g'2*36019.88+-0.12
60938.942697.22i'5*36019.71+-0.12

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

Our magnitude is consistent with other optical observations (Wortley et al., GCN 41907

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, Strausbaugh et al., GCN 41911, Ghosh et al., GCN 41913, Lipunov et al., GCN 41916, Kuin et al., GCN 41920).

The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.

GCN Circular 41924

Subject
GRB 250920C: Swift-BAT refined analysis
Date
2025-09-21T02:16:24Z (17 days ago)
From
Rahul Gupta at NASA GSFC <rahul.gupta@nasa.gov>
Via
Web form
R. Gupta (GSFC), S. D. Barthelmy (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),
T. Sakamoto (AGU) (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 250920C (trigger #1351039)
(R. Gupta, et al., GCN Circ. 41904).  The BAT ground-calculated position is
RA, Dec = 51.465, -29.646 deg which is 
   RA(J2000)  =  03h 25m 51.6s 
   Dec(J2000) = -29d 38' 46.7" 
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 15%.
 
The BAT mask-weighted light curve exhibits a bright initial peak followed by multiple
structured emission episodes, lasting for about 40 sec from the trigger time.
T90 (15-350 keV) is 38.02 +- 1.27 sec (estimated error including systematics).
 
The time-averaged spectrum from T-0.44 to T+40.49 sec is best fit by a power law
with an exponential cutoff.  This fit gives a photon index 1.09 +- 0.18, 
and Epeak of 133.4 +- 43.9 keV (chi squared 45.87 for 56 d.o.f.).  For this
model the total fluence in the 15-150 keV band is 1.3 +- 0.0 x 10^-05 erg/cm2
and the 1-sec peak flux measured from T-0.12 sec in the 15-150 keV band is
16.9 +- 1.0 ph/cm2/sec.  A fit to a simple power law gives a photon index
of 1.48 +- 0.04 (chi squared 60.21 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/1351039

GCN Circular 41922

Subject
GRB 250920C: Enhanced Swift-XRT position
Date
2025-09-20T22:43:41Z (17 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
M.R. Goad, J.P. Osborne, A.P. Beardmore and P.A. Evans (U. Leicester) 
report on behalf of the Swift-XRT team.

Using 915 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 250920C, 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 = 51.46554, -29.65537 which is equivalent
to:

RA (J2000): 03h 25m 51.73s
Dec (J2000): -29d 39' 19.3"

with an uncertainty of 2.2 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 41920

Subject
GRB 250920C: Swift/UVOT Detection
Date
2025-09-20T21:48:29Z (17 days ago)
From
Paul Kuin at MSSL <npkuin@gmail.com>
Via
email
N. P. M. Kuin (UCL-MSSL) and R. Gupta (GSFC) report on behalf of the
Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250920C 124 s
after the BAT trigger (Gupta et al., GCN Circ. 41904).  A source consistent
with the XRT position is detected in the initial UVOT exposures.

The preliminary UVOT position is:

 RA  (J2000)= 03h 25m 51.76s, 51.46568 deg
 Dec (J2000) = -29d 39m 20.2s,  -29.65562 deg

consistent with the detection by GOTO (Wortley et al. GCN Circ. 41907).

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

white_FC           146          207         60.5          14.55 +/- 0.04
v                  124          133         10.1          14.56 +/- 0.09

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.019 in the direction of the burst
(Schlegel et al. 1998).

GCN Circular 41917

Subject
GRB 250920C: Fermi GBM Observation
Date
2025-09-20T20:46:55Z (17 days ago)
From
Bagrat Mailyan at Florida Tech <mbagrat@gmail.com>
Via
Web form
B. Mailyan (Florida Tech) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 15:25:17.06 UT on 20 September 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250920C (trigger 780074722/250920643)
which was also detected by Swift BAT (R. Gupta et al. 2025, GCN 41904).
The Fermi GBM Final Real-time Localization (GCN 41903) is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 94 degrees.

The GBM light curve consists of multiple emission episodes with a duration (T90)
of about 36.6 s (50-300 keV). The time-averaged spectrum
from T0+0.003 to T0+37.377 s is best fit by
a Band function with Epeak = 117 +/- 4 keV,
alpha = -0.76 +/- 0.04, and beta = -2.34 +/- 0.05.

The event fluence (10-1000 keV) in this time interval is
(2.97 +/- 0.04)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+26 s in the 10-1000 keV band is 25.1 +/- 0.5 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 41916

Subject
Swift GRB 250920C: Global MASTER-Net observations report
Date
2025-09-20T19:34:22Z (17 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-SAAO robotic telescope  (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L)  located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB 250920C ( R. Gupta et al., GCN 41904) errorbox  13198 sec after notice time and 13225 sec after trigger time at 2025-09-20 19:05:42 UT, with upper limit up to  18.1 mag. The observations began at zenith distance = 85 deg. The sun  altitude  is -32.4 deg. 

The galactic latitude b = -56 deg., longitude l = 226 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2996190

We obtain a following upper limits.  

Tmid-T0  |          Site       |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________

   13315 |         MASTER-SAAO |   C |   180 | 14.4 |        
   14583 |         MASTER-SAAO |   C |   180 | 15.6 |        
   14583 |         MASTER-SAAO |   C |   180 | 18.1 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

GCN Circular 41913

Subject
GRB 250920C: Optical counterpart detection by LCO
Date
2025-09-20T18:32:10Z (17 days ago)
From
ankur ghosh at CAPP, University of Johannesburg <ghosh.ankur1994@gmail.com>
Via
Web form
Ankur Ghosh, Soebur Razzaque (CAPP, University of Johannesburg), Rahul Gupta (NASA GSFC),  Alexander Moskvitin, Yulia Sotnikova (SAO RAS), Naveen Dukiya (ARIES) on behalf of a larger collaboration. 

We observed the field of the GRB 250920C triggered by the Swift (Gupta et al., GCN 41904) in the r, V filter of the 1-meter Sinistro telescope at the Las Cumbres Observatory Global Telescope (LCOGT) node located at Siding Spring is near Coonabarabran, New South Wales, Australia . The 1-m Sinistro telescope is equipped with a 4K x 4K CCD (FOV: 26 x 26 arcmin, scale: 0.39 arcsec/pixel).
Observations began on September 20, 2025, starting 1.47 hours after the GRB trigger. Further observations are currently ongoing.

We clearly detect the optical transient (OT) reported by GCNs (Wortley et al., GCN 41907, Strausbaugh et al., GCN 41911) in our  r and V band image.

|Date|		|JDstart|	|t-T0 (hours)|	|Exp (sec)|	|Filter|	|Magnitude|  
----------------------------------------------------------------------------------------------------     
2025-09-20	2460939.20389	1.47		1 x 600 	r		r = 18.47 +/- 0.01
2025-09-20	2460939.22762	2.04		1 x 600 	r		r = 18.75 +/- 0.01

The field was calibrated against nearby APASS stars, with magnitudes converted using Lupton (2005) equations, and has not been corrected for Galactic extinction.

GCN Circular 41911

Subject
GRB250920C: LCOGT Optical Detection
Date
2025-09-20T17:10:36Z (17 days ago)
From
Robert Strausbaugh at Eastern Illinois University <rstrausbaugh@eiu.edu>
Via
email
R. Strausbaugh (Eastern Illinois University), A. Cucchiara (NASA) report on behalf of a larger collaboration:

We observed the Swift GRB 250920C field (Gupta et al., GCN 41904)  with the LCOGT 1-meter Sinistro instrument at the Siding Spring Observatory, Australia site, on September 20, from 16:29 to 16:51 UT (corresponding to 1.07 to 1.26 hours after the GRB trigger time) with the SDSS r and i filters.

We performed a series of 3x200s exposures in i-band and r-band.  We detect an uncatalogued source consistent with GOTO optical counterpart candidate (Wortley et al., GCN 41907) in both bands.

The following magnitudes are calculated using the PanSTARRS catalog as reference:

r = 18.54 +/- 0.05
i = 18.25 +/- 0.01

These magnitudes are not corrected for galactic extinction.

GCN Circular 41907

Subject
GRB 250920C: GOTO optical counterpart candidate
Date
2025-09-20T16:16:16Z (17 days ago)
From
d.s.oneill@bham.ac.uk
Via
Web form

M. E. Wortley, D. O'Neill, B. P. Gompertz, G. Ramsay, R. Starling, K. Ackley, M. J. Dyer, J. Lyman, K. Ulaczyk, A. Kumar, B. Godson, D. Steeghs, D. K. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. P. Breton, L. K. Nuttall, and J. Casares report on behalf of the GOTO collaboration:

We report on observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to GRB 250920C (GCN  #41903, Fermi GBM team, #41904, Gupta et al.). Targeted observations were performed at 2025-09-20 15:33:10 (starting 0.13 hours after trigger). Each observation consisted of 4x90s exposures in the GOTO L-band (400-700 nm).

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

We identify a candidate optical counterpart GOTO25hju with L = 15.91±0.01 AB mag, within the Swift/BAT error region with coordinates:

RA,DEC (J2000) = 51.465773, -29.65547 | 03:25:51.79, -29:39:19.67

We find no evidence of these sources prior to the GRB trigger time in previous GOTO observations taken at 2025-09-17 16:39:05 UT (2.95d pre-trigger) down to a depth of L<19.7 AB mag, the ZTF observations provided by the Lasair broker (Smith et al. 2019), or the ATLAS forced photometry server (Shingles et al. 2021). There is a faint, point-like object coincident at the source location in the Legacy catalogue.

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

Observations are ongoing.

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

GCN Circular 41904

Subject
GRB 250920C: Swift detection of a burst
Date
2025-09-20T15:39:55Z (17 days ago)
From
David Palmer at LANL <palmer@lanl.gov>
Via
email
R. Gupta (NASA GSFC), S. Dichiara (PSU), C. Gronwall (PSU),
J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), K. L. Page (U Leicester)
and D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 15:25:17 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250920C (trigger=1351039).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec 51.459, -29.646 which is 
   RA(J2000) = 03h 25m 50s
   Dec(J2000) = -29d 38' 44"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 40 sec.  The peak count rate
was ~8500 counts/sec (15-350 keV), at ~0 sec after the trigger.

The XRT began observing the field at 15:27:21.2 UT, 123.8 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 51.4644, -29.6566 which is equivalent to:
   RA(J2000)  = 03h 25m 51.46s
   Dec(J2000) = -29d 39' 23.8"
with an uncertainty of 5.3 arcseconds (radius, 90% containment). This
location is 41 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 1.50e-09 erg cm^-2 s^-1 (0.2-10
keV).

There are no UVOT data available at this time.

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 41903

Subject
GRB 250920C: Fermi GBM Final Real-time Localization
Date
2025-09-20T15:36:01Z (17 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 15:25:17 UT on 20 Sep 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250920C (trigger 780074722.062672 / 250920643).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 47.6, Dec = -26.7 (J2000 degrees, equivalent to J2000 03h 10m, -26d 41'), with a statistical uncertainty of 1.2 degrees.

The angle from the Fermi LAT boresight is 93.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920643/quicklook/glg_skymap_all_bn250920643.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/bn250920643/quicklook/glg_healpix_all_bn250920643.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250920643/quicklook/glg_lc_medres34_bn250920643.gif
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