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GRB 260924A

GCN Circular 45776

Subject
GRB 260924A: VLA detection
Date
2026-09-28T17:43:09Z (13 hours ago)
From
u6074340@umail.utah.edu
Via
Web form
A. Gulati (University of Utah), T. Laskar (University of Utah), K. D. Alexander (University of Arizona), D. Eappachen (University of Utah), C. Christy (University of Arizona), N. Franz (University of Arizona)

We observed GRB 260924A (Fermi GBM, GCN 45712; D’Elia et al., GCN 45713) with the Karl G. Jansky Very Large Array (VLA) at multiple frequencies beginning on 2026 Sep 26 at 14:36 UT (2.6 days after the burst). 

In preliminary analysis, we detect a radio counterpart at 15 GHz with a flux density of ~0.6 mJy at the position:

RA (J2000) = 08:55:03.106 +/- 0.001” 
Dec (J2000) = +12.02.35.24 +/- 0.03” 

This is consistent with the X-ray position (Evans et al., GCN 45715) and optical position (Izzo et al., GCN 45718; Klingler et al., GCN 45735). Further observations are planned. 

We thank the VLA staff for scheduling and executing these observations.

GCN Circular 45775

Subject
GRB 260924A: Swift/UVOT UV grism spectrum results
Date
2026-09-28T13:34:37Z (17 hours ago)
From
Paul Kuin at MSSL <npkuin@gmail.com>
Via
email
N. P. M. Kuin (UCL-MSSL), L. Izzo (INAF/OACN and DARK/NBI), D. B.
Malesani (DAWN/NBI and Radboud Univ.), Noel Klingler (NASA) and
V. D'Elia (SSDC and INAF-OAR) report on behalf of the Swift/UVOT
team:

Swift UVOT observed the bright GRB 260924A (D'Elia et al., GCN Circ.
45713) with an exposure time of 50 s in the UV grism, starting 258 s
after the trigger.

The data rate was large enough to cause the data buffers to overflow,
causing a drop at wavelengths longer than 4500 AA. The inherent
accuracy of the wavelength scale is limited by a 3 pixel uncertainty
and is typically 20 AA in the UV, but closer to 40 AA at 4000 AA.

The spectrum shows clear detection of continuum at wavelengths redder
than 4110 AA, but no flux at all to the blue. This is consistent with
a drop-out due to the Lyman limit at a redshift of z = 3.51 +- 0.04.
The unreliability of the spectrum above 4500 AA does not allow to look
for further features to the red. Our findings are thus overall fully
consistent with the higher-resolution spectroscopic measurement
z = 3.51 provided by Castro-Tirado (GCN Circ. 45740).

GCN Circular 45774

Subject
GRB 260924A: AbAO optical observations
Date
2026-09-28T08:59:15Z (a day ago)
From
Nicolai Pankov at IKI <colinsergesen@gmail.com>
Via
Web form
N. Pankov (IKI), A. Pozanenko (IKI), R. Ya. Inasaridze (AbAO), M. Zhuzhunadze (AbAO), A. Volnova (IKI), A. Moskvitin (SAO RAS) report on behalf of IKI-GRB-FuN:
 
We observed the field of a bright GRB 260924A (The Fermi GBM team, GCN 45712; D'Elia, GCN 45713; Salunke et al., GCN 45720; Holzmann Airasca et al., GCN  45721; Waratkar et al., GCN 45732; Yu et al., GCN 45741; Gupta et al., GCN 45750; Ridnaia et al., GCN  45751) at z=3.51 (Castro-Tirado et al., GCN 45740) with the AS-32 0.7m telescope of the Abastumani Astrophysical Observatory (AbAO) on 2 subsequent nights 2026-09-26 and 2026-09-27. The first observation has started at 00:31:30 UT, i.e, ~2 days since trigger. Both observations were taken in the R-band using a CCD-photometer. The optical afterglow (Hu et al., GCN 45717, Izzo et al., GCN 45718; Vijaykumar et al., GCN 45734; Klingler et al., GCN 45735; Ma et al., GCN 45736; Novotny et al., GCN 45743; Aryan et al., GCN 45748; Li et al., GCN 45728; Fortin et al., GCN 45759; Mo et al., GCN 45762) is detected in the co-add image of the first night, while absent in the second night co-add. Preliminary photometry and observation details are listed below:
 
Date       UTstart  Exptime t-T0     Filter OT   Err. UL(3sigma)
                    (nxs)   (days)                  
2026-09-26 00:31:30 68*60   1.99453  R      20.0 0.3  20.3
2026-09-27 00:16:21 90*60   2.99164  R      n/d  n/d  20.0
 
The photometry was calibrated using nearby stars from USNO-B1.0 (R2 magnitudes) and 
has not been corrected for the Galactic extinction.

GCN Circular 45768

Subject
GRB 260924A: Upper limits from a neutrino search with IceCube
Date
2026-09-27T00:54:35Z (2 days ago)
From
Sam Hori at IceCube/U Wisc-Madison <sahori@wisc.edu>
Via
email
The IceCube Collaboration (http://icecube.wisc.edu/) reports:

IceCube has performed a search for track-like muon neutrino events arriving from the direction of GRB 260924A (GCN 45712) at the position determined by NOT observations of the afterglow (GCN 45718) in a time range of (-1 hour, +2 hours) from the initial trigger (GCN 45723) reported by Fermi-GBM (T0=2026-09-24 01:13:23.56 UTC) during which IceCube was collecting good quality data.

Zero track-like events are found to coincide with the position of the GRB. We accordingly derive a time-integrated muon-neutrino flux upper limit of E^2 dN/ dE = 3.5e-2 GeV cm^-2 at 90% CL, for an E^-2 power law. 90% of events IceCube would detect from a source at this declination with an E^-2 spectrum have energies between approximately 940 GeV and 1.6 PeV.

A subsequent search was performed including 2 days of data centered on the Fermi-GBM trigger (2026-09-23 01:13:23.56 UTC to 2026-09-25 01:13:23.56 UTC). In this search, we report a p-value of 1.0, consistent with background expectation. We accordingly derive a time-integrated muon neutrino flux upper limit of E^2 dN/ dE = 4.1e-2 GeV cm^-2 at 90% CL, under the assumption of an E^-2 power law.

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc@icecube.wisc.edu.

[1] IceCube Collaboration, R. Abbasi  et al., ApJ 910 4 (2021)

GCN Circular 45762

Subject
GRB 260924A: Near-infrared detection with MDM/MIRAGE
Date
2026-09-26T17:03:38Z (3 days ago)
From
Geoffrey Mo at Caltech / Carnegie Observatories <gmo@mit.edu>
Via
Web form

Geoffrey Mo (Carnegie/Caltech), K. De, V. Karambelkar, S. Ibrahim, D. Schiminovich (Columbia University) report:

We observed the field of GRB 260924A (Fermi GBM Team, GCN 45712

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; D'Elia et al., GCN 45713; Evans et al., GCN 45715; Salunke et al., GCN 45720; Holzmann Airasca et al., GCN 45721; McDermott et al., GCN 45723; Dichiara et al., GCN 45730; Waratkar et al., GCN 45732; Gupta et al., GCN 45733; Vijaykumar et al., GCN 45734; Yu et al., GCN 45741; Gupta et al., GCN 45750; Ridnaia et al., GCN 45751) in the near-infrared J and Hs bands with the MDM InfraRed Astronomy inGaas Experiment (MIRAGE) instrument on the MDM 1.3 m telescope.

Observations began at 2026-09-26T12:08:38 (+2.46 days after the GRB trigger) in the J band and 2026-09-26T12:32:07 (+2.47 days) in the Hs band, lasting 1080 s in J and 1050 s in Hs. We detect the optical counterpart (Hu et al., GCN 45717

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; Izzo et al., GCN 45718; Klingler et al., GCN 45735; Ma et al., GCN 45736; Castro-Tirado et al., GCN 45740; Novotny et al., GCN 45743; Aryan et al., GCN 45748; Li et al., GCN 45758; Fortin et al., GCN 45759) in the J band and set an upper limit in Hs, with the following AB magnitudes: J = 19.81 ± 0.20, Hs > 19.2.

MIRAGE is a new robotic YJHs-band near-infrared imager for the MDM 1.3m telescope. We thank the Mt Cuba Astronomical Foundation for development of MIRAGE and the MDM Observatory staff for supporting the observations.

GCN Circular 45759

Subject
GRB 260924A: COLIBRÍ optical observations
Date
2026-09-26T13:04:25Z (3 days ago)
From
F. Fortin at IRAP <ffortin.sci.edu@gmail.com>
Via
Web form
Francis Fortin (IRAP), Edilberto Aguilar-Ruiz (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM),  Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM), Antonio de Ugarte Postigo (LAM),  Alan M. Watson (UNAM) report:

We imaged the field of the Swift GRB 260924A (D’Elia et al., GCN Circ. 45713) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed 2026-09-26T11:06:20 to 2026-09-26T12:19:36 UTC (from 57.88 to 59.12 hours after the trigger) and obtained 45 minutes of simultaneous exposure 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 Hu et al., GCN Circ. 45717, Izzo et al., GCN Circ. 45718 and Klingler et al., GCN Circ. 45735, at a preliminary magnitude of:

r = 20.40 +/- 0.03
z = 20.07 +/- 0.03

We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.

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 45758

Subject
GRB 260924A: Second-epoch J-band upper limit from the SYSU 80 cm infrared telescope
Date
2026-09-26T10:20:10Z (3 days ago)
From
Rui-Chen Gao <ayxzxn0100@gmail.com>
Via
Web form

Jin-Ji Li, Zhong-Nan Dong, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, Hao-Ran Zhang, P H Thomas Tam, Rong-Feng Shen, Bin Ma (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:

We carried out a second epoch of J-band observations of GRB 260924A using the Sun Yat-sen University (SYSU) 80 cm infrared telescope, following our previous detection reported in Ma et al. (GCN 45736

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). Our second-epoch observations started on 2026 September 25 at 22:00 UT, approximately 44.8 hours after the Swift/BAT trigger, and continued until 23:03 UT in the J band.

We do not detect the source at the position of the counterpart (Hu et al., GCN 45717

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; Izzo et al., GCN 45718; Vijaykumar et al., GCN 45734; Klingler et al., GCN 45735; Ma et al., GCN 45736; Castro-Tirado et al., GCN 45740; Yu et al., GCN 45741; Novotny et al., GCN 45743; Aryan et al., GCN 45748) in the stacked image, down to a 5-sigma limiting magnitude of J > 18.5 mag (Vega).

The photometry was calibrated against nearby stars from the 2MASS catalog and is reported in the Vega system, without correction for Galactic extinction. The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.

GCN Circular 45751

Subject
Konus-Wind detection of GRB 260924A
Date
2026-09-25T20:37:17Z (3 days ago)
From
Anna Ridnaia at Ioffe Institute <ridnaia@mail.ioffe.ru>
Via
Web form
A. Ridnaia, D. Frederiks, A. Lysenko, V. Panteleeva,
D. Svinkin, A. Tsvetkova,  M. Ulanov, and T. Cline,
on behalf of the Konus-Wind team, report:

The long-duration GRB 260924A
(Fermi-GBM detection: Fermi GBM team, GCN 45712;
McDermott & Scotton, GCN 45723;
Swift detection: D'Elia et al., GCN 45713;
Gupta et al., GCN 45733;
AstroSat-CZTI detection: Salunke et al., GCN 45720;
Fermi-LAT detection: Airasca et al., GCN 45721;
Nustar detection: Waratkar & Grefenstette, GCN 45732;
SVOM-GRM detection: Yu et al., GCN 45741)
triggered Konus-Wind at T0=04406.079 s UT (01:13:26.079).

The burst light curve shows two separated multipeaked emission episodes.
The first episode starts at ~T0-3.5 s and has a total duration of ~212 s,
the second starts at ~T0+328 s and lasts up to ~T0+361 s.
The emission is seen up to ~8 MeV.

The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB260924_T04406/

As observed by Konus-Wind, the burst
had the total fluence of 3.83(-0.40,+0.43)x10^-4 erg/cm2,
and the 64-ms peak flux, measured from T0+108.288 s,
of 1.10(-0.16,+0.16)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).

The time-averaged spectrum of the burst
(measured from T0 to T0+360.704 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 = -0.62(-0.11,+0.14),
the high energy photon index beta = -2.37(-0.31,+0.20),
the peak energy Ep = 497(-61,+59) keV
(chi2 = 91/97 dof).

The spectrum near the maximum count rate
(measured from T0+106.752 to T0+114.944 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 = -0.62(-0.09,+0.11),
the high energy photon index beta = -2.41(-0.23,+0.17),
the peak energy Ep = 596(-60,+63) keV
(chi2 = 82/86 dof).

Assuming the redshift z=3.510 (Castro-Tirado et al., GCN 45740)
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 1.00(-0.11,+0.11)x10^55 erg
(on a par with the most energetic to date GRB 221009A, Frederiks et al., 2023),
the peak luminosity L_iso is 1.30(-0.19,+0.19)x10^54 erg/s (one of the most 
luminous GRBs to date), the rest-frame peak energy of the time-averaged spectrum
Ep,i,z is 2242(-275,+267) keV and the spectrum near the maximum count rate
Ep,p,z is 2690(-270,+285) keV.

With the obtained estimates, GRB 260924A 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/GRB260924_T04406/GRB260924A_rest_frame.pdf

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

GCN Circular 45750

Subject
GRB 260924A: Extreme Isotropic-equivalent Gamma-ray Energy and encouragement for multiwavelength follow-up observations
Date
2026-09-25T19:48:04Z (3 days ago)
From
Rahul Gupta at NASA GSFC <rahul.gupta@nasa.gov>
Via
Web form
Rahul Gupta (NASA GSFC), Peter Veres (UAH), Vikas Chand (LSU), Eric Burns (LSU), Pádraig McDermott (University College Dublin), and L. Scotton (UAH) reports:

We have estimated the isotropic-equivalent gamma-ray energy of GRB 260924A (The Fermi GBM team, GCN 45712; D'Elia et al., GCN 45713; Salunke et al., GCN 45720; Holzmann Airasca et al., GCN 45721; McDermott et al., GCN 45723; Waratkar et al., GCN 45732; Gupta et al., GCN 45733; and Yu et al., GCN 45741) using the time-integrated Fermi-GBM spectrum and the measured redshift of z = 3.51 (Castro-Tirado et al. GCN 45740). The time-averaged spectrum from T0-11.6 to T0+382.6 s is best fit by a Band function with Epeak = 645 +/- 4 keV, alpha = -0.91 +/- 0.01, and beta = -2.36 +/- 0.01 (McDermott et al., GCN 45723). Using the spectral parameters from McDermott et al., GCN 45723 and measured fluence of 2.84 × 10^-4 erg cm^-2 in the 10-1000 keV observer-frame energy range, and applying a bolometric k-correction to the 1-10,000 keV rest-frame energy range, we obtain an isotropic-equivalent gamma-ray energy of approximately 9.74 × 10^54 erg.

We adopted a flat Lambda-CDM cosmology with Omega_m = 0.286, Omega_Lambda = 0.714, and H0 = 69.6 km s^-1 Mpc^-1.

The inferred isotropic-equivalent energy places GRB 260924A among the most energetic gamma-ray bursts known. Its energetics are comparable, within the uncertainties, to those of GRB 221009A, the “BOAT” (Brightest Of All Time).

Given its extreme energetics and broad observational coverage, continued multi-wavelength observations of GRB 260924A are strongly encouraged.

GCN Circular 45748

Subject
GRB 260924A: Optical follow-up observations with Kinder
Date
2026-09-25T17:19:12Z (4 days ago)
From
Amar Aryan at National Central University, Institute of Astronomy (NCUIA) <amararyan941@gmail.com>
Via
Web form
A. Aryan (NCU), R. Gupta (NASA GSFC), K. N.-T. Ho, T.-W. Chen, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang(HNAS), Y.-H. Lee, A. Sankar.K, A. Dutta, M.-H. Lee, Y.-C. Pan, C.-C. Ngeow, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, W.-J. Hou, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), K. W. Smith, H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:


We observed the field of the GRB 260924A (The Fermi GBM team, GCN 45712; D'Elia et al., GCN 45713; Evans et al., GCN 45715; Airasca et al., GCN 45721; McDermott et al., GCN 45723; Dichiara et al., GCN 45730; Gupta et al., GCN 45733) using the 40cm SLT and 1m LOT at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first SLT epoch of observations in r-band started at 19:42 UTC on the 24th of September 2026 (MJD 61307.8207), 18.47 hr after the Fermi GBM detection. 

For the SLT g-band observations, we utilized the astroalign (Beroiz et al. 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al. 2022, ApJ, 935, 167) packages to align and stack the individual frames. In the stacked image, we detect the optical counterpart reported by Hu et al., (GCN 45717) and confirmed by Izza et al., (GCN 45718); Vijaykumar et al., (GCN 45734); Klingler et al., (GCN 45735) and Novotny et al. GCN (45743). The NIR detection is  reported by Ma et al., GCN (45736). The GRB is also detected by NuSTAR (Waratkar et al., GCN 45732) and SVOM/GRM (SVOM/GRM team, GCN 45741). For the LOT r-band observations, the GRB is detected in all six individual frames, each of 300 seconds.

Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and the measured magnitudes (in the AB system) are as follows:

Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude     | avg. Seeing | med. Airmass*
SLT       | g      | 61307.8207  | 18.47     | 300 * 12     | 19.2 +/- 0.1  | 2".2        | 2.30
LOT       | r      | 61307.8328  | 18.76     | 300 * 1      | 18.38 +/- 0.04| 1".8        | 2.54

Our detections are in agreement with the reported optical counterpart magnitudes at various epochs. We aim to obtain further observations in the upcoming nights.

*for the single r-band frame, it's simply the airmass at which the frame was observed.

The presented magnitudes are calibrated using the field stars from the ATLAS-RefCat2 catalog from MAST (Tonry J. L. et al. 2018, ApJ, 867, 105). The reported magnitudes are not corrected for expected galactic extinction of A_g = 0.140 mag and A_r = 0.097 mag, in the direction of the transient (Schlafly & Finkbeiner 2011). The optical follow-up methodology of transients and details on the Lulin Observatory telescopes are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.

GCN Circular 45743

Subject
GRB 260924A: Ondrejov D50 optical afterglow detection
Date
2026-09-25T12:06:46Z (4 days ago)
From
Filip Novotný at Masaryk University <filip.novotny@talentovani.cz>
Via
Web form
F. Novotny, J. Strobl, M. Jelinek and R. Hudec (ASU CAS Ondrejov) report:

We observed the position of GRB 260924A (D'Elia et al., GCN 45713;
Evans et al., GCN 45715), detected by Swift (trigger=1513082), with the
D50 telescope of the Astronomical Institute Ondrejov, near Prague, Czech
Republic. We obtained 14x120s and 10x60s Sloan r'-band exposures between
03:15 and 03:57 UT on 2026 September 25, i.e. 26.0-26.7 h after the
trigger.

The optical afterglow (Hu et al., GCN 45717; Izzo et al., GCN 45718)
is detected in the combined image (mean epoch T0+26.4 h) at
RA, Dec = 08:55:03.11, +12:02:35.5 (J2000), consistent with the UVOT
position (Klingler et al., GCN 45735), with a magnitude of

r' = 19.12 +/- 0.08 (AB)

Photometry is calibrated in Sloan r' (AB system) against ATLAS-RefCat2
field stars, not corrected for Galactic extinction (E(B-V)=0.041 mag).

GCN Circular 45741

Subject
GRB 260924A: SVOM/GRM observation
Date
2026-09-25T09:22:35Z (4 days ago)
From
yzh807926@163.com
Via
Web form
SVOM/GRM team: Zheng-Hang Yu, Wen-Jun Tan, Yue Wang, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)

SVOM/ECLAIRs team: Olivier Godet (IRAP)

Report on behalf of the SVOM team:

SVOM/GRM was triggered in-flight by GRB 260924A (SVOM trigger reference: sb26092401) at 2026-09-24T01:13:28.000 (T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #45712), Swift (V. D'Elia et al., GCN #45713), AstroSat CZTI (S. Salunke, GCN #45720) and Fermi/LAT (A. Holzmann Airasca et al., GCN #45721).

With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of two main episodes multiple pulses followed by a single pulse with a T90 of 338.0 +/-1.0 s in the 15-5000 keV band.

The SVOM/GRM light curve can be found here:
https://www.bursthub.cn/gcn/2026/9/svgrb260924A.png

In addition, the position of this burst, as determined by Swift/XRT (RA = 133.76, Dec = 12.04, Error = 2.1 arcsec, GCN #45715), is located at about 78 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view. ECLAIRs detected the burst mostly through the shield above 50 keV.

With this localization, the time-averaged spectrum from T0-10 to T0+400 s is best fitted by a power law function with an exponential high-energy cutoff. The power law index is -0.90 +/-0.03 and the cutoff energy, parameterized as Epeak, is 535 +37/-33 keV. The event fluence (10-1000 keV) in this time interval is (3.35 +0.06/-0.07)E-04 erg/cm^2. 

The 1s peak spectrum, measured from T0+339 to T0+340 s, if fitted by a power law function with an exponential high-energy cutoff, the power law index is -0.88 +0.04/-0.05 and the cutoff energy, parameterized as Epeak, is 943 +113/-95 keV. The flux (10-1000 keV) in this time interval is (6.87 +/-0.16)E-06 erg/cm^2/s.

The localization of GRB 260924A in the 'Amati' relation diagram is shown at: https://www.bursthub.cn/gcn/2026/9/svgrb260924A_amati.png
The localization of GRB 260924A in the 'Yonetoku' relation diagram is shown at: 
https://www.bursthub.cn/gcn/2026/9/svgrb260924A_yonetoku.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/GRM point of contact for this burst is: Zheng-Hang Yu (IHEP) (zhyu@ihep.ac.cn)

GCN Circular 45740

Subject
GRB 260924A: 2.2m CAHA spectroscopic redshift z = 3.51
Date
2026-09-25T08:46:21Z (4 days ago)
From
I. Perez-Garcia at Instituto de Astrofísica de Andalucía <ipg@iaa.es>
Via
Web form
A. J. Castro-Tirado, I. Perez-Garcia, A. Sota, E. Fernandez-Garcia, M.D. Caballero-Garcia, S. Guziy and M. Gritsevich (IAA-CSIC), Y.-D. Hu (GXI), A. Fernandez-Martin and S. Gongora (CAHA) on behalf of a larger collaboration, report:

Following the detection of GRB 260924A by Fermi/GBM (Fermi Team, GCN 45712, McDermott, GCN 45723), Swift/BAT (D'Elia et al., GCN 45713, Gupta et al., GCN 45733) and Swift/XRT (enhanced XRT position, Evans et al., GCN 45715), AstroSAT (Salunke et al., GCN 45720) and NuSTAR (Waratkar et al. GCN 45732), we observed the optical afterglow (Hu et al. GCN 45717, Izzo et al. GCN 45718) with the 2.2m telescope at the Calar Alto Observatory (Almeria, Spain) equipped with CAFOS .

We obtained an acquisition image in the r’-band and long-slit spectroscopy with the grisms G-200 (2 x 500 s, mid-time 2026-09-25 04:16 UT, i.e. T0 + 27.0 h) and B-100 (3 x 500 s, mid-time 04:44 UT, T0 + 27.4 h), with a 2.0 arcsec slit set at the parallactic angle. The airmass decreased from 2.6 to 1.9 and the seeing was 1.1 arcsec.

In the acquisition image (30 s, 26.3 h post trigger) the afterglow is detected at  r’ = 19.04 +/- 0.18 (AB, calibrated against Pan-STARRS DR1) which, compared with r' = 18.68 +/- 0.04 at  21.50 h post trigger (GROWTH-India, GCN 45728), implies a decay index alpha = 1.6 +/- 0.8.

The combined spectrum covers 3700-8800 A and shows a well-detected continuum above 5500 A. Below 5480 +/- 40 A the flux drops by a factor of about 2-3, and below ~4100 A it falls to a few per cent of the extrapolated continuum. We interpret these as the Lyman-alpha break and the Lyman limit of the same absorber, giving a redshift estimate of z = 3.51 +/- 0.10

The break is present independently in both grisms, and is also seen in the raw count ratio between the afterglow and a field star that fell on the same slit, so it does not depend on the flux calibration. No metal absorption lines are detected due to the low signal-to-noise of these data.

This redshift confirms the photometric estimate z = 3.6 +/- 0.3 that we can derive from BVRI imaging with the 1.5m telescope of the Sierra Nevada Observatory (OSN) at 3.2 h post trigger, where the afterglow had R = 15.51 +/- 0.01 (Hu et al. , GCN 45717).

At z = 3.51 this corresponds to an absolute magnitude of about -31 at a rest-frame wavelength of 1400 A and a rest-frame epoch of 0.7 h, placing GRB 260924A among the most luminous optical afterglows observed till date.

We thank the staff of the Calar Alto and Sierra Nevada observatories for their excellent support.

GCN Circular 45736

Subject
GRB 260924A: J-band observations by the SYSU 80 cm infrared telescope
Date
2026-09-25T02:41:33Z (4 days ago)
From
linp@mail2.sysu.edu.cn
Via
Web form
Bin Ma, Jin-Ji Li, Zhong-Nan Dong, Jia-Qi Lin, Pu Lin, Yun Shi, Hao-Nan Yang, P H Thomas Tam, Rong-Feng Shen (Sun Yat-sen University) report on behalf of the SYSU 80cm infrared telescope team:

We observed the field of GRB 260924A, detected by Fermi/GBM (Fermi GBM team, GCN 45712) and Swift/BAT (D'Elia et al., GCN 45713), using the Sun Yat-sen University (SYSU) 80 cm infrared telescope. Our observations started on 2026 September 24 at 21:50 UT, 20.6 hours after the GRB trigger, and continued until 22:50 UT in the J band.

We clearly detect the source at the position of the optical counterpart (Hu et al., GCN 45717; Izzo et al., GCN 45718; Vijaykumar et al., GCN 45734; Klingler et al., GCN 45735), and we measure J = 17.13 +/- 0.02 Vega mag on the stacked image. The photometry was calibrated against nearby stars from the 2MASS catalog and is reported in the Vega system, without correction for Galactic extinction.

The SYSU 80 cm infrared telescope is operated and managed by the Department of Astronomy, Sun Yat-sen University.

GCN Circular 45735

Subject
GRB 260924A: Swift/UVOT Detection
Date
2026-09-25T01:37:57Z (4 days ago)
From
noelklin@umbc.edu
Via
Web form
N. Klingler (NASA-GSFC / UMBC / CRESST II) and V. D'Elia (SSDC & INAF-OAR) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 260924A 158 s after the BAT trigger (D'Elia et al., GCN Circ. 45713).  A source consistent with the XRT position (Evans et al., GCN Circ. 45715) is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  08:55:03.13 = 133.76304 (deg.)
    Dec (J2000) = +12:02:35.4  =  12.04318 (deg.)
with an estimated uncertainty of 0.50 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              158          308          147         14.68 +/- 0.02
v                 4254         4454          197         15.11 +/- 0.03
b                 3637         3837          197         15.71 +/- 0.03
u                  334          372           37         17.33 +/- 0.24
w1                4664         4864          197        >18.5
m2                4459         4658          197        >19.2
w2                4048         4247          197        >19.2

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.041 in the direction of the burst (Schlegel et al. 1998).

GCN Circular 45734

Subject
GRB 260924A: GROWTH-India Telescope optical observations
Date
2026-09-25T00:00:05Z (4 days ago)
Edited On
2026-09-25T00:08:29Z (4 days ago)
From
Aditya Pawan Saikia at IIT Bombay <adityaps@iitb.ac.in>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Aditya Pawan Saikia at IIT Bombay <adityaps@iitb.ac.in>
Via
Web form

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

We observed the field of GRB 260924A detected by Fermi (Fermi GBM Team, GCN 45712

Loading...
 
 
), with 0.7m GROWTH-India Telescope (GIT). We started the observation at 2026-09-24 22:37:28, i.e., 21.4 hours after the trigger, and obtained multiple exposures in the SDSS r', g' filters. We detect the optical counterpart in our images at the position reported by Evans et al., GCN 45715. The photometric results are as follows:

MJD (mid)Filtertmid-t0 (hours)Exposure Time (sec)Magnitude (AB)
61307.947r'21.4953x30018.68 +/- 0.04
61307.959g'21.7743x30019.63 +/- 0.05

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

Our results are consistent with other optical observations (Hu et al., GCN 45717

Loading...
 
 
and Izzo et al., GCN 45718)

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 45733

Subject
GRB 260924A: Swift-BAT refined analysis
Date
2026-09-24T21:08:37Z (4 days ago)
From
Amy <yarleen@gmail.com>
Via
email
R. Gupta (GSFC), S. D. Barthelmy (GSFC), V. D'Elia (SSDC & INAF-OAR), 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-180 to T+712 sec from the recent telemetry
downlink, we report further analysis of BAT GRB 260924A (trigger #1513082)
(D'Elia et al., GCN Circ. 45713).  The BAT ground-calculated position is
RA, Dec = 133.754, 12.041 deg which is
   RA(J2000)  =  08h 55m 00.9s
   Dec(J2000) = +12d 02' 27.0"
with an uncertainty of 1.0 arcmin, (radius, sys+stat, 90% containment). The
partial coding was 39%.

The mask-weighted light curve shows a bright multi-peaked structure that
starts at ~T-10 s and ends at ~T+450 s. There are roughly two distinct
structures. The first structure starts at ~T-10 s and ends at ~T+160 s and
consists of many overlapping pulses. The second structure is a single
FRED-like pulse and starts at ~T+330 s, peaked at ~T+340 s, and ends at
~T+400 s. T90 (15-350 keV) is 344.82 +- 1.78 sec (estimated error including
systematics).

The time-averaged spectrum from T-5.96 to T+455.68 sec is best fit by a
simple power-law model.  The power law index of the time-averaged spectrum
is 1.24 +- 0.02.  The fluence in the 15-150 keV band is 5.7 +- 0.1 x 10^-05
erg/cm2. The 1-sec peak photon flux measured from T+338.78 sec in the
15-150 keV band is 10.5 +- 0.4 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/1513082

GCN Circular 45732

Subject
GRB 260924A: NuSTAR detection of prompt emission
Date
2026-09-24T19:26:01Z (4 days ago)
From
Gaurav Waratkar at Caltech <gauravwaratkar@iitb.ac.in>
Via
Web form
G. Waratkar (Caltech) and B. Grefenstette (Caltech) report on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group:

The NuSTAR SINGS working group reports the detection of prompt emission from the long-duration GRB 260924A in both the NuSTAR CsI anti-coincidence shields. This GRB was identified through a blind search using the CsI shield rates. Details of the search algorithm will be described in a future paper.

The NuSTAR SINGS algorithm triggered at 2026-09-24T01:13:23.56 UTC shows a detection of GRB 260924A, also detected by Fermi-GBM (GCN 45712), Swift-BAT (GCN 45713), and AstroSat-CZTI (GCN 45720).

The NuSTAR CsI shield data are recorded at 1 Hz. We detect a multi-peaked burst lasting for about 150-s, consistent with the GBM, BAT and CZTI lightcurves. The peak count rate is ~1500-cps with a baseline rate of ~1000-cps during this time period. We also see a detection in the signal above 100 keV in the CZT detectors.

The Swift-XRT localization (GCN 45713) at RA = 133.7620, Dec = 12.0455 implies an offset from the NuSTAR boresight of 130-deg and an offset from the geocenter of 158-deg.

Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2026/260924A 

Information on NuSTAR SINGS can be found here: 
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/     

NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington.

GCN Circular 45731

Subject
GRB 260924A: Swift/UVOT Upper Limits
Date
2026-09-24T19:14:45Z (4 days ago)
Edited On
2026-09-24T21:14:42Z (4 days ago)
From
noelklin@umbc.edu
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of noelklin@umbc.edu
Via
Web form
***UPDATE***:  Please disregard this circular; an aspect solution issue caused an erroneous non-detection in the automated UVOT data processing system.  We will put out a revised circular shortly.

GCN Circular 45730

Subject
GRB 260924A: Swift-XRT refined Analysis
Date
2026-09-24T18:40:46Z (5 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
S. Dichiara (PSU), J.A. Kennea (PSU), K.L. Page (U. Leicester), A.P.
Beardmore (U. Leicester), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB),
R. Brivio (INAF-OAB), D.N. Burrows (PSU) and P.A. Evans report on
behalf of the Swift-XRT team:

We have analysed 6.4 ks of XRT data for GRB 260924A, from 99 s to 43.5
ks after the   trigger. The data comprise 365 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 late-time light curve (from T0+3.6 ks) can be modelled with an
initial power-law decay with an index of alpha=0.89 (+0.08, -0.07),
followed by a break at T+25.9 ks to an alpha of 2.4 (+/-0.7).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.491 (+0.017, -0.015). The
best-fitting absorption column is  consistent with the Galactic value
of 4.1 x 10^20 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 2.03 (+/-0.09) and a best-fitting absorption column
of 6.9 (+2.2, -2.0) x 10^20 cm^-2. The counts to observed (unabsorbed)
0.3-10 keV flux conversion factor deduced from this spectrum  is 3.3 x
10^-11 (3.9 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     6.9 (+2.2, -2.0) x 10^20 cm^-2
Galactic foreground: 4.1 x 10^20 cm^-2
Excess significance: 2.3 sigma
Photon index:	     2.03 (+/-0.09)

If the light curve continues to decay with a power-law decay index of
2.4, the count rate at T+24 hours will be 0.050 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.6 x
10^-12 (1.9 x 10^-12) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01513082.

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

GCN Circular 45723

Subject
GRB 260924A: Fermi GRB Observation
Date
2026-09-24T13:56:57Z (5 days ago)
From
Padraig McDermott at University College Dublin <padraig.mcdermott@ucdconnect.ie>
Via
Web form
Pádraig McDermott (University College Dublin), and L. Scotton (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:

"At 01:13:23.56 UT on 24 September 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260924A (trigger 811905208/260924051), which was also detected by Swift-BAT (D'Elia et al. 2026, GCN 45713), Swift-XRT (Evans et al. 2026, GCN 45715), AstroSat CZTI (Salunke et al. 2026, GCN 45720), and Fermi-LAT (Holzmann Airasca et al. 2026, GCN 45721)

An optical counterpart is identified by the 1.5m OSN (Hu et al. 2026, GCN 45717), and NOT (Izzo et al. 2026, GCN 45718).

The Fermi GBM real-time localization (GCN 45712) is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 62 degrees.

The GBM light curve consists of three seperate emission episodes with a duration (T90) of about 337.9 s (50-300 keV). The time-averaged spectrum from T0-11.6 to T0+382.6 s is best fit by a Band function with Epeak = 645 +/- 4 keV, alpha = -0.91 +/- 0.01, and beta = -2.36 +/- 0.01.

The event fluence (10-1000 keV) in this time interval is (2.84 +/- 0.01)E-04 erg/cm^2. The 1-sec peak photon flux measured starting from T0+343.5 s in the 10-1000 keV band is 19.2 +/- 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 45721

Subject
GRB 260924A: Fermi-LAT detection of a bright burst
Date
2026-09-24T12:00:30Z (5 days ago)
From
Rahul Gupta at NASA GSFC <rahul.gupta@nasa.gov>
Via
Web form
A. Holzmann Airasca (UniTrento and INFN Bari), D. Depalo (Politecnico & INFN Bari), S. Cutini (INFN Perugia), Tamador Khalil (University of Johannesburg), and R. Gupta (NASA GSFC) report on behalf of the Fermi-LAT collaboration:


At 01:13:23 UT on September 24th, 2026, Fermi-LAT detected high-energy emission from GRB 260924A, which was also detected by Fermi-GBM (trigger 811905208/260924051, GCN #45712), Swift-BAT (GCN #45713), AstroSat CZTI (GCN #45720) and followed by Swift-XRT (GCN #45715), OSN telescope (GCN #45716), and NOT (GCN #45718).

The best LAT on-ground location is found to be

RA, Dec = 133.84, 12.01 (J2000)

with an error radius of 0.02 deg (90 % containment, statistical error only).

This was 62 deg from the LAT boresight at the time of the trigger.

The data from the Fermi-LAT show a significant increase in the event rate that is spatially and temporally correlated with the trigger with high significance.

The photon flux above 100 MeV in the time interval 0 - 1.1 ks after the GBM trigger is (8.3 ± 0.5)E-5 ph/cm2/s. The estimated photon index above 100 MeV is -2.2 ± 0.1. The highest-energy photon has an energy of 43.3 GeV and occurs at 77.5 s after the trigger time.

The Fermi-LAT point of contact for this burst is Sara Cutini (sara.cutini@pg.infn.it ).

The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.

GCN Circular 45720

Subject
GRB 260924A: AstroSat CZTI detection
Date
2026-09-24T11:11:28Z (5 days ago)
From
ABHINAV GOYAL at Indian Institute of Technology, Bombay <abhigoyale2003@gmail.com>
Via
Web form
S. Salunke(IUCAA), Harsha K. H. (IUCAA), A. Goyal (IITB), A. Arya (IITB), S. Puri (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 GRB 260924A which was also detected by Fermi (Fermi GBM team, GCN Circ. 45712), Swift BAT (D'Elia et. al, GCN Circ. 45713).

The source was detected in the CZT detectors in the 20-200 keV energy range. The light curve showed two distinct peaks of emission separated by ~ 170 s with the strongest peak at 2026-09-24 01:14:33.76 UTC. The measured peak count rate associated with the burst is 249 (+20, -7) counts/s above the background in the combined data of all quadrants, with a total of 20185 (+885, -1029) counts. The local mean background count rate was 250 (+1, -1) counts/s. We measure a T90 of 336 (+1, -7) s from the cumulative CZT light curve. 

The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve showed two distinct peaks of emission separated by ~ 170 s with the strongest peak at 2026-09-24 01:15:16.62 UTC. The measured peak count rate associated with the burst is 1019 (+33, -35) counts/s above the background in the combined data of all quadrants, with a total of 71944 (+1628, -1717) counts. The local mean background count rate was 642 (+2, -2) counts/s. We measure a T90 of 336 (+1, -1) s from the cumulative Veto light curve.

CZTI data products like interactive and downloadable light curves for this GRB can be found at: https://astrosat.iucaa.in/cift/cift_products/ET20260924T011328/ET20260924T011328_details.html

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 45718

Subject
GRB 260924A: NOT optical counterpart confirmation
Date
2026-09-24T07:35:16Z (5 days ago)
From
Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute <daniele.malesani@nbi.ku.dk>
Via
Web form
L. Izzo (INAF/OACN and DARK/NBI), J. An (NAOC), A. Saccardi (CEA/Irfu), G. Corcoran (UCD), S.Y. Fu (HUST), D. Xu (NAOC), D. B. Malesani (DAWN/NBI), A. S. Mårtensson (NOT), report on behalf of a larger collaboration:

We observed the field of the Swift/BAT GRB 260924A (D'Elia et al., GCN 45713) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC instrument. The target had only a short visibility window from La Palma. Observations started at 05:38:57 UT on September 24 2026 (about 4.43 hr after the Swift trigger), and consisted of 3x300 s in the SDSS-r and 5x200 s in the SDSS-z band. 

Within the enhanced Swift/XRT localisation region reported by Evans et al. (GCN 45715), we detect the source first noted by Hu et al. (GCN 45717), for which we report the following coordinates:

RA (J2000) = 08:55:03.10
Dec (J2000) = +12:02:35.42

with an estimated error of ~0.5".

In our stacked images we measure the following AB magnitudes:

r = 16.28 +/- 0.02
z = 16.00 +/- 0.02

These magnitudes are calibrated using nearby objects from the Pan-STARRS catalog and are not corrected for Galactic extinction.

GCN Circular 45717

Subject
GRB 260924A: detection of a bright optical afterglow by the 1.5m OSN telescope
Date
2026-09-24T07:10:50Z (5 days ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
email

Y.-D. Hu (GXI), A. Sota, E. Fernandez-Garcia, A. J. Castro-Tirado, I. Perez-Garcia, M.D. Caballero-Garcia  and M. Gritsevich (IAA-CSIC), on behalf of a larger collaboration, report:

Following the detection of GRB 260924A by both Fermi (Fermi team, GCN 45712) and Swift (D'Elia et al. GCN 45713), we triggered the 1.5m telescope at Sierra Nevada Observatory (OSN) in Granada (Spain), starting on Sep 24, 04:24 UT (~3.18 h post trigger) in the BVRI filters. An uncatalogued object is detected within the XRT error circle (Evans et al. GCNC 45715) with R=15.5+-0.1 mag (calibrated with USNO B2), which we propose to be the GRB 260924A optical afterglow. Spectroscopic observations are encouraged.

We thank the staff at OSN for their excellent support.

GCN Circular 45715

Subject
GRB 260924A: Enhanced Swift-XRT position
Date
2026-09-24T05:11:47Z (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 1346 s of XRT Photon Counting mode data and 4 UVOT
images for GRB 260924A, 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 = 133.76280, +12.04325 which is equivalent
to:

RA (J2000): 08h 55m 03.07s
Dec (J2000): +12d 02' 35.7"

with an uncertainty of 2.1 arcsec (radius, 90% confidence).

This position may be improved as more data are received. The latest
position can be viewed at https://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 45713

Subject
GRB 260924A: Swift detection of a burst
Date
2026-09-24T01:41:46Z (5 days ago)
From
Jimmy DeLaunay at PSU <jjd330@psu.edu>
Via
email
V. D'Elia (SSDC & INAF-OAR), J. J. DeLaunay (PSU),
R. Gupta (NASA GSFC), A. Y. Lien (U Tampa), K. L. Page (U Leicester),
D. M. Palmer (LANL), T. M. Parsotan (GSFC) and
B. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 01:13:28 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB  260924A (trigger=1513082).  Swift slewed immediately to the burst.
The BAT on-board calculated location is 
RA, Dec (133.740, +12.046)  which is 
   RA(J2000) = 08h 54m 58s
   Dec(J2000) = +12d 02' 44"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The available BAT light curve (cut off at T+8s
while still active) showed a complex structure with a duration of at least
20 sec.  The peak count rate was ~3700 counts/sec (15-350 keV), at ~0 sec
after the trigger.

The XRT began observing the field at 01:15:18.8 UT, 110.6 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 133.7620, 12.0455 which is equivalent to:
   RA(J2000)  = 08h 55m 2.88s
   Dec(J2000) = +12d 02' 43.8"
with an uncertainty of 5.1 arcseconds (radius, 90% containment). This
location is 77 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.

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 123 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image processing
FAILED because of no aspect solution. Results from the list of sources
generated on-board are not available at this time. No correction has been made
for the expected extinction corresponding to E(B-V) of 0.041.

Burst Advocate for this burst is V. D'Elia (valerio.delia AT ssdc.asi.it).
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 45712

Subject
GRB 260924A: Fermi GBM Final Real-time Localization
Date
2026-09-24T01:23:59Z (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 01:13:23 UT on 24 Sep 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260924A (trigger 811905208.561746 / 260924051).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 139.9, Dec = 9.7 (J2000 degrees, equivalent to J2000 09h 19m, 9d 41'), with a statistical uncertainty of 1.6 degrees.

The angle from the Fermi LAT boresight is 62.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260924051/quicklook/glg_skymap_all_bn260924051.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260924051/quicklook/glg_healpix_all_bn260924051.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260924051/quicklook/glg_lc_medres34_bn260924051.gif
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