GRB 251026B
GCN Circular 42519
Subject
GRB 251026B: SOAR Classification of GOTO25jgl/SN2025absf as a Type Ia supernova
Date
2025-10-29T20:04:03Z (6 days ago)
From
jcarney@unc.edu
Via
Web form
Jonathan Carney (UNC), Xander J. Hall (CMU), Igor Andreoni (UNC), Brendan O’Connor (CMU), Malte Busmann (LMU), Daniel Gruen (LMU), and Antonella Palmese (CMU) report:
We observed the source SN 2025absf reported by Belkin et al. (GCN 42470), a possible counterpart to short GRB 251026B (GCN 42455), with the Goodman High Throughput Spectrograph (GHTS; Clemens 2004) on the Southern Astrophysical Telescope (SOAR). We used a grating of 400 lines/mm and a 1.0'' wide slit mask in the M1 spectroscopic setup (hereafter 400M1) with 2x2 binning using the GHTS Red Camera. The 400M1 spectra cover a wavelength range of 3800 -- 7040 Å. The spectra were reduced using Pypeit (Prochaska 2020), using arcs taken immediately after target observation and calibration images from the same night. Flux calibration was performed using observations of HR7596 taken on the same night as the observations with an identical 400M1 setup and 2x2 binning.
Using SNID (Blondin et al. 2011) and NGSF (AstroNote 2022-191), we find good template matches to a Ia-norm SN at z ~ 0.044. Thus, we classify SN 2025absf as a type Ia supernova. We therefore conclude that SN 2025absf is not the optical counterpart to GRB 251026B.
We thank the CTIO and NOIRLab staff for supporting these observations.
GCN Circular 42517
Subject
GRB 251026B: LBT near-infrared observations
Date
2025-10-29T17:44:10Z (6 days ago)
From
Yu-Han Yang at University of Rome Tor Vergata <yyang@roma2.infn.it>
Via
Web form
Yu-Han Yang, Eleonora Troja, Massine El Kabir, Muskan Yadav (U Rome) report on behalf of the ERC BHianca team:
Following the detection of GRB 251026B by Fermi/GBM (Dafčíková et al., GCN 42455), we observed the candidate optical counterpart GOTO25jgl/AT2025absf (Belkin et al., GCN 42470) in the J filter with the LUCI near-infrared imager mounted on the Large Binocular Telescope (LBT). Observations started at 2025-10-29T11:28:17 (~ 3.2 d after the GBM trigger) with an average airmass of 1.5.
We detect the source previously reported as a candidate from GOTO (Belkin et al., GCN 42470) with a preliminary magnitude of J ~ 19.2 AB mag, calibrated using nearby stars from the 2MASS catalog (Skrutskie et al. 2006), uncorrected for Galactic extinction. Our measurement is consistent with the near-infrared upper limit derived by PRIME (Kabir et al. GCN 42501).
Compared to the predicted optical magnitude L~19.3 at the same epoch using the magnitudes and decay trend reported by GOTO (Belkin et al., GCN 42470), we find no evidence for a red color excess. Based on the lack of X-ray emission (Osborne et al. GCN 42491) and blue color, the properties of GOTO25jgl do not appear consistent with standard afterglow or kilonova emission, therefore its association with GRB 251026B cannot be confirmed.
We acknowledge excellent support from the LBTO and LBT-INAF staff, particularly Ester Marini.
GCN Circular 42513
Subject
GRB 251026B: Fermi GBM Observation
Date
2025-10-29T14:40:22Z (7 days ago)
From
Marianna Dafčíková at Masaryk University <500025@mail.muni.cz>
Via
Web form
M. Dafcikova (MUNI) reports on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 07:18:53.33 UT on 26 October 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251026B (trigger 783155938/251026305).
The angle from the Fermi LAT boresight is 19 degrees.
The GBM light curve consists of a single emission episode with a duration (T90)
of about 6 s (50-300 keV). The time-averaged spectrum
from T0-1.3 to T0+3.7 s is best fit by
a simple power law function with index -1.38 +/- 0.04.
The event fluence (10-1000 keV) in this time interval is
(7.2 +/- 0.7)E-07 erg/cm^2. The 1-sec peak photon flux measured
starting from T0-0.32 s in the 10-1000 keV band is 1.6 +/- 0.2 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 42501
Subject
GRB251026B: PRIME H-band upper limit
Date
2025-10-28T10:04:28Z (8 days ago)
From
Massine El Kabir <m.elkabir@campus.unimib.it>
Via
Web form
M. El Kabir (U Rome), N. Passaleva (U Rome), J. Durbak (UMD), O. Guiffreda (UMD), E. Troja (U Rome), A. S. Kutyrev (NASA/GSFC), S. B. Cenko (NASA/GSFC)
Following the detection of a candidate optical counterpart (Belkin et al., GCN 42470), we observed the field of GOTO GOTO25jgl/AT2025absf in the H filter with PRIME. Observations started ~ 2 d after the initial Fermi trigger (Dafčíková et al., GCN 42455).
At the GOTO position (Belkin et al., GCN 42470), no obvious counterpart is detected on the top of the bright nearby galaxy. Due to the bright galaxy background, it is hard to assess the presence of a faint transient source without a template. Using GALFIT, we modelled the galaxy with a single Sersic profile and, after subtraction, derive a 3-sigma upper limit of H > 19 AB mag
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 42491
Subject
GRB 251026B: Swift-XRT observations of GOTO25jgl/AT2025absf
Date
2025-10-27T20:04:15Z (8 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
J.P. Osborne (U. Leicester), C. Salvaggio (INAF-OAB), M. Ferro
(INAF-OAB), R. Brivio (INAF-OAB), D.N. Burrows (PSU), S. Lanava (PSU),
S. Dichiara (PSU), K.L. Page (U. Leicester) and P.A. Evans (U.
Leicester) reports on behalf of the Swift-XRT team:
Swift-XRT has performed follow-up observations of the
Fermi/GBM-detected burst GRB 251026B, collecting 2.0 ks of Photon
Counting (PC) mode data between T0+116.7 ks and T0+122.7 ks, centred on
the tentative GOTO optical counterpart candidate (GCN Circ. 42470).
No X-ray sources have been detected consistent with the estimated
3-sigma GOTO error region (0.8 arcsec). The 3-sigma upper limit in the
field ranges from ~0.004 to ~0.005 ct s^-1, corresponding to a 0.3-10
keV observed flux of 1.7e-13 to 2.1e-13 erg cm^-2 s^-1 (assuming a
typical GRB spectrum).
The results of the XRT-team automatic analysis of the XRT observations,
including a position-specific upper limit calculator, are available at
https://www.swift.ac.uk/ToO_GRBs/00021872.
This circular is an official product of the Swift-XRT team.
GCN Circular 42483
Subject
GRB 251026B: Swift ToO observations
Date
2025-10-27T15:40:49Z (9 days ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
P. A. Evans (U. Leicester) reports on behalf of the Swift team:
Swift has initiated a ToO observation of the Fermi/GBM-detected event
GRB 251026B. Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021872
Any uncatalogued X-ray sources detected in this analysis will be
reported on this website and via GCN COUNTERPART notices. These are
not necessarily related to the Fermi/GBM event. Any X-ray source
considered to be a probable afterglow candidate will be reported via a
GCN Circular after manual consideration.
Details of the XRT automated analysis methods are detailed in Evans et
al. (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8).
This circular is an official product of the Swift-XRT team.
GCN Circular 42470
Subject
GRB 251026B: Tentative GOTO optical counterpart candidate
Date
2025-10-27T10:39:39Z (9 days ago)
From
Sergey Belkin at Monash University <sergey.belkin@monash.edu>
Via
Web form
S. Belkin, G. Ramsay, B. P. Gompertz, D. O’Neill, M. Wortley, R. Starling K. Ackley, M. Dyer, J. Lyman, K. Ulaczyk, D. Steeghs, D. Galloway, V. Dhillon, P. O'Brien, K. Noysena, R. Kotak, R. Breton, J. Casares, L. Nuttall, B. Godson, T. Killestein, A. Kumar, M. Pursiainen report on behalf of 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 the short GRB 251026B (Fermi GBM team, GCN 42455). Targeted observations were performed beginning at 2025-10-26 17:35:03 UT, (+10.3h post trigger) and continued through to 2025-10-27 05:29:04 UT (+22.2h post trigger). 88 images were taken, across 8 unique pointings, covering 417.8 square degrees within the 90% localisation contour. ~75.1% of the total 2D localisation probability was covered, with an average 5-sigma depth of 20.1 mag. 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.
We detect a new optical source, GOTO25jgl/AT2025absf, with coordinates:
RA,DEC (J2000) = 133.2284, -6.25815 | 08:52:54.8, -06:15:29.4
This position lies on the 38% probability contour of the Fermi/GBM localisation. The source was initially detected with AB magnitude L = 18.45 ± 0.10 mag (+10.3h), before fading to L = 18.80 ± 0.06 mag (+22.2 h). Forced photometry on the difference images indicates excess flux corresponding to L = 19.06 ± 0.28 AB mag at 2025-10-23 18:09:10 (−2.55 d), but visual inspection does not reveal an obvious point source, meaning the marginally significant residual may be due to an imperfect subtraction. Earlier coverage at 2025-10-23 04:48:34 (−3.10 d) yields a 3-sigma upper limit of L > 20.50 AB. The source is coincident with the galaxy LEDA 1034956 with a redshift of z=0.055 (247.26 ± 17.34 Mpc).
The two post-trigger detections are consistent with a power-law decay with index: alpha ~0.41. The temporal coincidence and fast decay suggest that GOTO25jgl could be the optical counterpart to GRB 251026B. However, given the possible excess flux observed pre-trigger, we can not definitively rule out that it is an unrelated, fast-evolving transient. Further follow-up to determine the nature of the source is encouraged.
Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and are not corrected for Galactic extinction.
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 42464
Subject
Fermi GRB 251026B: Global MASTER-Net observations report
Date
2025-10-27T01:31:39Z (9 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-Kislovodsk robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) started inspect of the Fermi GRB 251026B ( Fermi GBM team, GCN 42455) errorbox 59309 sec after notice time and 62633 sec after trigger time at 2025-10-27 00:42:47 UT, with upper limit up to 18.4 mag. The observations began at zenith distance = 68 deg. The sun altitude is -32.5 deg.
The galactic latitude b = 20 deg., longitude l = 239 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3023975
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
62664 | 2025-10-27 00:42:47 | MASTER-Kislovodsk | (08h 46m 04.47s , -11d 51m 14.2s) | C | 60 | 17.9 |
62664 | 2025-10-27 00:42:47 | MASTER-Kislovodsk | (08h 50m 15.56s , -12d 14m 29.7s) | C | 60 | 18.3 |
63317 | 2025-10-27 00:53:40 | MASTER-Kislovodsk | (08h 46m 04.23s , -11d 50m 49.0s) | C | 60 | 17.8 |
63317 | 2025-10-27 00:53:40 | MASTER-Kislovodsk | (08h 50m 17.20s , -12d 14m 14.5s) | C | 60 | 18.1 |
63959 | 2025-10-27 01:04:22 | MASTER-Kislovodsk | (08h 46m 02.23s , -11d 50m 25.1s) | C | 60 | 18.2 |
63959 | 2025-10-27 01:04:22 | MASTER-Kislovodsk | (08h 50m 16.29s , -12d 13m 53.6s) | C | 60 | 18.4 |
64626 | 2025-10-27 01:15:29 | MASTER-Kislovodsk | (08h 46m 09.08s , -11d 49m 58.6s) | C | 60 | 18.2 |
64626 | 2025-10-27 01:15:29 | MASTER-Kislovodsk | (08h 50m 18.58s , -12d 13m 27.0s) | C | 60 | 18.4 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 42455
Subject
GRB 251026B: Fermi GBM Final Localization
Date
2025-10-26T09:48:27Z (10 days ago)
From
Marianna Dafčíková at Masaryk University <500025@mail.muni.cz>
Via
Web form
The Fermi GBM team reports the detection of a likely SHORT GRB
"At 07:18:53.33 UT on 26 October 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251026B (trigger 783155938/251026305).
The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 132.32, Dec = -12.21 (J2000 degrees, equivalent to J2000 8h 49m, -12d 12'),
with a statistical uncertainty of 8.04 degrees.
The angle from the Fermi LAT boresight is 19 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251026305/quicklook/glg_skymap_all_bn251026305.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/bn251026305/quicklook/glg_healpix_all_bn251026305.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251026305/quicklook/glg_lc_medres34_bn251026305.gif"