GRB 251001B
GCN Circular 42102
Subject
GRB 251001B: NIR observations with WINTER
Date
2025-10-04T00:28:40Z (2 months ago)
From
Geoffrey Mo at Caltech / Carnegie Observatories <gmo@mit.edu>
Via
Web form
Geoffrey Mo (Caltech/Carnegie), Tomas Ahumada (Caltech), Viraj Karambelkar (Columbia/CCA), 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 251001B (Fermi GBM Team, GCN 42038; Beardmore et al., GCN 42039; Beardmore et al., GCN 42045; Neights et al., GCN 42047; Dichiara et al., GCN 42052; Krimm et al., GCN 42058) 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 2025-10-02T06:59:33 UTC in the J band (~16.7 hours after the GRB trigger), consisting of 15 x 120 s exposures. The images were processed using the WINTER data reduction pipeline implemented with mirar (https://github.com/winter-telescope/mirar, https://doi.org/10.5281/zenodo.13352565).
We do not detect a source at the optical counterpart location (Gompertz et al., GCN 42040; Liu et al., GCN42041; Strausbaugh et al., GCN 42043; O’Neill et al., GCN 42046; Lipunov et al., GCN 42048; De Ugarte Postigo et al., GCN 42050; Ma et al., GCN 42051; Sanchez-Ramirez et al., GCN 42053; Schneider et al., GCN 42054; Breeveld et al., GCN 42055; Pankov et al., GCN 42083; Becerra et al., GCN 42097). We obtain the following 5-sigma upper limit: J ~ 19.3 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 42097
Subject
GRB 251001B: COLIBRÍ further observations and evidence of a multi-wavelength shallow decay
Date
2025-10-03T17:59:09Z (2 months ago)
From
Rosa L. Becerra at Tor Vergata, Roma <rosa.becerra@roma2.infn.it>
Via
Web form
Rosa L. Becerra (U Roma), Benjamin Schneider (LAM), Camila Angulo (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), and Antonio de Ugarte Postigo (LAM):
We reobserved the field of the GRB 251001B (Fermi GBM team, GCN Circ. 42038; Beardmore et al. GCN Circ. 42039) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-03 12:00 to 12:19 UTC (from 45.7 to 46.0 hours after the trigger) and obtained 16 minutes of simultaneous exposure in the r and z filters.
The data were reduced and coadded with custom software 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 previously reported by Gompertz et al. (GCN Circ. 42040), An et al. (GCN Circ. 42041), Strausbaugh & Cucchiara (GCN Circ. 42043), O'Neill et al. (GCN Circ. 42046), de Ugarte Postigo et al. (GCN Circ. 42050), Ma et al. (GCN Circ. 42051), Sanchez-Ramirez et al. (GCN Circ. 42053), Schneider et al. (GCN Circ. 42054), and Breeveld et al. (GCN Circ. 42055) at preliminary magnitudes of:
r = 22.28 +/- 0.14.
z = 22.08 +/- 0.41 (with a 3-sigma detection limit of z = 21.85).
By comparing the optical data reported by the aforementioned facilities and COLIBRÍ, we estimate a spectral index of approximately -0.5. This shallow decay is consistent with the temporal evolution observed in the X-ray band after 10ks from the trigger (https://www.swift.ac.uk/xrt_curves/01400467/).
Further observations are planned.
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 42083
Subject
GRB 251001B: AbAO optical upper limit
Date
2025-10-03T09:51:25Z (2 months ago)
From
Nicolai Pankov at HSE, IKI RAS <colinsergesen@gmail.com>
Via
Web form
N. Pankov (HSE, IKI), A. Pozanenko (IKI), R. Ya. Inasaridze (AbAO) report on behalf of IKI-GRB-FuN:
We performed optical observations of the field of GRB 251001B detected by Fermi/GBM (The Fermi GBM team et. al, GCN 42038), and Swift/BAT (Beardmore et. al, GCN 42039) with the AS-32 0.7m telescope of the Abastumani Observatory (AbAO). The R-filter observations began on 2025-10-01 22:37 UT, i.e. ~0.35 days since trigger and consisted of 55x30 s exposures. The optical source at z = 2.162 (Sanchez-Ramirez et. al, GCN 42053) found by GOTO (Gompertz et. al, GCN 42040) and also observed by (An et. al, GCN 42041; Strausbaugh & Cucchiara, GCN 42043; O'Neill et. al, GCN 42046; de Ugarte Postigo et. al, GCN 42050; Ma et. al, GCN 42051; Sanchez-Ramirez et. al, GCN 42053; Schneider et. al, GCN 42054; Breeveld et. al, GCN 42055) is not detected in the co-add image of 51x30 s. Preliminary photometry and observation details are presented below:
Date UT start t-T0 Exp. Filter OT Err. UL
(mid, days) (s) (3sigma)
2025-10-01 22:37:48 0.35641 51x30 R n/d n/d 19.9
The photometry was calibrated using nearby stars from USNO-B1.0 (R2 magnitudes) and not corrected for the Galactic extinction.
Ref. stars
USNO-B1.0
RA Dec R2
42.63445 -23.02161 14.13
42.77496 -23.05166 13.47
GCN Circular 42058
Subject
GRB 251001B: Swift-BAT refined analysis
Date
2025-10-02T20:10:01Z (2 months ago)
From
Mike Moss at NASA GSFC <mikejmoss3@gmail.com>
Via
Web form
H. A. Krimm (NSF), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), R. Gupta (GSFC),
S. Laha (GSFC/UMBC), A. Y. Lien (U Tampa),
C. B. Markwardt (GSFC), M. J. Moss (GSFC),
D. M. Palmer (LANL), T. Parsotan (GSFC),
D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU)
(i.e. the Swift-BAT team):
Using the data set from T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 251001B (trigger #1400467)
(Beardmore, et al., GCN Circ. 42039). The BAT ground-calculated position is
RA, Dec = 42.668, -22.990 deg which is
RA(J2000) = 02h 50m 40.2s
Dec(J2000) = -22d 59' 23.9"
with an uncertainty of 1.1 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 91%.
The mask-weighted light curve shows a single FRED pulse from ~T0-5 s to ~T0+20 s.
Statistically significant emission is found to occur out to ~T0+90 s.
The T90 (15-350 keV) is 75.33 +- 46.90 sec (estimated error including systematics).
The time-averaged spectrum from T-9.98 to T+91.72 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.49 +- 0.14. The fluence in the 15-150 keV band is 1.1 +- 0.1 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+2.29 sec in the 15-150 keV band
is 0.8 +- 0.1 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/1400467
GCN Circular 42055
Subject
GRB 251001B: Swift/UVOT Detection
Date
2025-10-02T12:12:59Z (2 months ago)
From
Alice Breeveld at MSSL-UCL <a.breeveld@ucl.ac.uk>
Via
email
A.A. Breeveld (UCL-MSSL), S.P.R. Shilling (Lancaster U.) and A.P. Beardmore (U. Leicester) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 251001B 119 s after the BAT trigger (Beardmore et al., GCN Circ. 42039). A source consistent with the enhanced XRT position (Beardmore et al., GCN Circ. 42045) and also with the optical counterpart seen by Gompertz et al. (GCN Circ. 42040), An et al. (GCN Circ. 2041), Strausbaugh and Cucchiara (GCN Circ. 42043) and de Ugarte Postigo et al. (GCN Circ. 42050) is detected in the initial UVOT exposures.
Preliminary detections using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first finding chart (FC) exposures and subsequent exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
white_FC 119 269 147 18.76 +/- 0.06
u_FC 331 581 246 18.86 +/- 0.15
white 611 630 10 19.0 +/- 0.25
white 861 1010 147 19.6 +/- 0.2
b 587 779 39 > 19.0
v 661 681 20 > 17.4
The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.023 in the direction of the burst (Schlegel et al. 1998).
GCN Circular 42054
Subject
GRB 251001B: COLIBRÍ optical observations
Date
2025-10-02T10:03:11Z (2 months ago)
From
Benjamin Schneider at MIT <bschn@mit.edu>
Via
Web form
Benjamin Schneider (LAM), Camila Angulo (UNAM), Rosa L. Becerra (U Roma), Antonio de Ugarte Postigo (LAM), Jean-Grégoire Ducoin (CPPM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Alan M. Watson (UNAM), Jean-Luc Atteia (IRAP), Dalya Akl (NYUAD), Sarah Antier (OCA), Nathaniel R. Butler (ASU) , Damien Dornic (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), and Fredd Sánchez Álvarez (UNAM):
We imaged the field of the GRB 251001B (Fermi GBM team, GCN Circ. 42038; Beardmore et al. GCN Circ. 42039) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-10-02T06:13:31 to 08:49:34 UTC (from 15.94 to 18.54 hours after the trigger) and obtained 64 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 previously reported by Gompertz et al. GCN Circ. 42040; An et al. GCN Circ. 42041; Strausbaugh & Cucchiara GCN Circ. 42043; O'Neill et al. GCN Circ. 42046; A. de Ugarte Postigo et al., GCN Circ. 42050; Ma et al., GCN Circ. 42051 and Sanchez-Ramirez et al., GCN 42053 at preliminary magnitudes of:
r = 21.56 +/- 0.06
z = 21.28 +/- 0.13
Our measured r-band magnitude is consistent with the magnitude reported by the NOT (Ugarte Postigo et al., GCN Circ. 42050), indicating a possible flattening in the optical afterglow.
Further observations and analysis are ongoing in g, r, i, z and y filters.
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 42053
Subject
GRB 251001B: GTC/OSIRIS+ spectroscopic redshift z = 2.162
Date
2025-10-02T09:33:41Z (2 months ago)
From
Ruben Sanchez-Ramirez at IAA-CSIC <ruben@iaa.es>
Via
Web form
R. Sanchez-Ramirez (IAA-CSIC), Y.-D. Hu (GXI), A. J. Castro-Tirado, S. Guziy, M.D. Caballero-Garcia, S.-Y. Wu and I. Perez-Garcia (IAA-CSIC), R. Scarpa (GTC, IAC), D. Gonzalez (GTC), A. Cabrera-Lavers (GTC, IAC), S. B. Pandey (ARIES), M. Gritsevich (Univ. of Helsinki), J. Becerra-Gonzalez (IAC), L. Piro (INAF/IAPS) and B.-B. Zhang (NJU), on behalf of a larger collaboration, report:
Following the detection of GRB 251001B by Fermi (Fermi Team, GCN 42038, Neights et al. GCN 42047) and Swift (Beardmore et al. GCN 42039), we observed the optical afterglow (Gompertz et al. GCN 42040, An et al. GCN 42041, Strausbaugh et al. GCN 42043, O'Neill et al. GCN 42046, de Ugarte Postigo et al., GCN 42050, Ma et al., GCN 42051) 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 high airmass and consisted of 3x900s exposures using grism R1000B, with a spectral coverage between 3,600 and 7,700 A (R~600). The observations started on Oct 2, 02:39 UT (i.e. 12.35 h after the burst trigger).
From a preliminary reduction, we find a strong DLA at ~4000A, as well as several metal absorption lines that we interpret as coming from NV, SiII, SiII*, OI, CII, SiIV, CIV, FeII, FeII*, AlII, AlIII, ZnII, CrII, all at z=2.162. The detection of SiII* and FeII* links this system to the GRB. We also note the presence of Lya in emission at the same redshift.
GCN Circular 42052
Subject
GRB 251001B: Swift-XRT refined Analysis
Date
2025-10-02T07:52:16Z (2 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
S. Dichiara (PSU), J.A. Kennea (PSU), D.N. Burrows (PSU), K.L. Page (U.
Leicester), A.P. Beardmore (U. Leicester), T. Sbarrato (INAF-OAB), P.
D'Avanzo (INAF-OAB), M.G. Bernardini (INAF-OAB) and P.A. Evans report
on behalf of the Swift-XRT team:
We have analysed 7.8 ks of XRT data for GRB 251001B, from 99 s to 40.1
ks after the trigger. The data comprise 540 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 light curve can be modelled with a power-law decay with a decay
index of alpha=1.34 (+/-0.06).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.92 (+0.08, -0.07). The
best-fitting absorption column is 8.8 (+2.0, -1.9) x 10^20 cm^-2, in
excess of the Galactic value of 2.6 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.74 (+0.19, -0.18)
and a best-fitting absorption column of 8.6 (+5.7, -4.8) x 10^20 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor
deduced from this spectrum is 3.9 x 10^-11 (4.5 x 10^-11) erg cm^-2
count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 8.6 (+5.7, -4.8) x 10^20 cm^-2
Galactic foreground: 2.6 x 10^20 cm^-2
Excess significance: 2.1 sigma
Photon index: 1.74 (+0.19, -0.18)
If the light curve continues to decay with a power-law decay index of
1.34, the count rate at T+24 hours will be 9.6 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.8 x
10^-14 (4.3 x 10^-14) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/01400467.
This circular is an official product of the Swift-XRT team.
GCN Circular 42051
Subject
GRB 251001B: SVOM/VT optical observation
Date
2025-10-02T07:32:26Z (2 months ago)
From
Yinuo Ma <mayn@bao.ac.cn>
Via
Web form
Y. N. Ma, L. P. Xin, Z. H. Yao, Y. L. Qiu, C. Wu, H. L. Li, X. H. Han, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM/VT performed a Target of Opportunity observation of GRB 251001B detected by Fermi/GBM and Swift/BAT (Fermi GBM team, GCN 42038; Beardmore et al., GCN 42039; Beardmore et al., GCN 42045; Neights & Meegan, GCN 42047). SVOM/VT began observing the field at 2025-10-01T15:03:04 UTC, 0.762 hours after the trigger, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
With X-band data available, the optical counterpart (Gompertz et al., GCN 42040; An et al., GCN 42041; Strausbaugh & Cucchiara, GCN 42043; O'Neill et al., GCN 42046; Lipunov et al., GCN 42048; de Ugarte Postigo et al., GCN 42050) was clearly detected in both VT_B and VT_R bands. The magnitudes are:
mid time (h) | exposure time (s) | band | mag (AB) | mag err
-------------|-------------------|------|----------|--------
3.790 | 38*70 | VT_B | 22.06 | 0.09
3.819 | 35*70 | VT_R | 21.31 | 0.07
Our photometry was not corrected for Galactic extinction.
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. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.
GCN Circular 42050
Subject
GRB 251001B: NOT optical observations
Date
2025-10-02T07:25:01Z (2 months ago)
From
Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com>
Via
email
A. de Ugarte Postigo (LAM), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), B. Schneider (LAM), B. P. Gompertz (Birmingham), A. Martin-Carrillo (UCD) and J. Maiz Apellaniz (IAA-CSIC) report:
We observed the field of GRB 251001B (Fermi GBM team, GCN 42038; Beardmore et al. GCN 42039) using the 2.5 m Nordic Optical Telescope (NOT) equipped with ALFOSC. Observations started on 2 October 2025 at 02:37:23 UT (12.33 hr after the trigger) and consisted of 3x300 s and 5x200 s in the SDSS r and z bands, respectively.
The combined images reveal the presence of the afterglow identified by Gompertz et al. (GCN 42040; An et al. GCN 42041; Strausbaugh & Cucchiara GCN 42043; O'Neill et al. GCN 42046) at a magnitude of r = 21.54 +/- 0.06 (AB) calibrated against field stars of the Pan-STARRS catalogue and not corrected by Galactic extinction.
GCN Circular 42048
Subject
Swift GRB 251001B: Global MASTER-Net observations report
Date
2025-10-02T00:16:59Z (2 months 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) was pointed to the Swift GRB 251001B ( A. P. Beardmore et al., GCN 42039) errorbox 34585 sec after notice time and 35427 sec after trigger time at 2025-10-02 00:07:47 UT, with upper limit up to 18.4 mag. The observations began at zenith distance = 67 deg. The sun altitude is -32.7 deg.
The galactic latitude b = -62 deg., longitude l = 211 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3004463
We obtain a following upper limits.
Tmid-T0 | Site |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________
35517 | MASTER-Kislovodsk | C | 180 | 18.4 |
35517 | MASTER-Kislovodsk | C | 180 | 18.2 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 42047
Subject
GRB 251001B: Fermi GBM Observation
Date
2025-10-01T21:33:18Z (2 months ago)
From
eliza.neights@gmail.com
Via
Web form
Eliza Neights (GWU, NASA GSFC) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 14:17:19.05 UT on 01 October 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251001B (trigger 781021044/251001595)
which was also detected by Swift BAT (A. P. Beardmore et al. 2025, GCN 42039).
The Fermi GBM on-ground location is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 53 degrees.
The GBM light curve consists of a single emission episode with a duration (T90)
of about 19 s (50-300 keV). The time-averaged spectrum
from T0-3.7 to T0+16.7 s is best fit by
a Comptonized function with index 0.22 +/- 0.03 and peak energy 107 +/- 5 keV.
The event fluence (10-1000 keV) in this time interval is
(1.3 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+0.7 s in the 10-1000 keV band is 1.4 +/- 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 42046
Subject
GRB 251001B: Further GOTO observations of optical counterpart.
Date
2025-10-01T20:34:36Z (2 months ago)
From
d.s.oneill@bham.ac.uk
Via
Web form
D. O'Neill, B. P. Gompertz, G. Ramsay, R. Starling, M. Kennedy, B. Godson, J. Lyman, K. Ackley, M. J. Dyer, K. Ulaczyk, A. Kumar,, 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 new observations with the Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022, Dyer et al. 2024) in response to GRB 251001B (Fermi GBM team, GCN 42038; Beardmore et al. GCN 42039).
Following the detection of the optical counterpart of GRB 251001B (Gompertz et al. GCN 42040, An et al. GCN 42041, Strausbaugh & Cucchiara, GCN 42043) we detect the counterpart again in an additional epoch of GOTO L-band (400-700 nm) imaging taken at 2025-10-01 15:44:25 UT (1.45h post-trigger) consisting of 4x90s exposures. We find the counterpart has faded to L=20.6±0.3 AB mag, consistent with a power law decay with index: alpha = -0.75.
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 42045
Subject
GRB 251001B: Enhanced Swift-XRT position
Date
2025-10-01T18:10:10Z (2 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
A.P. Beardmore, P.A. Evans, M.R. Goad and J.P. Osborne (U. Leicester)
report on behalf of the Swift-XRT team.
Using 700 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 251001B, 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 = 42.65089, -23.02048 which is equivalent
to:
RA (J2000): 02h 50m 36.21s
Dec (J2000): -23d 01' 13.7"
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 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 42043
Subject
GRB 251001B: LCOGT Optical Observations
Date
2025-10-01T15:44:16Z (2 months 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 251001B field (Beardmore et al., GCN 42039) with the LCOGT 1-meter Sinistro instrument at the Siding Spring Observatory, Australia site, on October 1, from 14:51 to 15:23 UT (corresponding to 0.57 to 1.10 hours after the GRB trigger time) with the SDSS i and r filters.
We performed a series of 3x300s exposures in i-band and r-band. We detect an uncatalogued source consistent with other optical counterpart observations (Gompertz et al., GCN 42040; An et al., GCN 42041) in both bands. We note that our r-band measurement implies a rapidly fading source.
The following magnitudes are calculated using the PanSTARRS catalog as reference:
r = 20.26 +/- 0.04
i = 19.81 +/- 0.03
These magnitudes are not corrected for galactic extinction.
GCN Circular 42041
Subject
GRB 251001B: TRT optical afterglow detection and potential host galaxy
Date
2025-10-01T15:17:04Z (2 months ago)
From
sqjiang at NAOC <sqjiang@bao.ac.cn>
Via
Web form
J. An, X. Liu (NAOC), K. Noysena, K. Chanchaiworawit, S. Tinyanont (NARIT), S.Q. Jiang, L.B. He, Z.P. Zhu, D. Xu (NAOC), S.Y. Fu (HUST) report on behalf of a large collaboration:
We observed the field of GRB 251001B detected by Swift/BAT (Beardmore et al., GCN 42039), using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO). A series of images were obtained in the R band.
A varying optical transient (OT) is detected within the Swift/XRT error circle at coordinates
R.A. (J2000) = 2:50:36.33
Dec.(J2000) = -23:01:11.96
with an uncertainty of ~ 0.6 arcsec. It has R ~ 18.9 mag at 21.1 min after the Swift/BAT trigger. We thus think the OT is the optical afterglow of the burst.
We also note that there is a galaxy at the OT position from Legacy Survey DR10 with g ~ 25.16, r ~ 24.93, i ~25.06, z ~24.79, and a phot_z ~ 1.14 +/- 0.53, which is presumably the host galaxy of the burst.
GCN Circular 42040
Subject
GRB 251001B: GOTO optical counterpart detection
Date
2025-10-01T15:09:24Z (2 months ago)
From
Ben Gompertz at U of Birmingham <b.gompertz@bham.ac.uk>
Via
email
B. P. Gompertz, D. O'Neill, R. Starling, M. Kennedy, B. Godson, J. Lyman, K. Ackley, M. J. Dyer, K. Ulaczyk, A. Kumar, G. Ramsay, 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 251001B (Fermi GBM team, GCN 42038; Beardmore et al. GCN 42039). Targeted observations were performed at 2025-10-01 14:33:37 (starting 16.3 minutes 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.
At a position consistent with the Swift/XRT localisation, we detect a new optical source, GOTO25icy, with L = 19.33 ± 0.12 AB mag, at coordinates:
RA,DEC (J2000) = 42.651328, -23.02001 | 02:50:36.32, -23:01:12.03
We find no evidence of this source prior to the GRB trigger time in the previous GOTO observation taken at 2025-10-01 14:01:01 UT (16.3 min pre-trigger) down to a 3-sigma depth of L > 20.1 AB mag. We note the source is coincident (~0.4”) with a faint g~25 AB mag source in Legacy Survey DR10, which may be the host galaxy of the GRB.
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 42039
Subject
GRB 251001B: Swift detection of a burst
Date
2025-10-01T14:44:42Z (2 months ago)
From
K.L. Page at U Leicester <klp5@leicester.ac.uk>
Via
email
A. P. Beardmore (U Leicester), J. A. Kennea (PSU), M. J. Moss (GSFC)
and K. L. Page (U Leicester) report on behalf of the Neil Gehrels
Swift Observatory Team:
At 14:17:20 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251001B (trigger=1400467). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 42.665, -23.020 which is
RA(J2000) = 02h 50m 40s
Dec(J2000) = -23d 01' 09"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a single-peaked
structure with a duration of about 25 sec. The peak count rate
was ~2,000 counts/sec (15-350 keV), at ~4 sec after the trigger.
The XRT began observing the field at 14:19:17.0 UT, 116.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 42.65098, -23.02014
which is equivalent to:
RA(J2000) = 02h 50m 36.24s
Dec(J2000) = -23d 01' 12.5"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 46 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.65
x 10^20 cm^-2 (Willingale et al. 2013).
UVOT took a finding chart exposure of nominal 150.000 seconds with the White
filter starting 860 seconds after the BAT trigger. No credible afterglow
candidate has been found in the initial data products. Data from the 2.7'x2.7'
sub-image are not available at this time. The 8'x8' region for the list of
sources generated on-board covers 100% of the XRT error circle. The list of
sources is typically complete to about 18 mag. No correction has been made for
the expected extinction corresponding to E(B-V) of 0.023.
Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk).
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 42038
Subject
GRB 251001B: Fermi GBM Final Real-time Localization
Date
2025-10-01T14:28:02Z (2 months ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB
At 14:17:19 UT on 1 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251001B (trigger 781021044.051374 / 251001595).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 52.0, Dec = -11.2 (J2000 degrees, equivalent to J2000 03h 28m, -11d 11'), with a statistical uncertainty of 5.6 degrees.
The angle from the Fermi LAT boresight is 67.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251001595/quicklook/glg_skymap_all_bn251001595.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/bn251001595/quicklook/glg_healpix_all_bn251001595.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251001595/quicklook/glg_lc_medres34_bn251001595.gif