GRB 251201B
GCN Circular 42965
Subject
GRB 251201B: Fermi GBM Observation
Date
2025-12-02T20:57:29Z (3 hours ago)
From
Eva Palafox at INAOE <eva.palafox@gmail.com>
Via
Web form
Eva Palafox (INAOE) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 16:26:24.22 UT on 01 December 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251201B (trigger 786299189/251201685),
which was also detected by Swift BAT (M. J. Moss et al. 2025, GCN 42924), and Swift XRT (J.P. Osborne et al. 2025, GCN 42927).
The Fermi GBM on-ground location (GCN 42923) is consistent with the Swift BAT position.
The angle from the Fermi LAT boresight is 119 degrees.
The GBM light curve consists of a single emission episode with a duration (T90)
of about 91 s (50-300 keV).
The time-averaged spectrum from T0-22.5 to T0+105.5 s is best fit by
a simple power law function with index -1.83 +/- 0.04.
A power law function with an exponential high-energy cutoff fits equally well with power
law index = -1.71 +/- 0.08 and Epeak = 269.8 +/- 166.0.
The event fluence (10-1000 keV) in this time interval is
(1.2 +/- 0.1)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.7 s in the 10-1000 keV band is 4 +/- 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 42955
Subject
GRB 251201B: Kilonova-Catcher optical afterglow detection
Date
2025-12-02T16:14:17Z (8 hours ago)
From
Damien Turpin at CEA-Saclay <dturpin-astro@hotmail.com>
Via
Web form
M. Freeberg (KNC), D. Turpin (CEA-Saclay/Irfu), C. Andrade(UMN), S. Antier (OCA/IJCLab), M. Coughlin (UMN), S. Karpov (FZU), P. Hello (IJCLAB), M. Pillas (IAP) on behalf of the GRANDMA/Kilonova-Catcher collaboration:
We observed the field of GRB 251201B detected by Swift/BAT (Moss et al., GCN 42924) and Fermi/GBM (Fermi GBM team, GCN 42923) with the GRANDMA citizen science project Kilonova-catcher (KNC). Our observations were performed with the iTelescope 72 located in Chile and operated by M. Freeberg. Our observations started at TGRB+8.4h and were taken with Rc filter.
In our stacked frames, subtracted from the Legacy Survey DR10 template image, we detect the optical counterpart reported by Swift/UVOT(Moss et al., GCN 42924, Kuin et al., GCN 42954), LCO/1m (Cerón et al., GCN 42929), MASTER (Buckley et al., GCN 42930), SVOM/VT (Li et al., GCN 42931).
We report our follow-up results in the table below:
+---------------+-----------+-----------+----------------+-------------+
| Tmid-TGRB (hr)| Exp (s) | Filter | Magnitude | Instrument |
+===============+===========+===========+================+=============+
| 8.89 | 17 x 180s | Rc (Vega) | 20.33 +/- 0.09 | iT72 |
+---------------+-----------+-----------+----------------+-------------+
All the data have been reduced by a single data processing pipeline, STDPipe (Karpov et al., 2022). Images obtained with the Johnson-Cousins filters were calibrated using the GAIA DR3 Synphot catalog.
We use the SkyPortal application (skyportal.io) to monitor our observational campaign (Coughlin et al. 2023).
GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).
GCN Circular 42954
Subject
GRB 251201B: Swift/UVOT Detection
Date
2025-12-02T15:52:28Z (8 hours ago)
From
Paul Kuin at MSSL <npkuin@gmail.com>
Via
email
N.P.M. Kuin (UCL-MSSL) and M. J. Moss (GSFC) report on behalf of the
Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 251201B 101
s after the BAT trigger (Moss et al., GCN Circ. 42924). A source
consistent with the XRT position is detected in the initial UVOT exposures.
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 101 251 147 17.31 +/- 0.03
v 644 663 19 15.97 +/- 0.12
b 570 589 20 16.71 +/- 0.10
u 314 564 246 16.48 +/- 0.04
w1 693 1808 117 18.35 +/- 0.18
m2 668 1093 39 >18.7
w2 1196 1562 39 >18.8
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.008 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 42952
Subject
GRB 251201B: GECAM-B observation of a long burst
Date
2025-12-02T15:41:14Z (8 hours ago)
From
Yue Wang <m18509381757@163.com>
Via
Web form
Yue Wang, Chen-Wei Wang, Hao-Xuan Guo, Shao-Lin Xiong (IHEP) report on behalf of GECAM team:
GECAM-B was triggered on-ground by GRB 251201B, at 2025-12-01T16:26:32.950 UTC (denoted as T0), which is also detected by Fermi/GBM (Fermi GBM team, GCN #42923).
According to the GECAM-B light curves in about 70-6000 keV, this burst mainly consists of a long pulse with a duration (T90) of 65 +6/-5 s.
The GECAM-B light curve can be found here:
https://www.bursthub.cn//admin/static/gecambgrb251201B.png
Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation, GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).
GCN Circular 42936
Subject
GRB 251201B: Swift-XRT refined Analysis
Date
2025-12-02T03:23:04Z (21 hours ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
V. D'Elia (SSDC & INAF-OAR), B. Sbarufatti (INAF-OAB), D.N. Burrows
(PSU), S. Lanava (PSU), S. Dichiara (PSU), J.P. Osborne (U. Leicester),
K.L. Page (U. Leicester), C. Salvaggio (INAF-OAB) and P.A. Evans report
on behalf of the Swift-XRT team:
We have analysed 6.3 ks of XRT data for GRB 251201B, from 86 s to 23.3
ks after the trigger. The data comprise 362 s in Windowed Timing (WT)
mode (the first 6 s were taken while Swift was slewing) with the
remainder in Photon Counting (PC) mode.
The late-time light curve (from T0+6.3 ks) can be modelled with a
power-law decay with a decay index of alpha=1.6 (+/-0.3).
A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.40 (+/-0.04). The
best-fitting absorption column is 2.30 (+/-0.12) x 10^21 cm^-2, in
excess of the Galactic value of 1.8 x 10^20 cm^-2 (Willingale et al.
2013). The PC mode spectrum has a photon index of 1.93 (+0.16, -0.15)
and a best-fitting absorption column of 10.0 (+4.5, -4.0) x 10^20
cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 3.5 x 10^-11 (4.3 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 10.0 (+4.5, -4.0) x 10^20 cm^-2
Galactic foreground: 1.8 x 10^20 cm^-2
Excess significance: 3.4 sigma
Photon index: 1.93 (+0.16, -0.15)
If the light curve continues to decay with a power-law decay index of
1.6, the count rate at T+24 hours will be 3.5 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x
10^-13 (1.5 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01418729.
This circular is an official product of the Swift-XRT team.
GCN Circular 42931
Subject
GRB 251201B: SVOM/VT optical fading
Date
2025-12-02T00:45:58Z (a day ago)
Edited On
2025-12-02T17:05:57Z (7 hours ago)
From
Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>
Via
Web form
H.L. Li, L.P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, Y. N. Ma, X. H. Han, J. Wang, Y. Xu, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. R. Xu, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA) report on behalf of the SVOM/VT team.
SVOM/VT performed ToO observation to the field of GRB 251201B (Fermi GBM team, GCN 42923; Moss et al., GCN 42924; Osborne et al., GCN 42927; Goyal et al., GCN 42925). The observation stared at 2025-12-01T17:48:58 UTC, I.e., 1.40 hours post trigger in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.
The optical counterpart (Cerón et al., GCN 42929;Buckley et al., GCN 42930) was detected Within the error box of Swift-UVOT (Moss et al., GCN 42924) and Swift-XRT (Osborne et al., GCN 42927).
The position is at R.A., Dec. = 6.39414, -44.23374 degrees, equivalent to:
R.A. (J2000) = 0:25:34.59
Dec. (J2000) = -44:14:01.46
with an uncertainty of 0.5 arcsec.
The measurements in AB magnitude without correction for Galactic extinction are given below:
Mid_time Band Exposure Time Magnitude (AB)
1.40 hours VT_B 70 sec 18.56+/-0.03 mag
1.40 hours VT_R 70 sec 18.08+/-0.03 mag
3.04 hours VT_B 70 sec 19.46+/-0.04 mag
3.04 hours VT_R 70 sec 18.97+/-0.04 mag
It was fading with a slope of about -1.0 with a single power law.
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 42930
Subject
Swift GRB 251201B : MASTER optical counterpart detection
Date
2025-12-01T23:18:16Z (a day ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
D.Buckley (SAAO), V.M. Lipunov (Lomonosov MSU), A.Kuznetsov, I.Panchenko, G.Antipov, P.Balanutsa, K.Zhirkov, V.Senik, N. Tiurina, Ya.Kechin, A.Chasovnikov, D.Vlasenko, E.Gorbovskoy, Yu.Tselik(Lomonosov MSU, SAI, Moscow),
O.A.Gress, N.M.Budnev, O.Ershova (Irkutsk State University),
R. Podesta, C.Francile, F. Podesta, E. Gonzalez (OAFA, San Juan Uni.,Argentina);
A. Sosnovskij (CrAO RAS),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino, J.Martinez, A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysic Observatory, Mexico)
V.M.Pillet, R.Rebolo Lopez (The Instituto de Astrofisica de Canarias,Spain),
MASTER-SAAO robotic telescope of MASTER Global Robotic Net (Lipunov et al. [1-4], http://observ.pereplet.ru ),
started Swift GRB 251201B (Moss et al GCN 42924, Osborne et al. GCN 42927) inspect at 2025-12-01 22:00:57 UT (20067s after trigger time), Lipunov et al. GCN 42929
There is optical counterpart MASTER OT J002534.28-441352.2
at Swift UVOT position (Moss et al. GCN 42924)
with unfiltered m_OT~20.4m at summary image at 22:00:57 UT (420s exposition, mlim=21m).
Real time updated cover map and OT position are available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3054567
Observation and reduction will continue.
[1] Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L
[2] Lipunov et al. 2022, Universe, Vol. 8(5), id.271
[3] Lipunov et a. 2019, ARep, vol.63, 293
[4] Lipunov V., Kornilov V., Gorbovskoy E., Tiurina N., Kuznetsov A. 2023,
Astronomical Robotic Networks and Operative Multichanel Astrophysics,Lomonosov MSU PRESS, 591pp.
http://www.pereplet.ru/lipunov/625.html#625
GCN Circular 42929
Subject
GRB 251201B: ULL-ASTRO-MASTER detection of the optical afterglow with LCO 1-m telescope at Sutherland Observatory
Date
2025-12-01T22:57:16Z (a day ago)
From
Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es>
Via
Web form
A. Cerón, J. Basurto Merino, P.G. Berdayes, A. Caballero-Almagro, M. Contreras,
F. Díaz-Segado, T. Ferrer-Laviña, B. Gandolfi, V. Ghiraldo, J. Hernández Fung, L. Juliá-Maroto, E. Lekaroz-Urriza, M. Manzano García, E. Mejía-Martínez, J. Prieto Polo, M. Pulido-Torres, M. Quintana-Ansaldo, A. Schenone-Zanuzzi, A. Selezneva, T. Tundidor Rodríguez, E. Urquijo-Rodríguez (all ULL), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL).
Following the detection of GRB 251201B by Fermi (Fermi GBM team, GCN Circ. 42923) and Swift (Moss et al., GCN Circ. 42924; Osborne et al., GCN Circ. 42927), we observed the field with one of the three Las Cumbres Observatory (LCO) 1-m telescopes equipped with Sinistro cameras located at the LCO node at Sutherland Observatory, South Africa. The observation, a single exposure of 300 sec in the SDSS r' filter, started on 2025-12-01 at 22:00:22 UT, about 5.73 hours after the Fermi and Swift trigger. The optical counterpart detected by Swift UVOT (Moss et al., GCN Circ. 42924) is clearly detected in our image with a magnitude of r' = 19.78 +/- 0.11 (AB), calibrated against
the Legacy Imaging Surveys DR10 catalog and not corrected for Galactic extinction.
This work makes use of observations from the Las Cumbres Observatory global telescope network (LCO program IAC2025B-010). These observations are part of a course in Astrophysical Techniques of the Master in Astrophysics of the Astrophysics Department of the University of La Laguna in collaboration with the Instituto de Astrofísica de Canarias (La Laguna, Tenerife, Canary Islands, Spain).
This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5).
GCN Circular 42928
Subject
Swift GRB 251201B: Global MASTER-Net observations report
Date
2025-12-01T22:31:04Z (a day ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
email
V.Lipunov, E.Gorbovskoy, A.Kuznetsov, K.Zhirkov, I.Panchenko, N.Tiurina, P.Balanutsa, V.Topolev, D.Vlasenko,
G.Antipov, A.Sankovich, Yu.Tselik, Ya.Kechin, V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile, F. Podesta, R.Podesta, E. Gonzalez (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A.Sosnovskij (CrAO),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity),
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory)
MASTER-SAAO robotic telescope [1] located in South Africa (South African Astronomical Observatory) was pointed to the Swift GRB 251201B ( M. J. Moss et al., GCN 42924) errorbox 20045 sec after notice time and 20067 sec after trigger time at 2025-12-01 22:00:57 UT, with upper limit up to 17.9 mag. The observations began at zenith distance = 45 deg. The sun altitude is -35.7 deg.
The galactic latitude b = -73 deg., longitude l = 317 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3057197
We obtain a following upper limits.
Tmid-T0 | Site |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________
20158 | MASTER-SAAO | C | 180 | 17.9 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
[1] - V.M. Lipunov, V.G. Kornilov, E.S. Gorbovskoy, N.A. Tiurina & A.S.Kuznetsov, 2023, Astronomical Robotic Networks and Operative Multichanel Astrophysics, Lomonosov MSU PRESS, 591pp.
http : // www.pereplet.ru/lipunov/625.html
GCN Circular 42927
Subject
GRB 251201B: Enhanced Swift-XRT position
Date
2025-12-01T21:24:42Z (a day ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
J.P. Osborne, A.P. Beardmore, P.A. Evans and M.R. Goad (U. Leicester)
report on behalf of the Swift-XRT team.
Using 1078 s of XRT Photon Counting mode data and 3 UVOT
images for GRB 251201B, 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 = 6.39463, -44.23233 which is equivalent
to:
RA (J2000): 00h 25m 34.71s
Dec (J2000): -44d 13' 56.4"
with an uncertainty of 3.8 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 42924
Subject
GRB 251201B: Swift detection of a burst with an optical counterpart
Date
2025-12-01T16:38:08Z (a day ago)
From
Tyler Parsotan at NASA GSFC <tyler.parsotan@nasa.gov>
Via
email
M. J. Moss (GSFC), S. Dichiara (PSU), R. Gupta (NASA GSFC),
J. A. Kennea (PSU), K. L. Page (U Leicester) and T. M. Parsotan (GSFC)
report on behalf of the Neil Gehrels Swift Observatory Team:
At 16:26:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251201B (trigger=1418729). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 6.409, -44.237 which is
RA(J2000) = 00h 25m 38s
Dec(J2000) = -44d 14' 12"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a complex
structure with a duration of about 40 sec. The peak count rate
was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger.
The XRT began observing the field at 16:28:03.7 UT, 93.4 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 6.3925, -44.2312 which is equivalent to:
RA(J2000) = 00h 25m 34.20s
Dec(J2000) = -44d 13' 52.3"
with an uncertainty of 7.0 arcseconds (radius, 90% containment). This
location is 47 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy.
The initial flux in the 0.1 s image was 8.63e-09 erg cm^-2 s^-1 (0.2-10
keV).
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 101 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
RA(J2000) = 00:25:34.59 = 6.39413
DEC(J2000) = -44:14:01.1 = -44.23364
with a 90%-confidence error radius of about 0.75 arc sec. This position is 9.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.28 with a 1-sigma error of about 0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.008.
Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)
GCN Circular 42923
Subject
GRB 251201B: Fermi GBM Final Real-time Localization
Date
2025-12-01T16:37:01Z (a day 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 16:26:24 UT on 1 Dec 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251201B (trigger 786299189.217446 / 251201685).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 13.8, Dec = -40.0 (J2000 degrees, equivalent to J2000 00h 55m, -40d 00'), with a statistical uncertainty of 1.1 degrees.
The angle from the Fermi LAT boresight is 118.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251201685/quicklook/glg_skymap_all_bn251201685.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/bn251201685/quicklook/glg_healpix_all_bn251201685.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251201685/quicklook/glg_lc_medres34_bn251201685.gif