GRB 251031A
GCN Circular 42530
U. Pathak (IITB), R. Sonawane (IISER TVM) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 07:21:00.49 UT on 31 October 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251031A (trigger 783588065/251031306).
which was also detected by Swift/BAT-GUANO (DeLaunay et al. 2025, GCN 42528).
The Fermi GBM on-ground location is consistent with the Swift/BAT-GUANO position.
The angle from the Fermi LAT boresight is 49 degrees.
The GBM light curve consists of multiple spikes from a single emission episode
with a duration (T90) of about 45 s (50-300 keV). The time-averaged
spectrum from T0-1.5 to T0+47.1 s is best fit by a power law
function with an exponential high-energy cutoff.
The power law index is -0.7 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 130 +/- 1 keV.
The event fluence (10-1000 keV) in this time interval is
(8 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+23 s in the 10-1000 keV band is 8 +/- 1 ph/s/cm^2.
A Band function fits the spectrum equally well
with Epeak= 116 +/- 1 keV, alpha = -0.59 +/- 0.01 and beta = -2.51 +/- 0.04.
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 42528
James DeLaunay (PSU), Samuele Ronchini (GSSI), Aaron Tohuvavohu (Cosmic Frontier), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC), Maia Williams (Northwestern) report:
Swift/BAT did not localize GRB 251031A onboard (T0: 2025-10-31T07:21:00.49 UTC, Fermi Trig 783588065)
The Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).
Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.
The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), performed on the temporal window [T0-20 s, T0+20 s], detects the burst with a sqrt(TS) of 8.4 in a 16.384 s analysis time bin, starting at T0 + 8.192 s.
Using the NITRATES analysis, parameter estimation was performed to obtain the localization of this burst in the form of a HEALPIX Multi-Order Coverage (MOC) skymap. This localization accounts for both statistical and systematic errors. More details in the creation and calibration of these maps will soon be published (DeLaunay et al. 2025. in prep)
The 90% credible area is 8,620 deg2 and the 50% credible area is 269 deg2. The integrated probability inside the coded field of view is 1%.
The NITRATES skymap is consistent with the Fermi localization reported in the final position notice (GCN 42525). The combined Fermi/GBM+NITRATES 90% credible area is 168 deg2 and the 50% credible area is 47 deg2.
A plot of the probability skymap can be viewed here:
The probability skymap and joint skymap files can be downloaded from the links here
Instructions on how to read and manipulate this map can be found here:
https://guano.swift.psu.edu/documentation
More details about this burst can be found on the trigger report page here:
https://guano.swift.psu.edu/trigger_report?id=783588097
GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.
A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/
GCN Circular 42526
T. Preis (University of Innsbruck) & J. Greiner (MPE Garching) report:
The public trigdat data of the Fermi Gamma-ray Burst Monitor (GBM) trigger
783588065 at 07:21:00 on 31 Oct. 2025 were automatically fitted for spectrum
and sky location with BALROG (Burgess et al. 2018, MNRAS 476, 1427;
Berlato et al. 2019, ApJ 873, 60).
The best-fit position is:
RA(2000.0) = 188.3 deg
Decl.(2000.0) = -17.9 deg
The 1 sigma statistical error radius is 2.8 deg.
We estimate an additional systematic error of 1 deg.
Further details are available at:
https://grb.mpe.mpg.de/grb/GRB251031306/
The Healpix map can be downloaded from:
https://grb.mpe.mpg.de/grb/GRB251031306/healpix
The location parameters are available as JSON at:
https://grb.mpe.mpg.de/grb/GRB251031306/json
GCN Circular 42525
The Fermi GBM team reports the detection of a likely LONG GRB
At 07:21:00 UT on 31 Oct 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251031A (trigger 783588065.486224 / 251031306).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 190.3, Dec = -17.6 (J2000 degrees, equivalent to J2000 12h 41m, -17d 36'), with a statistical uncertainty of 3.3 degrees.
The angle from the Fermi LAT boresight is 49.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251031306/quicklook/glg_skymap_all_bn251031306.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/bn251031306/quicklook/glg_healpix_all_bn251031306.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251031306/quicklook/glg_lc_medres34_bn251031306.gif