GRB 251208B

Francesco Magnani (CPPM), Diego López-Cámara (UNAM), Alan M. Watson (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), Francis Fortin (IRAP), Leonardo García-García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), Kin Ocelotl López (UNAM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM):

We imaged the field of the Fermi/Swift GRB 251208B (Fermi GBM Team, GCN Circ. 43028
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 ; DeLaunay et al., GCN Circ. 43033
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 ; Bissaldi et al., GCN Circ. 43034
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 ) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-12-09 12:13 to 13:13 UTC (from 25.9 to 26.9 hours after the Fermi/GBM trigger) and obtained 45 minutes of simultaneous exposure in the r and z filters.

The data were reduced, coadded, and analyzed with the ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.

In our stacked images, at the position of the XRT afterglow candidate (Kennea et al., GCN Circ. 43050
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 ) we do not detect any new source to the following 3-sigma limits:

r > 23.7
z > 22.7

Furthermore, we do not detect any new source at the LAT uncertainty region (Bissaldi et al., GCN Circ. 43034
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 ) down to the following 5-sigma limits:

r > 23.1
z > 22.1

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.

J.A. Kennea (PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester),
J.P. Osborne (U. Leicester), M. Ferro (INAF-OAB), R. Brivio (INAF-OAB),
A. D'Ai (INAF-IASFPA), S. Lanava (PSU) and P.A. Evans (U. Leicester)
reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the GUANO (DeLaunay
et al. GCN 43033
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 ) and Fermi/LAT-detected (Bissaldi et al. GCN 43034
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 )
burst GRB 251208B. We searched for X-ray sources in  4.8 ks of Photon
Counting (PC) mode data. The total exposure at the position of the
afterglow (see below) is 4.8 ks, obtained between T0+33.1 ks and
T0+45.4 ks.

Three uncatalogued X-ray sources are detected within the estimated
3-sigma Fermi/LAT error region (414 arcsec), of which one ("Source 1")
is 2.9 sigma above the RASS limit and it is fading with 2.2 sigma
significance and thus is believed to be the GRB afterglow. Using 4576 s
of PC mode data and 5 UVOT images, we find an enhanced XRT position
(using the XRT-UVOT alignment and matching UVOT field sources to the
USNO-B1 catalogue): RA, Dec = 228.48108, +29.04575 which is equivalent
to:

RA (J2000): 15h 13m 55.46s
Dec(J2000): +29d 02' 44.7"

with an uncertainty of 2.3 arcsec (radius, 90% confidence). This
position is 87 arcsec from the Fermi/LAT position.  The light curve is
consistent with a constant source of mean count rate 5.0e-02 ct/sec. A
power-law fit gives an index of 1.2 (+/-1.2).

A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.2 (+0.4, -0.3). The
best-fitting absorption column is  1.4 (+1.1, -0.8) x 10^21 cm^-2, in
excess of the Galactic value of 2.6 x 10^20 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum  is 3.1 x 10^-11 (4.4 x 10^-11) erg
cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     1.4 (+1.1, -0.8) x 10^21 cm^-2
Galactic foreground: 2.6 x 10^20 cm^-2
Excess significance: 2.3 sigma
Photon index:	     2.2 (+0.4, -0.3)

The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/00021893.
The results of the full analysis of the XRT observations are available
at https://www.swift.ac.uk/ToO_GRBs/00021893.

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

A. von Kienlin (MPE) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 10:17:17.29 UT on 08 December 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251208B (trigger 786881842/251208429).
which was also detected by Fermi-LAT (E. Bissaldi et al. 2025, GCN 43034
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 )
and Swift/BAT-GUANO (GCN #43033
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 ). The Fermi GBM on-ground location was reported
by the Fermi-GBM team in GCN 43028
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  and is consistent with the Fermi-LAT
and Swift/BAT positions.

The GBM light curve shows a structured emission episode with a duration (T90)
of about 62.5 s (50-300 keV). The time-averaged spectrum
from T0-4.1 to T0+61.4 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -0.67 +/- 0.08 and the cutoff energy,
parameterized as Epeak, is 250 +/- 20 keV.

The event fluence (10-1000 keV) in this time interval is
(9.5 +/- 0.5)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+55 s in the 10-1000 keV band is 2 +/- 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/"

P. A. Evans (U. Leicester) reports on behalf of the Swift team:

Swift has initiated a ToO observation of the Fermi/LAT-detected event
GRB 251208B. Automated analysis of the XRT data will be presented online at
https://www.swift.ac.uk/ToO_GRBs/00021893
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/LAT 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.

E. Bissaldi (Politecnico and INFN Bari), N. Di Lalla (Stanford University), R. Gupta (NASA/GSFC), A. Holzmann Airasca (UniTrento and INFN Bari), C. Bartolini (UniTrento and INFN Bari) and F. Longo (University and INFN, Trieste) report on behalf of the Fermi-LAT team:

At 10:17:17.29 UT on December, 08, 2025 Fermi-LAT detected high-energy emission from GRB 251208B, which was also detected by Fermi-GBM (trigger 786881842 / 251208429, GCN #43028
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 ) and Swift/BAT-GUANO (GCN #43033
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 ).

The best LAT on-ground location is found to be

RA, Dec = 228.48, 29.07 (J2000)

with an error radius of 0.07 deg (90 % containment, statistical error only), which is consistent with the Swift/BAT-GUANO localization.

This was 39 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 ks after the GBM trigger is (3.13 ± 0.09)E-6 ph/cm2/s. The estimated photon index above 100 MeV is -2.01 ± 0.23. The highest-energy photon is a 17 GeV event which is observed 419 seconds after the GBM trigger.

A Swift ToO has been approved for this burst.

The Fermi-LAT point of contact for this burst is Chiara Bartolini (chiara.bartolini@ba.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.

James DeLaunay (PSU), Aaron Tohuvavohu (Cosmic Frontier), Samuele Ronchini (GSSI), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC), Maia Williams (Northwestern)  report:

Swift/BAT did not localize GRB 251208B onboard (T0: 2025-12-08T10:17:17.29 UTC, Fermi Trig 786881842).

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), detects the burst in a 8.192 s analysis time bin starting at T0 - 2.048 s with a sqrt(TS) of 17.6.
An arcminute localization is found with DeltaLLHOut of 43 and a DeltaLLHPeak of 30.0.

See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut.

The BAT position is
RA, Dec = 228.453, 29.044 deg which is
   RA(J2000)  = 15h 13m 48.72s
   Dec(J2000) = 29d 02’ 38.4″
with an estimated uncertainty of 5 arcmin radius.

More details about this burst can be found on the trigger report page here:

https://guano.swift.psu.edu/trigger_report?id=786881874

XRT and UVOT follow-up has been requested.
Results of follow-up observations will be reported in future circulars.

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/

The Fermi GBM team reports the detection of a likely LONG GRB

At 10:17:17 UT on 8 Dec 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251208B (trigger 786881842.294214 / 251208429).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 212.3, Dec = 30.1 (J2000 degrees, equivalent to J2000 14h 09m, 30d 06'), with a statistical uncertainty of 5.8 degrees.

The angle from the Fermi LAT boresight is 25.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251208429/quicklook/glg_skymap_all_bn251208429.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/bn251208429/quicklook/glg_healpix_all_bn251208429.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251208429/quicklook/glg_lc_medres34_bn251208429.gif