Gamma-ray Burst 251208B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 08 Dec 25 10:17:46 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      47
TRIGGER_NUM:     786881842
GRB_RA:          244.700d {+16h 18m 48s} (J2000),
                 244.931d {+16h 19m 43s} (current),
                 244.255d {+16h 17m 01s} (1950)
GRB_DEC:         +37.783d {+37d 46' 60"} (J2000),
                 +37.722d {+37d 43' 19"} (current),
                 +37.903d {+37d 54' 12"} (1950)
GRB_ERROR:       31.33 [deg radius, statistical plus systematic]
GRB_INTEN:       51 [cnts/sec]
DATA_SIGNIF:     5.10 [sigma]
INTEG_TIME:      4.096 [sec]
GRB_DATE:        21017 TJD;   342 DOY;   25/12/08
GRB_TIME:        37037.29 SOD {10:17:17.29} UT
GRB_PHI:          52.00 [deg]
GRB_THETA:        50.00 [deg]
DATA_TIME_SCALE: 4.0960 [sec]
HARD_RATIO:      1.21
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      85%  GRB
2nd_MOST_LIKELY:   9%  Distant Particles
DETECTORS:       1,0,1, 0,0,0, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:       255.42d {+17h 01m 40s}  -22.76d {-22d 45' 29"}
SUN_DIST:         61.28 [deg]   Sun_angle= 0.7 [hr] (West of Sun)
MOON_POSTN:      127.40d {+08h 29m 37s}  +22.32d {+22d 19' 26"}
MOON_DIST:        96.07 [deg]
MOON_ILLUM:      84 [%]
GAL_COORDS:       60.37, 45.56 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      230.65, 57.82 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251208429/quicklook/glg_lc_medres34_bn251208429.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 309.87,25.65 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 08 Dec 25 10:18:04 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      57
TRIGGER_NUM:     786881842
GRB_RA:          204.810d {+13h 39m 14s} (J2000),
                 205.107d {+13h 40m 26s} (current),
                 204.238d {+13h 36m 57s} (1950)
GRB_DEC:         +30.460d {+30d 27' 36"} (J2000),
                 +30.329d {+30d 19' 45"} (current),
                 +30.713d {+30d 42' 48"} (1950)
GRB_ERROR:       10.34 [deg radius, statistical only]
DATA_SIGNIF:     7.70 [sigma]
DATA_INTERVAL:   4.096 [sec]
GRB_DATE:        21017 TJD;   342 DOY;   25/12/08
GRB_TIME:        37037.29 SOD {10:17:17.29} UT
GRB_PHI:          25.00 [deg]
GRB_THETA:        19.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4173 (Gnd S/W Version number)
SUN_POSTN:       255.42d {+17h 01m 40s}  -22.76d {-22d 45' 29"}
SUN_DIST:         71.76 [deg]   Sun_angle= 3.4 [hr] (West of Sun)
MOON_POSTN:      127.41d {+08h 29m 38s}  +22.32d {+22d 19' 23"}
MOON_DIST:        68.78 [deg]
MOON_ILLUM:      84 [%]
GAL_COORDS:       53.40, 79.02 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      189.45, 37.52 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251208429/quicklook/glg_lc_medres34_bn251208429.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_786881842.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        In the LAT Field-of-view.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 08 Dec 25 10:18:23 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      86
TRIGGER_NUM:     786881842
GRB_RA:          229.233d {+15h 16m 56s} (J2000),
                 229.471d {+15h 17m 53s} (current),
                 228.776d {+15h 15m 06s} (1950)
GRB_DEC:         +40.933d {+40d 55' 60"} (J2000),
                 +40.839d {+40d 50' 21"} (current),
                 +41.116d {+41d 06' 57"} (1950)
GRB_ERROR:       20.27 [deg radius, statistical plus systematic]
GRB_INTEN:       64 [cnts/sec]
DATA_SIGNIF:     7.30 [sigma]
INTEG_TIME:      4.096 [sec]
GRB_DATE:        21017 TJD;   342 DOY;   25/12/08
GRB_TIME:        37037.29 SOD {10:17:17.29} UT
GRB_PHI:          53.00 [deg]
GRB_THETA:        35.00 [deg]
DATA_TIME_SCALE: 4.0960 [sec]
HARD_RATIO:      0.73
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      93%  GRB
2nd_MOST_LIKELY:   2%  Distant Particles
DETECTORS:       1,0,1, 0,0,0, 0,0,0, 0,0,0, 0,0,
SUN_POSTN:       255.42d {+17h 01m 41s}  -22.76d {-22d 45' 29"}
SUN_DIST:         68.02 [deg]   Sun_angle= 1.7 [hr] (West of Sun)
MOON_POSTN:      127.41d {+08h 29m 38s}  +22.32d {+22d 19' 19"}
MOON_DIST:        84.14 [deg]
MOON_ILLUM:      84 [%]
GAL_COORDS:       67.24, 57.15 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      208.19, 55.97 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251208429/quicklook/glg_lc_medres34_bn251208429.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 309.87,25.65 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 08 Dec 25 10:19:14 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      1
TRIGGER_NUM:     786881842
GRB_RA:          212.520d {+14h 10m 05s} (J2000),
                 212.804d {+14h 11m 13s} (current),
                 211.973d {+14h 07m 53s} (1950)
GRB_DEC:         +32.020d {+32d 01' 12"} (J2000),
                 +31.898d {+31d 53' 54"} (current),
                 +32.255d {+32d 15' 20"} (1950)
GRB_ERROR:       6.35 [deg radius, statistical only]
DATA_SIGNIF:     13.20 [sigma]
DATA_INTERVAL:   17.408 [sec]
GRB_DATE:        21017 TJD;   342 DOY;   25/12/08
GRB_TIME:        37037.29 SOD {10:17:17.29} UT
GRB_PHI:          33.00 [deg]
GRB_THETA:        25.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       255.42d {+17h 01m 41s}  -22.76d {-22d 45' 29"}
SUN_DIST:         68.18 [deg]   Sun_angle= 2.8 [hr] (West of Sun)
MOON_POSTN:      127.42d {+08h 29m 41s}  +22.32d {+22d 19' 10"}
MOON_DIST:        74.70 [deg]
MOON_ILLUM:      84 [%]
GAL_COORDS:       53.64, 72.24 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      196.17, 41.90 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251208429/quicklook/glg_lc_medres34_bn251208429.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_786881842.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        In the LAT Field-of-view.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 08 Dec 25 10:18:58 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      0
TRIGGER_NUM:     786881842
GRB_RA:          210.780d {+14h 03m 07s} (J2000),
                 211.068d {+14h 04m 16s} (current),
                 210.225d {+14h 00m 54s} (1950)
GRB_DEC:         +30.940d {+30d 56' 24"} (J2000),
                 +30.816d {+30d 48' 58"} (current),
                 +31.180d {+31d 10' 48"} (1950)
GRB_ERROR:       7.00 [deg radius, statistical only]
DATA_SIGNIF:     10.50 [sigma]
DATA_INTERVAL:   11.264 [sec]
GRB_DATE:        21017 TJD;   342 DOY;   25/12/08
GRB_TIME:        37037.29 SOD {10:17:17.29} UT
GRB_PHI:          30.00 [deg]
GRB_THETA:        24.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       255.42d {+17h 01m 41s}  -22.76d {-22d 45' 29"}
SUN_DIST:         68.40 [deg]   Sun_angle= 3.0 [hr] (West of Sun)
MOON_POSTN:      127.42d {+08h 29m 40s}  +22.32d {+22d 19' 13"}
MOON_DIST:        73.60 [deg]
MOON_ILLUM:      84 [%]
GAL_COORDS:       50.87, 73.89 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      195.05, 40.26 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251208429/quicklook/glg_lc_medres34_bn251208429.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_786881842.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        In the LAT Field-of-view.  
COMMENTS:        This is likely a Long GRB.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file will not be created/available until ~15 min after the trigger.  
COMMENTS:        The POS_MAP_URL file will not be created/available until ~1.5 min after the notice.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 08 Dec 25 10:26:41 UT
NOTICE_TYPE:     Fermi-GBM Final Position
RECORD_NUM:      0
TRIGGER_NUM:     786881842
GRB_RA:          212.340d {+14h 09m 22s} (J2000),
                 212.628d {+14h 10m 31s} (current),
                 211.785d {+14h 07m 09s} (1950)
GRB_DEC:         +30.070d {+30d 04' 12"} (J2000),
                 +29.948d {+29d 56' 54"} (current),
                 +30.306d {+30d 18' 21"} (1950)
GRB_ERROR:       5.75 [deg radius, statistical only]
GRB_DATE:        21017 TJD;   342 DOY;   25/12/08
GRB_TIME:        37037.29 SOD {10:17:17.29} UT
GRB_PHI:          28.00 [deg]
GRB_THETA:        25.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       255.43d {+17h 01m 42s}  -22.76d {-22d 45' 31"}
SUN_DIST:         66.85 [deg]   Sun_angle= 2.9 [hr] (West of Sun)
MOON_POSTN:      127.49d {+08h 29m 59s}  +22.30d {+22d 17' 50"}
MOON_DIST:        75.08 [deg]
MOON_ILLUM:      84 [%]
GAL_COORDS:       47.30, 72.67 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      197.14, 40.08 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251208429/quicklook/glg_lc_medres34_bn251208429.gif
LOC_URL:         http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251208429/quicklook/glg_locplot_all_bn251208429.png
COMMENTS:        Fermi-GBM Final Position.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        The LC_URL file should be available by the time this FINAL notice is produced.  
COMMENTS:        This notice was generated completely by automated pipeline processing.  
COMMENTS:        In the LAT Field-of-view.  
COMMENTS:        This is likely a Long GRB.

GCN Circular #43028
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
GCN Circular #43033
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/

GCN Notice

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Mon 08 Dec 25 17:25:11 UT
NOTICE_TYPE:     Fermi-LAT Offline Position
TRIGGER_NUM:     786881842
GRB_RA:          228.480d {+15h 13m 55s} (J2000),
                 228.752d {+15h 15m 01s} (current),
                 227.955d {+15h 11m 49s} (1950)
GRB_DEC:         +29.070d {+29d 04' 12"} (J2000),
                 +28.975d {+28d 58' 28"} (current),
                 +29.256d {+29d 15' 20"} (1950)
GRB_ERROR:       4.20 [arcmin radius, 90% containment, statistical only]
GRB_DATE:        21017 TJD;   342 DOY;   25/12/08
GRB_TIME:        37037.29 SOD {10:17:17.29} UT
TRIGGER_ID:      0x2000
MISC:            0x40002000
SUN_POSTN:       255.74d {+17h 02m 59s}  -22.79d {-22d 47' 16"}
SUN_DIST:         57.92 [deg]   Sun_angle= 1.8 [hr] (West of Sun)
MOON_POSTN:      131.67d {+08h 46m 42s}  +20.99d {+20d 59' 27"}
MOON_DIST:        85.82 [deg]
MOON_ILLUM:      81 [%]
GAL_COORDS:       44.96, 58.64 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      215.10, 44.92 [deg] ecliptic lon,lat of the burst (or transient)
COMMENTS:        Fermi LAT Offline position.  
COMMENTS:        This Notice was ground-generated -- not flight-generated.  
COMMENTS:        It is the result of human-in-the-loop processing.  
COMMENTS:        This is a human generated position of a LAT ground detection.  
COMMENTS:        This source corresponds to GBM trigger.

GCN Circular #43034
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) and Swift/BAT-GUANO (GCN #43033).
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.
GCN Circular #43035
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.
GCN Circular #43039
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 [1] located in Russia (Lomonosov MSU, Kislovodsk Solar Station of Pulkovo observatory) was pointed to the LAT GRB251208.43 (trigger No 786881842,15h 13m 55.20s , +29d 04m 12.0s, R=0.07) errorbox 23885 sec after notice time and 49564 sec after trigger time at 2025-12-09 00:03:21 UT, with upper limit up to 12.9 mag. The observations began at zenith distance = 82 deg. The sun altitude is -47.7 deg.
The galactic latitude b = 58 deg., longitude l = 45 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3064689
We obtain a following upper limits.
```
Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   49654 | 2025-12-09 00:03:21 |   MASTER-Kislovodsk | (15h 12m 35.31s , +29d 07m 16.4s) |   C |   180 | 12.9 |        
   49654 | 2025-12-09 00:03:21 |   MASTER-Kislovodsk | (15h 12m 31.18s , +28d 44m 43.8s) |   C |   180 | 12.6 |        
```
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 #43045
A. Kumar, T. Mohan, V.Swain, S. Patil, A.P. Saikia, V. Bhalerao (IITB), G.C. Anupama, S. Barway (IIA) and K. Angail (IAO) report on behalf of the GIT team:
We observed the field of Fermi GBM transient (Fermi GBM Team, GCN 43028), with 0.7m GROWTH-India Telescope (GIT). We started the observation at 2025-12-08T23:43:57 UT, i.e., about 13.4 hours after the Fermi GBM Trigger. Exposures were obtained in r' and i' filters. We did not detect any transient in our images within the Fermi-LAT on-ground localization region (Bissaldi et al., GCN 43034). Additionally, we checked the source 1 and 5 of Swift-XRT ToO observations (Evans at al., GCN 43035), and did not detect any optical counterpart at their respective position and uncertainty. The photometric upper limits are as follows:
```
| MJD (mid)    | Filter | tmid-t0 (hrs) | Exposure Time (sec) | Upper limit (AB) |
| ------------ | ------ | ------------- | ------------------- | ---------------- |
| 61018.007928 | r'     | 13.90        | 3 x 360                 | 20.6            |
| 61018.020746 | i'     | 14.21        | 2 x 360                 | 17.4            |
```
The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.
The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.
GCN Circular #43046
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) and Swift/BAT-GUANO (GCN #43033). The Fermi GBM on-ground location was reported by the Fermi-GBM team in GCN 43028 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/"
GCN Circular #43050
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) and Fermi/LAT-detected (Bissaldi et al. GCN 43034) 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.
GCN Circular #43057
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; DeLaunay et al., GCN Circ. 43033; Bissaldi et al., GCN Circ. 43034) 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) 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) 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.

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Jochen Greiner, last update: 09-Dec-2025 [Disclaimer]