Gamma-ray Burst 251220B

(All information courtesy of the instrument teams.)

Previous IAU Circulars

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Results of Observations

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 20 Dec 25 23:38:23 UT
NOTICE_TYPE:     Fermi-GBM Ground Position
RECORD_NUM:      59
TRIGGER_NUM:     787966689
GRB_RA:          268.860d {+17h 55m 26s} (J2000),
                 269.045d {+17h 56m 11s} (current),
                 268.503d {+17h 54m 01s} (1950)
GRB_DEC:         +45.570d {+45d 34' 12"} (J2000),
                 +45.567d {+45d 34' 02"} (current),
                 +45.576d {+45d 34' 35"} (1950)
GRB_ERROR:       1.85 [deg radius, statistical only]
DATA_SIGNIF:     25.70 [sigma]
DATA_INTERVAL:   0.128 [sec]
GRB_DATE:        21029 TJD;   354 DOY;   25/12/20
GRB_TIME:        85084.63 SOD {23:38:04.63} UT
GRB_PHI:         253.00 [deg]
GRB_THETA:        28.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   4173 (Gnd S/W Version number)
SUN_POSTN:       269.29d {+17h 57m 09s}  -23.43d {-23d 26' 00"}
SUN_DIST:         69.00 [deg]   Sun_angle= 0.0 [hr] (West of Sun)
MOON_POSTN:      280.77d {+18h 43m 06s}  -27.54d {-27d 32' 31"}
MOON_DIST:        73.88 [deg]
MOON_ILLUM:      1 [%]
GAL_COORDS:       72.35, 28.45 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      267.77, 69.00 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251220985/quicklook/glg_lc_medres34_bn251220985.gif
POS_MAP_URL:     http://gcn.gsfc.nasa.gov/notices_f/gbm_gnd_loc_map_787966689.fits
COMMENTS:        Fermi-GBM Ground-calculated Coordinates.  
COMMENTS:        In the LAT Field-of-view.  
COMMENTS:        Bright hard burst in the GBM.  
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:     Sat 20 Dec 25 23:38:08 UT
NOTICE_TYPE:     Fermi-GBM Flight Position
RECORD_NUM:      49
TRIGGER_NUM:     787966689
GRB_RA:          272.800d {+18h 11m 12s} (J2000),
                 272.991d {+18h 11m 58s} (current),
                 272.432d {+18h 09m 44s} (1950)
GRB_DEC:         +44.383d {+44d 22' 60"} (J2000),
                 +44.391d {+44d 23' 26"} (current),
                 +44.371d {+44d 22' 14"} (1950)
GRB_ERROR:       5.42 [deg radius, statistical plus systematic]
GRB_INTEN:       1210 [cnts/sec]
DATA_SIGNIF:     23.20 [sigma]
INTEG_TIME:      0.128 [sec]
GRB_DATE:        21029 TJD;   354 DOY;   25/12/20
GRB_TIME:        85084.63 SOD {23:38:04.63} UT
GRB_PHI:         252.00 [deg]
GRB_THETA:        25.00 [deg]
DATA_TIME_SCALE: 0.1280 [sec]
HARD_RATIO:      0.42
LOC_ALGORITHM:   3 (version number of)
MOST_LIKELY:      80%  GRB
2nd_MOST_LIKELY:  16%  Cyg X-1
DETECTORS:       0,0,0, 1,0,0, 0,1,1, 0,0,0, 0,0,
SUN_POSTN:       269.29d {+17h 57m 09s}  -23.43d {-23d 26' 00"}
SUN_DIST:         67.91 [deg]   Sun_angle= -0.2 [hr] (East of Sun)
MOON_POSTN:      280.77d {+18h 43m 05s}  -27.54d {-27d 32' 32"}
MOON_DIST:        72.28 [deg]
MOON_ILLUM:      1 [%]
GAL_COORDS:       71.72, 25.47 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      275.30, 67.77 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251220985/quicklook/glg_lc_medres34_bn251220985.gif
COMMENTS:        Fermi-GBM Flight-calculated Coordinates.  
COMMENTS:        This trigger occurred at longitude,latitude = 225.42,-20.38 [deg].  
COMMENTS:        The LC_URL file will not be created until ~15 min after the trigger.

GCN NOTICE

TITLE:           GCN/FERMI NOTICE
NOTICE_DATE:     Sat 20 Dec 25 23:47:22 UT
NOTICE_TYPE:     Fermi-GBM Final Position
RECORD_NUM:      0
TRIGGER_NUM:     787966689
GRB_RA:          251.350d {+16h 45m 24s} (J2000),
                 251.531d {+16h 46m 07s} (current),
                 251.002d {+16h 44m 00s} (1950)
GRB_DEC:         +47.920d {+47d 55' 12"} (J2000),
                 +47.874d {+47d 52' 26"} (current),
                 +48.010d {+48d 00' 35"} (1950)
GRB_ERROR:       6.40 [deg radius, statistical only]
GRB_DATE:        21029 TJD;   354 DOY;   25/12/20
GRB_TIME:        85084.63 SOD {23:38:04.63} UT
GRB_PHI:         257.00 [deg]
GRB_THETA:        40.00 [deg]
E_RANGE:         44.032 - 279.965 [keV]
LOC_ALGORITHM:   41731 (Gnd S/W Version number)
SUN_POSTN:       269.29d {+17h 57m 10s}  -23.43d {-23d 26' 00"}
SUN_DIST:         73.07 [deg]   Sun_angle= 1.2 [hr] (West of Sun)
MOON_POSTN:      280.86d {+18h 43m 26s}  -27.53d {-27d 32' 00"}
MOON_DIST:        79.88 [deg]
MOON_ILLUM:      1 [%]
GAL_COORDS:       74.17, 40.55 [deg] galactic lon,lat of the burst (or transient)
ECL_COORDS:      233.28, 68.99 [deg] ecliptic lon,lat of the burst (or transient)
LC_URL:          http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251220985/quicklook/glg_lc_medres34_bn251220985.gif
LOC_URL:         http://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251220985/quicklook/glg_locplot_all_bn251220985.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:        Bright hard burst in the GBM.  
COMMENTS:        This is likely a Short GRB.

GCN Circular #43160
The Fermi GBM team reports the detection of a likely SHORT GRB
At 23:38:04 UT on 20 Dec 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251220B (trigger 787966689.62881 / 251220985).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 251.4, Dec = 47.9 (J2000 degrees, equivalent to J2000 16h 45m, 47d 53'), with a statistical uncertainty of 6.4 degrees.
The angle from the Fermi LAT boresight is 40.0 degrees.
The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251220985/quicklook/glg_skymap_all_bn251220985.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/bn251220985/quicklook/glg_healpix_all_bn251220985.fit
The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251220985/quicklook/glg_lc_medres34_bn251220985.gif
GCN Circular #43161
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) started inspect of the SVOM GRB251220.98 (trigger No 1766273885,18h 57m 31.51s , +68d 19m 14.2s, R=1) errorbox 616 sec after notice time and 649 sec after trigger time at 2025-12-20 23:48:54 UT, with upper limit up to 18.2 mag. The observations began at zenith distance = 67 deg. The sun altitude is -51.7 deg.
The galactic latitude b = 25 deg., longitude l = 99 deg.
Real time updated cover map and OT discovered available here: https://master.sai.msu.ru/site/master2/observ.php?id=3078836
We obtain a following upper limits.
```
Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

     740 | 2025-12-20 23:48:54 |   MASTER-Kislovodsk | (18h 56m 08.59s , +68d 09m 08.0s) |   C |   180 | 18.2 |        
     740 | 2025-12-20 23:48:54 |   MASTER-Kislovodsk | (19h 06m 49.88s , +67d 44m 25.1s) |   C |   180 | 14.9 |        
```
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 #43171
SVOM/GRM team: Chen-Wei Wang, Yue Huang, Shi-Jie Zheng, Shao-Lin Xiong, Shuang-Nan Zhang (IHEP)
SVOM/ECLAIRs team: Marius Brunet, Sebastien Guillot (IRAP)
Report on behalf of the SVOM team:
SVOM/GRM was triggered in-flight by a short burst GRB 251220B (SVOM trigger reference: sb25122004) at 2025-12-20T23:38:04.700 UTC (T0), which is also detected by Fermi/GBM (Fermi GBM Team, GCN#43160).
With the event-by-event data downloaded through the X-band ground station, the GRM light curve shows that this burst consists of a single pulse with a T90 of 0.28 +0.15/-0.10 s in the 15-5000 keV band.
The SVOM/GRM light curve can be found here: https://www.bursthub.cn//admin/static/svgrb251220B.png
In addition, the position of this burst, as determined by Fermi/GBM(RA= 251.4, DEC= 47.9, ERR=6.4), is located at about 135 degrees from the SVOM optical axis, which is outside the ECLAIRs field of view.
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. GRM is developed by the Institute of High Energy Physics (IHEP) of CAS.
The SVOM point of contact for this burst is: Chen-Wei Wang (IHEP)(cwwang@ihep.ac.cn)
GCN Circular #43192
A.S. Kozyrev, D.V. Golovin, M.L. Litvak, I.G. Mitrofanov, and A.B. Sanin on behalf of the HEND/Mars Odyssey team,
A. Ridnaia, D. Frederiks, A. Lysenko, D. Svinkin, and T. Cline on behalf of the Konus-Wind team,
A. Goldstein, M. S. Briggs, C. Wilson-Hodge, and E. Burns on behalf of the Fermi GBM team,
S. Barthelmy, J. Cummings, H. Krimm, D. Palmer, A. Tohuvavohu, and J. DeLaunay on behalf of the Swift-BAT team,
and
W. Boynton, C. Fellows, K. Harshman, H. Enos, R. Starr, and A.S. Gardner on on behalf of the GRS-Odyssey GRB team, report:
The short-duration GRB 251220B (Fermi/GBM detection: The Fermi GBM team, GCN 43160; SVOM/GRM observation: Wang et al., GCN 43171) was detected by Fermi(GBM trigger 787966689), Konus-Wind, Swift(BAT), SVOM (GRM), and Mars-Odyssey (HEND), at about 85085 s UT (23:38:05). The burst was outside the coded field of view of the BAT.
We have triangulated it to a preliminary, 3 sigma error box whose coordinates are:
```
 ---------------------------------------------
  RA(2000), deg                 Dec(2000), deg
 ---------------------------------------------
 Center:
  246.020 (16h 24m 05s) +44.550 (+44d 33' 00")
 Corners:
  242.980 (16h 11m 55s) +43.398 (+43d 23' 51")
  248.478 (16h 33m 55s) +45.405 (+45d 24' 19")
  249.037 (16h 36m 09s) +45.540 (+45d 32' 24")
  243.550 (16h 14m 12s) +43.587 (+43d 35' 13")
 ---------------------------------------------
```
The error box area is 597 sq. arcmin, and its maximum dimension is 4.82 deg (the minimum one is 2.3 arcmin). The Sun distance was 71 deg.
This localization may be improved.
A triangulation map and HEALPix FITS file are posted at http://www.ioffe.ru/LEA/GRBs/GRB251220_T85082/IPN/ The HEALPix triangulation map is the multi-order HEALPix in units of probability density.
The Konus-Wind time history and spectrum will be given in a forthcoming GCN Circular.

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Jochen Greiner, last update: 22-Dec-2025 [Disclaimer]