GRB 251220B

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
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 ;
SVOM/GRM observation: Wang et al., GCN 43171
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 )
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.

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
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 ).

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)

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

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