GRB 251221A
GCN Circular 43234
Subject
GRB 251221A: Swift-BAT refined analysis
Date
2025-12-25T05:09:07Z (9 months ago)
From
Mike Moss at NASA GSFC <mikejmoss3@gmail.com>
Via
Web form
A. Y. Lien (U Tampa), E. Ambrosi (INAF-IASFPA),
S. D. Barthelmy (GSFC), R. Gupta (GSFC),
H. A. Krimm (NSF), S. Laha (GSFC/UMBC),
C. B. Markwardt (GSFC), M. J. Moss (GSFC),
D. M. Palmer (LANL), T. Parsotan (GSFC),
D. Sadaula (GSFC/UMBC), T. Sakamoto (AGU)
(i.e. the Swift-BAT team):
Using the data set from T-240 to T+962 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 251221A (trigger #1426088)
(Ambrosi, et al., GCN Circ. 43166). The BAT ground-calculated position is
RA, Dec = 101.422, 0.738 deg which is
RA(J2000) = 06h 45m 41.4s
Dec(J2000) = +00d 44' 15.8"
with an uncertainty of 1.4 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 42%.
The mask-weighted light curves displays a complicated multi-pulsed light curve.
The T90 (15-350 keV) is 56.86 +- 7.05 sec (estimated error including systematics).
The time-averaged spectrum from T-38.10 to T+27.74 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
1.25 +- 0.13. The fluence in the 15-150 keV band is 2.0 +- 0.2 x 10^-06 erg/cm2.
The 1-sec peak photon flux measured from T+2.91 sec in the 15-150 keV band
is 1.1 +- 0.2 ph/cm2/sec. All the quoted errors are at the 90% confidence
level.
The results of the batgrbproduct analysis are available at
https://swift.gsfc.nasa.gov/results/batgrbcat/BAT_refined_circular/1426088
GCN Circular 43206
Subject
GRB251221A: 7DT optical upper limits
Date
2025-12-23T06:57:34Z (9 months ago)
Edited On
2025-12-23T16:19:59Z (9 months ago)
From
Donggeun Tak <donggeun.tak@gmail.com>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of Donggeun Tak <donggeun.tak@gmail.com>
Via
Web form
Donggeun Tak (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), YoungPyo Hong (SNU ARC/SNU), Hyeonho Choi (SNU ARC/SNU), Donghwan Hyun (SNU ARC/SNU), Gregory S.H. Paek (IfA, SNU ARC/SNU), Seo-Won Chang (SNU ARC/SNU), and Ji Hoon Kim (SNU ARC/SNU) report on behalf of the 7 Dimensional Telescope (7DT) team:
We report optical follow-up observations of GRB251221A (Fermi/GBM GCN 43164; Swift GCN 43166) conducted with the 7-Dimensional Telescope (7DT), located in Chile.
The 7DT follow-up observations began at 06:25:41 UTC on 2025-12-21, corresponding to approximately 1 hour after the trigger. The observations covered the target localization provided by Swift-XRT at RA, Dec = 101.42801 deg, 0.73241 deg, with an uncertainty radius of 2.1 arcseconds.
Initial observations were carried out using 12 telescope units with 20 medium-band filters as well as g-, r-, i-band filters (23 filters in total).
The following table summarizes (the observations and) the derived 5-sigma upper limits:
Filter Mag Mag_err Exposure Date Time
----------------------------------------------------------------------------------------------------
g >20.32 N/A 600s 2025-12-21T06:27:52.833
r >19.54 N/A 600s 2025-12-21T06:34:05.833
i >18.54 N/A 600s 2025-12-21T06:27:56.167
m400 >18.61 N/A 600s 2025-12-21T06:34:09.167
m425 >19.09 N/A 600s 2025-12-21T06:34:04.667
m450 >19.23 N/A 600s 2025-12-21T06:27:50.667
m475 >19.21 N/A 600s 2025-12-21T06:34:05.167
m500 >19.36 N/A 600s 2025-12-21T06:27:57.333
m525 >19.05 N/A 600s 2025-12-21T06:34:08.167
m550 >18.66 N/A 600s 2025-12-21T06:34:12.833
m575 >18.94 N/A 600s 2025-12-21T06:28:00.500
m600 >18.88 N/A 600s 2025-12-21T06:27:58.500
m625 >18.48 N/A 600s 2025-12-21T06:34:30.000
m650 >18.42 N/A 600s 2025-12-21T06:27:52.667
m675 >18.35 N/A 600s 2025-12-21T06:34:26.333
m700 >18.40 N/A 600s 2025-12-21T06:27:55.000
m725 >17.92 N/A 600s 2025-12-21T06:34:13.500
m750 >17.75 N/A 1200s 2025-12-21T06:31:10.667
m775 >17.28 N/A 600s 2025-12-21T06:34:04.000
m800 >17.14 N/A 600s 2025-12-21T06:27:53.333
m825 >16.85 N/A 600s 2025-12-21T06:34:06.000
m850 >16.69 N/A 600s 2025-12-21T06:27:51.667
m875 >16.14 N/A 600s 2025-12-21T06:27:54.167
----------------------------------------------------------------------------------------------------
No significant transient event was identified in the preliminary result. Preliminary analysis yields 5 sigma limiting magnitudes in the range of 16.14 – 20.32 mag depending on sky brightness and airmass.
Photometric flux calibration was performed using synthetic photometry based on the Gaia DR3 XP catalog (Gaia Collaboration et al. 2022) within the AB magnitude system. Note that no extinction correction has been applied.
The 7-Dimensional Telescope (7DT), located in Chile and comprising 20 wide-field telescopes equipped with 40 medium-bandwidth (~25nm) filters, aims to detect optical counterparts of GW sources and conduct the 7-Dimensional Sky Survey (7DS) of the Southern Hemisphere. Further information about the 7DT is available at http://7ds.snu.ac.kr/ and http://gwuniverse.snu.ac.kr/.
GCN Circular 43183
Subject
GRB 251221A: PRIME near-infrared observation
Date
2025-12-22T16:02:42Z (9 months ago)
Edited On
2025-12-22T21:29:47Z (9 months ago)
From
N. Passaleva at Sapienza University of Rome <niccolo.passaleva@uniroma1.it>
Edited By
Vidushi Sharma at NASA GSFC/UMBC <vidushi.sharma@nasa.gov> on behalf of N. Passaleva at Sapienza University of Rome <niccolo.passaleva@uniroma1.it>
Via
Web form
N. Passaleva (U Rome), O. Guiffreda (UMD), M. El Kabir (U Rome), J. Durbak (UMD), E. Troja (U Rome), A. S. Kutyrev (NASA/GSFC), S. B. Cenko (NASA/GSFC)
Following the Fermi/Swift (Fermi GBM team, GCN 43164; Ambrosi et al., GCN 43166) detection, we observed the transient field in J and H filter with PRIME. Observations started ~15.5 hours after the initial trigger.
Using Two Micron All Sky Survey (2MASS) nearby stars for preliminary calibration we detect an uncatalogued source at the position reported by Rakotondrainibe et al. (GCN 43168) and Volnova et al. (GCN 43180). We derived the following magnitudes not corrected for galactic extinction:
J = 20.27 ± 0.09
PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023, Durbak et al. 2024).
We thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations.
GCN Circular 43180
Subject
GRB 251221A: UAFO and Mondy optical upper limit
Date
2025-12-22T12:03:11Z (9 months ago)
From
Alina Volnova at IKI RAS <alinusss@gmail.com>
Via
Web form
A. A. Volnova (IKI), A. Kochergin (UAFO IAA), E. Klunko (ISTP), A. S. Pozanenko (IKI), N. S. Pankov (HSE, IKI) report on behalf of IKI-GRB-FuN:
We observed the field of GRB 251221A (Fermi GBM team, GCN 43164; Ambrosi et al., GCN 43166) with the RC-500 (0.5m) telescope of Ussuriysk Astrophysical Observatory (UAFO) and with the AZT-33IK 1.5m telescope of the Sayan Solar Observatory (Mondy) taking several frames in R-band. The observations started on Dec. 21 at UT 15:18:04 with RC-500 and at UT 17:25:26, i.e., ~ 0.4-0.5 days after the Swift/BAT trigger. In the stacked images in the Swift/XRT error circle (Goad et al., GCN 43175) we do not detect the optical counterpart reported previously by Rakotondrainibe et al. (GCN 43168). Preliminary photometry and observational details are the following:
Date UT start t-T0 Exp. Filter Obj. Err. UL Site/Telescope fwhm"
(mid,days) (n*s) (3sigma)
2025-12-21 15:18:04 0.42900 60*60 R n/d n/d 19.7 UAFO/RC-500 2.7
2025-12-21 17:25:26 0.51314 18*120 R n/d n/d 22.1 Mondy/AZT-33IK 2.6
The photometry is based on several nearby stars from the USNO-B1 catalogue (R2 magnitudes) and is not corrected for the Galactic extinction.
GCN Circular 43179
Subject
GRB 251221A: Swift-XRT refined Analysis
Date
2025-12-22T05:38:12Z (9 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), S. Campana
(INAF-OAB), A. Melandri (INAF-OAR), T. Sbarrato (INAF-OAB), S. Dichiara
(PSU), J.A. Kennea (PSU), D.N. Burrows (PSU) and P.A. Evans report on
behalf of the Swift-XRT team:
We have analysed 4.3 ks of XRT data for GRB 251221A, from 122 s to 56.3
ks after the trigger. The data comprise 8 s in Windowed Timing (WT)
mode (taken while Swift was slewing), with the remainder in Photon
Counting (PC) mode.
The light curve can be modelled with a power-law decay with a decay
index of alpha=1.13 (+0.17, -0.14).
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 1.49 (+0.19, -0.18). The
best-fitting absorption column is 1.11 (+0.24, -0.22) x 10^22 cm^-2,
in excess of the Galactic value of 7.3 x 10^21 cm^-2 (Willingale et al.
2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 6.8 x 10^-11 (9.5 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 1.11 (+0.24, -0.22) x 10^22 cm^-2
Galactic foreground: 7.3 x 10^21 cm^-2
Excess significance: 2.8 sigma
Photon index: 1.49 (+0.19, -0.18)
If the light curve continues to decay with a power-law decay index of
1.13, the count rate at T+24 hours will be 1.9 x 10^-3 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.3 x
10^-13 (1.8 x 10^-13) erg cm^-2 s^-1.
The results of the XRT-team automatic analysis are available at
https://www.swift.ac.uk/xrt_products/01426088.
This circular is an official product of the Swift-XRT team.
GCN Circular 43177
Subject
GRB 251221A: Swift/UVOT Upper Limits
Date
2025-12-22T01:04:18Z (10 months ago)
From
Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>
Via
Web form
M.H. Siegel (PSU) and Ambrosi (INAF-IASFPA) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 251221A 143 s after the BAT trigger (Ambrosi et al., GCN Circ. 43166). No optical afterglow consistent with the XRT position (Goad et al., GCN Circ 43175) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits using the UVOT photometric system(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
white_FC 143 292 147 >20.5
white 143 1541 366 >21.1
v 631 1584 65 >19.6
b 570 1521 45 >19.4
u 301 1497 169 >19.5
w1 680 1647 40 >19.1
m2 655 847 39 >19.5
w2 606 1572 78 >19.6
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 1.088 in the direction of the burst
(Schlegel et al. 1998).
GCN Circular 43175
Subject
GRB 251221A: Enhanced Swift-XRT position
Date
2025-12-21T20:38:47Z (10 months ago)
From
Phil Evans at U of Leicester <pae9@star.le.ac.uk>
Via
email
M.R. Goad, J.P. Osborne, A.P. Beardmore and P.A. Evans (U. Leicester)
report on behalf of the Swift-XRT team.
Using 1499 s of XRT Photon Counting mode data and 2 UVOT
images for GRB 251221A, we find an astrometrically corrected X-ray
position (using the XRT-UVOT alignment and matching UVOT field sources
to the USNO-B1 catalogue): RA, Dec = 101.42771, +0.73217 which is equivalent
to:
RA (J2000): 06h 45m 42.65s
Dec (J2000): +00d 43' 55.8"
with an uncertainty of 2.1 arcsec (radius, 90% confidence).
This position may be improved as more data are received. The latest
position can be viewed at https://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).
This circular was automatically generated, and is an official product of the
Swift-XRT team.
GCN Circular 43174
Subject
GRB 251221A: Fermi GBM Observation
Date
2025-12-21T20:13:03Z (10 months ago)
From
A. Holzmann Airasca at University of Trento and INFN Bari <a.holzmannairasca@unitn.it>
Via
Web form
A. Holzmann Airasca (UniTrento and INFN Bari) and S. Bala (USRA) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:
"At 05:24:28.61 UT on 21 December 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 251221A (trigger 787987473/251221225).
which was also detected by Swift (Ambrosi et al. 2025, GCN 43166).
The Fermi GBM on-ground location is consistent with the Swift position.
The angle from the Fermi LAT boresight is 96 degrees.
The GBM light curve consists of a single peak with a duration (T90)
of about 28 s (50-300 keV). The time-averaged spectrum
from T0-13.3 to T0+23.6 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.1 +/- 0.1 and the cutoff energy,
parameterized as Epeak, is 330 +/- 100 keV.
The event fluence (10-1000 keV) in this time interval is
(4.2 +/- 0.5)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+1.4 s in the 10-1000 keV band is 2.0 +/- 0.2 ph/s/cm^2.
A Band function fits the spectrum equally well
with Epeak= 240 +/- 130 keV, alpha = -1.0 +/- 0.2 and beta = -1.8 +/- 0.2.
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 43168
Subject
GRB 251221A: COLIBRÍ detection of the optical counterpart
Date
2025-12-21T06:43:18Z (10 months ago)
From
Ny Avo Rakotondrainibe at LAM <nyavo.rakotobe@gmail.com>
Via
Web form
Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (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), Enrique Moreno Méndez (UNAM), Kin Ocelotl López (UNAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Nikos Mandarakas (LAM), Margarita Pereyra (UNAM), Benjamin Schneider (LAM), and Antonio de Ugarte Postigo (LAM):
We imaged the field of the Fermi/Swift GRB 251221A (Fermi GBM team, GCN Circ. 43164; Ambrosi et al., GCN Circ. 43166) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2025-12-21 05:25:46 to 06:08 UTC (from 75 seconds to 43 minutes after the trigger) and obtained 1690 seconds of simultaneous exposure in the r and z filters.
The data were reduced and coadded 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.
We detected an uncatalogued, fading source at
RA (J2000) = 06:45:42.73 = 101.42804 deg
Dec (J2000) = +00:43:54.8 = 0.73189 deg
with an uncertainty of 0.5 arcsec and preliminary magnitudes of:
r > 24.2 (3-sigma)
z = 22.26 +/- 0.18
The XRT uncertainty region (Ambrosi et al., GCN Circ. 43166) has a 90% confidence radius of 1.8 arcsec, and this source is 2.3 arcsec from the center. We suggest therefore that this is the optical counterpart of the GRB.
The lack of detection in r is consistent with the high Galactic extinction in this direction of A_V = 3.1 mag (Schlafly et al. 2011).
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.
GCN Circular 43166
Subject
GRB 251221A: Swift detection of a burst
Date
2025-12-21T05:38:42Z (10 months ago)
From
Elena Ambrosi at INAF-IASF <elena.ambrosi@inaf.it>
Via
email
E. Ambrosi (INAF-IASFPA), C. Gronwall (PSU), R. Gupta (NASA GSFC),
A. Melandri (INAF-OAR), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:
At 05:24:31 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 251221A (trigger=1426088). Swift slewed immediately to the burst.
The BAT on-board calculated location is
RA, Dec 101.408, +0.729 which is
RA(J2000) = 06h 45m 38s
Dec(J2000) = +00d 43' 45"
with an uncertainty of 3 arcmin (radius, 90% containment, including
systematic uncertainty). The BAT light curve showed a single-peaked
structure with a duration of about 50 sec. The peak count rate
was ~1300 counts/sec (15-350 keV), at ~1 sec after the trigger.
The XRT began observing the field at 05:26:50.3 UT, 138.4 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 101.42801,
0.73241 which is equivalent to:
RA(J2000) = 06h 45m 42.72s
Dec(J2000) = +00d 43' 56.7"
with an uncertainty of 2.1 arcseconds (radius, 90% containment). This
location is 73 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper.
A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 7.34
x 10^21 cm^-2 (Willingale et al. 2013).
UVOT took a finding chart exposure of 150 seconds with the White filter
starting 142 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag.
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the large, but uncertain, extinction expected.
Burst Advocate for this burst is E. Ambrosi (elena.ambrosi AT inaf.it).
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)
GCN Circular 43165
Subject
Swift GRB251221A: Global MASTER-Net observations report
Date
2025-12-21T05:36:25Z (10 months ago)
Edited On
2025-12-21T20:19:02Z (10 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Edited By
Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of Vladimir Lipunov at Lomonosov Moscow State University <lipunov@sai.msu.ru>
Via
email
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-OAFA robotic telescope [1] located in Argentina (OAFA observatory of San Juan National University) was pointed to the Swift GRB251221.23 (trigger No 1426088,06h 45m 37.92s , +00d 43m 44.4s, R=0.05) errorbox 19 sec after notice time and 67 sec after trigger time at 2025-12-21 05:25:39 UT, with upper limit up to 18.2 mag. The observations began at zenith distance = 32 deg. The sun altitude is -34.1 deg.
The galactic latitude b = -0 deg., longitude l = 212 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=3079192
We obtain a following upper limits.
Tmid-T0 | Site |Filt.| Expt. | Limit| Comment
_________|_____________________|_____|_______|______|________
72 | MASTER-OAFA | C | 10 | 18.2 |
95 | MASTER-OAFA | C | 20 | 17.6 |
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 43164
Subject
GRB 251221A: Fermi GBM Final Real-time Localization
Date
2025-12-21T05:35:02Z (10 months ago)
From
Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>
Via
email
The Fermi GBM team reports the detection of a likely LONG GRB
At 05:24:28 UT on 21 Dec 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251221A (trigger 787987473.613216 / 251221225).
The on-ground calculated location, using the Fermi GBM trigger data, is RA = 96.6, Dec = 1.6 (J2000 degrees, equivalent to J2000 06h 26m, 1d 36'), with a statistical uncertainty of 5.1 degrees.
The angle from the Fermi LAT boresight is 100.0 degrees.
The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251221225/quicklook/glg_skymap_all_bn251221225.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/bn251221225/quicklook/glg_healpix_all_bn251221225.fit
The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251221225/quicklook/glg_lc_medres34_bn251221225.gif