GRB 251209A

G. Waratkar (Caltech) and B. Grefenstette (Caltech) report on behalf of the NuSTAR Search for INteresting Gamma-ray Signals (SINGS) working group:

The NuSTAR SINGS working group reports the detection of prompt emission from the long-duration GRB 251209A in both the NuSTAR CsI anti-coincidence shields. This GRB was identified through a blind search using the CsI shield rates. Details of the search algorithm will be described in a future paper.

The NuSTAR SINGS algorithm triggered at 2025-12-09 13:24:26 (with a resolution ~5-seconds). This is consistent with the detections of GRB 251209A by Fermi/GBM (Fermi GBM Team, GCN Circ. 43048
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 ) and AstroSat/CZTI (Tembhurnikar et al., GCN Circ. 43066
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 ). The NusTAR trigger is consistent with triggering on the longer-duration “main burst” as seen in the Fermi-GBM lightcurves.

The NuSTAR CsI shield data are recorded at 1 Hz. We detect two peaks lasting for ~30-s, consistent with the detections of Fermi/GBM & AstroSat/CZTI.  A fainter, narrower initial burst at 2025-12-09 13:24:03 is also seen by eye in the NuSTAR lightcurves but did not trigger the automated algorithm. The peak count rate is ~3000-cps over a baseline rate of ~1,000-cps during this time period. 

The Fermi/GBM localization (GCN Circ. 43048
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 ) at RA =  214.2, Dec = 43.1 implies an offset from the NuSTAR boresight of 92-deg (i.e. from the side of the instrument) and an offset from the geocenter of 120-deg.

Lightcurves and analysis for this GRB can be found here:
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/reports/2025/251209A  

Information on NuSTAR SINGS can be found here: 
https://nustarsoc.caltech.edu/NuSTAR_Public/grbs/   

NuSTAR is a NASA Small Explorer mission led by Caltech and managed by JPL for NASA's Science Mission Directorate in Washington.

E. Bissaldi (Politecnico and INFN Bari) 
reports on behalf of the Fermi GBM Team:


"At 13:24:03.99 UT on 9 December 2025, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 251209A (trigger 786979448 / 251209558).
The Fermi GBM Final Real-time Localization is reported in GCN 43048
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 .

The angle from the Fermi LAT boresight at the GBM trigger time is 52 degrees.

The GBM light curve consists of several structured emission episodes
with a duration (T90) of about 57 s (50-300 keV).
The time-averaged spectrum from T0-4 s to T0+81 s is
best fit by a Band function with Epeak = 105 +/- 4 keV,
alpha = -1.63 +/- 0.01, and beta = -2.37 +/- 0.06.

The event fluence (10-1000 keV) in this time interval is
(6.89 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+49.7 s in the 10-1000 keV band
is 38.4 +/- 0.6 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/"

M. Tembhurnikar (IUCAA), S. Salunke (IUCAA), Harsha K. H. (IUCAA), A. Arya (IITB), A. Goyal (IITB), G. Waratkar (Caltech/IITB), A. Vibhute (IUCAA), V. Bhalerao (IITB), D. Bhattacharya (Ashoka University/IUCAA), A. R. Rao (IUCAA/TIFR), and S. Vadawale (PRL) report on behalf of the AstroSat CZTI collaboration:

Analysis of AstroSat CZTI data with the CIFT framework (Sharma et al., 2021, JApA, 42, 73) showed the detection of a long, bright GRB 251209A which was also detected by Fermi GBM (Fermi GBM Team, GCN Circ. 43048
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 ).

The source was clearly detected in the CZT detectors in the 20-200 keV energy range. The light curve peaks at 2025-12-09 13:24:31.50 UTC. The measured peak count rate associated with the burst is 358 (+38, -34) counts/s above the background in the combined data of three quadrants (out of four), with a total of 5908 (+328, -277) counts. The local mean background count rate was 208 (+1, -2) counts/s. Using cumulative rates, we measure a T90 of 38 (+4, -3) s. In the preliminary analysis, we find 594 Compton events associated with this event.

The source was also clearly detected in the CsI anticoincidence (Veto) detector in the 100-500 keV energy range. The light curve peaks at 2025-12-09 13:24:35.75 UTC. The measured peak count rate associated with the burst is 435 (+68, -32) counts/s above the background in the combined data of all quadrants, with a total of 6675 (+484, -624) counts. The local mean background count rate was 1160 (+3, -3) counts/s. We measure a T90 of 33 (+5, -3) s from the cumulative Veto light curve.

CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, URSC, IUCAA, SAC, and PRL. The Indian Space Research Organisation funded, managed, and facilitated the project.

CZTI GRB detections are reported regularly on the payload site at:
http://astrosat.iucaa.in/czti/?q=grb

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 Fermi GRB 251209A ( Fermi GBM team, GCN 43048
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 ) errorbox  4838 sec after notice time and 4882 sec after trigger time at 2025-12-09 14:45:26 UT, with upper limit up to  15.1 mag. Observations started at twilight.  The observations began at zenith distance = 80 deg. The sun  altitude  is -12.3 deg. 

The galactic latitude b = 66 deg., longitude l = 80 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=3065758

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

    4912 | 2025-12-09 14:45:26 |   MASTER-Kislovodsk | (14h 17m 04.16s , +45d 00m 13.7s) |   C |    60 | 15.1 |        
    4912 | 2025-12-09 14:45:26 |   MASTER-Kislovodsk | (14h 22m 26.41s , +44d 40m 31.0s) |   C |    60 | 15.0 |        
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 LONG GRB

At 13:24:03 UT on 9 Dec 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251209A (trigger 786979448.987734 / 251209558).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 214.2, Dec = 43.1 (J2000 degrees, equivalent to J2000 14h 16m, 43d 06'), with a statistical uncertainty of 1.0 degrees.

The angle from the Fermi LAT boresight is 52.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251209558/quicklook/glg_skymap_all_bn251209558.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/bn251209558/quicklook/glg_healpix_all_bn251209558.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251209558/quicklook/glg_lc_medres34_bn251209558.gif