TITLE:   GCN CIRCULAR
NUMBER:  43173
SUBJECT: GRB 251221B: Fermi GBM Final Real-time Localization
DATE:    25/12/21 18:56:51 GMT
FROM:    Fermi GBM Team at MSFC/Fermi-GBM <do_not_reply@GIOC.nsstc.nasa.gov>

The Fermi GBM team reports the detection of a likely LONG GRB

At 18:46:21 UT on 21 Dec 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 251221B (trigger 788035586.997792 / 251221782).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 274.7, Dec = -21.4 (J2000 degrees, equivalent to J2000 18h 18m, -21d 23'), with a statistical uncertainty of 7.2 degrees.

The angle from the Fermi LAT boresight is 86.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251221782/quicklook/glg_skymap_all_bn251221782.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/bn251221782/quicklook/glg_healpix_all_bn251221782.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn251221782/quicklook/glg_lc_medres34_bn251221782.gif