TITLE:   GCN CIRCULAR
NUMBER:  43164
SUBJECT: GRB 251221A: Fermi GBM Final Real-time Localization
DATE:    25/12/21 05:35:02 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 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