TITLE:   GCN CIRCULAR
NUMBER:  44360
SUBJECT: GRB 260421B: Fermi GBM Final Real-time Localization
DATE:    26/04/21 04:22:36 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 04:13:21 UT on 21 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260421B (trigger 798437606.166588 / 260421176).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 188.8, Dec = -17.8 (J2000 degrees, equivalent to J2000 12h 35m, -17d 48'), with a statistical uncertainty of 2.4 degrees.

The angle from the Fermi LAT boresight is 58.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421176/quicklook/glg_skymap_all_bn260421176.png

The HEALPix FITS file, including the estimated localization systematic, can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421176/quicklook/glg_healpix_all_bn260421176.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421176/quicklook/glg_lc_medres34_bn260421176.gif