TITLE:   GCN CIRCULAR
NUMBER:  44369
SUBJECT: GRB 260421C: Fermi GBM Final Real-time Localization
DATE:    26/04/21 08:35:58 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 08:25:19 UT on 21 Apr 2026, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 260421C (trigger 798452724.59066 / 260421351).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 295.0, Dec = 64.2 (J2000 degrees, equivalent to J2000 19h 40m, 64d 12'), with a statistical uncertainty of 3.1 degrees.

The angle from the Fermi LAT boresight is 59.0 degrees.

The skymap can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421351/quicklook/glg_skymap_all_bn260421351.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/bn260421351/quicklook/glg_healpix_all_bn260421351.fit

The GBM light curve can be found here:
https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2026/bn260421351/quicklook/glg_lc_medres34_bn260421351.gif