TITLE: GCN CIRCULAR NUMBER: 39825 SUBJECT: GRB 250321C: Fermi GBM Final Real-time Localization DATE: 25/03/21 21:57:57 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 21:47:36 UT on 21 Mar 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250321C (trigger 764286461.175894 / 250321908).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 89.3, Dec = -28.5 (J2000 degrees, equivalent to J2000 05h 57m, -28d 30'), with a statistical uncertainty of 5.0 degrees.

The angle from the Fermi LAT boresight is 53.0 degrees.

The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250321908/q...

The HEALPix FITS file, including the estimated localization systematic, can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250321908/q...

The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250321908/q...

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