TITLE: GCN CIRCULAR NUMBER: 39635 SUBJECT: GRB 250309B: Fermi GBM Final Localization DATE: 25/03/09 15:47:40 GMT FROM: Padraig McDermott at University College Dublin padraig.mcdermott@ucdconnect.ie

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

"At 07:38:30.66 UT on 09 March 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250309B (trigger 763198715/250309318).

The on-ground calculated location, using the Fermi GBM trigger data, is RA = 210.58, Dec = -10.86 (J2000 degrees, equivalent to J2000 14h 02m, -10d 51'), with a statistical uncertainty of 1.2 degrees.

The angle from the Fermi LAT boresight is 77 degrees.

The skymap can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250309318/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/bn250309318/q...

The GBM light curve can be found here: https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2025/bn250309318/q..."

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