TITLE: GCN CIRCULAR NUMBER: 39724 SUBJECT: GRB 250312B: Fermi GBM Observation DATE: 25/03/15 00:56:42 GMT FROM: Lorenzo Scotton at UAH lscottongcn@outlook.com
L. Scotton (UAH) and C. Meegan (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 20:36:56.64 UT on 12 March 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250312B (trigger 763504621/250312859), which was also localized by Swift/BAT-GUANO (J. DeLaunay et al. 2025, GCN 39708). The Fermi GBM on-ground location was given in GCN 39692 and it is consistent with the Swift/BAT-GUANO localization skymap.
The angle from the Fermi LAT boresight is 105 degrees.
The GBM light curve consists of single emission with two over lapping pulses, with a duration (T90) of about 5.4 s (50-300 keV). The time-averaged spectrum from T0-1.0 to T0+5.5 s is best fit by a power law function with an exponential high-energy cutoff. The power law index is -1.1 +/- 0.1 and the cutoff energy, parameterized as Epeak, is 260 +/- 50 keV.
The event fluence (10-1000 keV) in this time interval is (2.7 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+0.003 s in the 10-1000 keV band is 4.4 +/- 0.3 ph/s/cm^2.
The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog: https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html
For Fermi GBM data and info, please visit the official Fermi GBM Support Page: https://fermi.gsfc.nasa.gov/ssc/data/access/gbm/"
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