TITLE:   GCN CIRCULAR
NUMBER:  39726
SUBJECT: GRB 250313A: Fermi GBM Detection
DATE:    25/03/15 02:17:47 GMT
FROM:    eliza.neights@gmail.com

Eliza Neights (George Washington University, NASA Goddard Space Flight Center), M. Godwin (UAH), C. Meegan (UAH) and O.J. Roberts (USRA) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 14:34:33.69 UT on 13 March 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250313A (trigger 763569278/250313607), which was also detected by Fermi LAT (Di Lalla et al. 2025, GCN 39711), AstroSat CZTI (Dasgupta et al. 2025, GCN 39714), 
SVOM/GRM (Wang et al. 2025, GCN 39716), and Konus-Wind (Frederiks et al. 2025, GCN 39721).

The Fermi GBM Final Position location is consistent with the Fermi LAT position.

The angle from the Fermi LAT boresight is 4 degrees.

The GBM light curve consists of two emission episodes, a bright pulse lasting 12 s after the trigger time and a dimmer pulse about 280 s later. The duration (T90)
is about 295.2 s (50-300 keV). The time-averaged spectrum
from T0-2.2 to T0+389.0 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.36 +/- 0.01 and the cutoff energy,
parameterized as Epeak, is 320 +/- 20 keV.

The event fluence (10-1000 keV) in this time interval is
(9.4 +/- 0.1)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+4.7 s in the 10-1000 keV band is 55.4 +/- 0.5 ph/s/cm^2.

The spectrum is fit about equally well by a Band function with Epeak = 280 +/- 20 keV, alpha = -1.34 +/- 0.02, and beta = -2.23 +/- 0.13.

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/"