TITLE: GCN CIRCULAR NUMBER: 40030 SUBJECT: GRB 250402A: Fermi GBM Observation DATE: 25/04/03 17:29:10 GMT FROM: Matt Godwin msg0028@uah.edu
Jacob Smith (UAH), Utkarsh Pathak(IIT Bombay), and Matt Godwin (UAH) report on behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 23:08:09.09 UT on 02 April 2025, the Fermi Gamma-ray Burst Monitor (GBM) triggered and located GRB 250402A (trigger 765328094/250402964). which was also detected by SVOM (Chen et al. 2025, GCN 40014). The Fermi GBM on-ground location is consistent with the SVOM position.
The angle from the Fermi LAT boresight is 57 degrees.
The GBM light curve consists of a double peak with a duration (T90) of about 44 s (50-300 keV). The time-averaged spectrum from T0-5.1 to T0+45.1 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 115 +/- 10 keV.
The event fluence (10-1000 keV) in this time interval is (4.1 +/- 0.2)E-06 erg/cm^2. The 1-sec peak photon flux measured starting from T0+23 s in the 10-1000 keV band is 2.4 +/- 0.2 ph/s/cm^2.
A Band function fits the spectrum equally well with Epeak= 60 +/- 10 keV, alpha = -0.4 +/- 0.3 and beta = -2.1 +/- 0.1.
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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