TITLE:   GCN CIRCULAR
NUMBER:  40067
SUBJECT: GRB 250404A: Fermi GBM Detection
DATE:    25/04/04 19:52:20 GMT
FROM:    oindabimukherjee@gmail.com

O. Mukherjee (USRA) and C. Meegan (UAH) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 14:19:46.35 UT on 04 April 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250404A (trigger 765469191/250404597).

The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 123.37, Dec = 34.10 (J2000 degrees, equivalent to
J2000 8h 13m, +34d 6'), with a statistical uncertainty of 1.20 degrees.
(radius, 1-sigma containment, statistical only; there is additionally a
systematic error which we have characterized as a mixture of two Gaussians,
one with a radius of 1.8 degrees (52% contribution) and one with a radius
of 4.1 degrees (47% contribution) [A. Goldstein et al. 2020, ApJ, 895, 1]).

The angle from the Fermi LAT boresight is 42 degrees.

The GBM light curve consists of multiple peaks with a duration (T90)
of about 91.4 s (50-300 keV). The time-averaged spectrum
from T0-3.6 to T0+124.4 s is best fit by
a power law function with an exponential high-energy cutoff.
The power law index is -1.10 +/- 0.02 and the cutoff energy,
parameterized as Epeak, is 55.7 +/- 0.5 keV.

The event fluence (10-1000 keV) in this time interval is
(3.33 +/- 0.02)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+21 s in the 10-1000 keV band is 12.9 +/- 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/"