TITLE:   GCN CIRCULAR
NUMBER:  40422
SUBJECT: GRB 250509A: Fermi GBM Observation
DATE:    25/05/10 16:51:18 GMT
FROM:    Ava Myers at NASA GSFC <ava.myers@nasa.gov>

A. Myers (NPP/GSFC), C. Meegan (UAH) and A. Holzmann Airasca (UniTrento and INFN Bari) report on behalf of
the Fermi Gamma-ray Burst Monitor Team:

"At 22:33:29.29 UT on 09 May 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250509A (trigger 768522814/250509940).
which was also detected by Swift BAT (M. H. Siegel et al. 2025, GCN 40407).
The Fermi GBM on-ground location is consistent with the Swift BAT position.

The angle from the Fermi LAT boresight is 89 degrees.

The GBM light curve consists of main emission episode with a duration (T90)
of about 69 s (50-300 keV). The time-averaged spectrum
from T0+0.000 to T0+69.120 s is best fit by
a power law function.
The power law index is -2.19 +/- 0.03.

The event fluence (10-1000 keV) in this time interval is
(6.18 +/- 0.29)E-06 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+6.2 s in the 10-1000 keV band is 3.6 +/- 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: