TITLE:   GCN CIRCULAR
NUMBER:  41890
SUBJECT: GRB 250919A: Fermi GBM Observation
DATE:    25/09/19 18:56:13 GMT
FROM:    oindabimukherjee@gmail.com

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

"At 00:28:52.28 UT on 19 September 2025, the Fermi Gamma-ray Burst Monitor (GBM)
triggered and located GRB 250919A (trigger 779934537/250919020)
which was also detected by Fermi-LAT ( A. Holzmann Airasca et al. 2025, GCN 41884), 
SVOM-GRM (Chen-Wei Wang et al. 2025, GCN 41882), 
Swift-XRT (J.P. Osborne et al., 2025, GCN 41885), 
EP-WXT (Y.F. Liang et al. 2025, GCN 41879), 
Swift-UVOT (S.R. Oates et al. 2025, GCN 41886),
Gemini/GMOS-S spectroscopic redshift z = 1.145 (A. J. Levan et al. 2025, GCN 41883) 
The Fermi GBM on-ground location is consistent with the position measured by other instruments.

The angle from the Fermi LAT boresight is 56 degrees.

The GBM light curve consists of a very bright emission episode, 
followed by a fainter second emission episode, with a total duration (T90)
of about 129.3 s (50-300 keV). The time-averaged spectrum
from T0+3 to T0+190 s is best fit by
a Band function with Epeak = 476 +/- 6 keV,
alpha = -0.88 +/- 0.01, and beta = -2.39 +/- 0.04.

The event fluence (10-1000 keV) in this time interval is
(4.36 +/- 0.002)E-04 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+33 s in the 10-1000 keV band is 115.2 +/- 0.7 ph/s/cm^2.

The time-averaged spectrum of the main pulse
from T0+3 to T0+58 s is best fit by
a Band function with Epeak = 475 +/- 5 keV,
alpha = -0.7 +/- 0.01, and beta = -2.38 +/- 0.03.
The event fluence (10-1000 keV) in this time interval is
(2.56 +/- 0.01)E-04 erg/cm^2.

The time-averaged spectrum of the second pulse
from T0+111 to T0+190 s is best fit by
a Band function with Epeak = 220 +/- 12 keV,
alpha = -1.12 +/- 0.02, and beta = -1.93 +/- 0.03.
The event fluence (10-1000 keV) in this time interval is
(5.77 +/- 0.05)E-05 erg/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/"