TITLE:   GCN CIRCULAR
NUMBER:  33652
SUBJECT: GRB 230418A: Fermi GBM Observation
DATE:    23/04/19 03:51:00 GMT
FROM:    Oliver J Roberts at USRA/NASA <oliver.roberts@nasa.gov>

O.J. Roberts (USRA/NASA-MSFC) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 21:11:33.49 UT on 18 April 2023, the Fermi Gamma-Ray Burst Monitor (GBM)
triggered and located GRB 230418A (trigger 703545098 / 230418883).
The on-ground calculated location, using the Fermi GBM trigger data,
is RA = 47.4, Dec = -12.5 (J2000 degrees, equivalent to
J2000 03h 10m, -12d 30'), with a statistical uncertainty of 5.9 degrees
(radius, 1-sigma containment, statistical only; there is additionally a systematic error
which we have characterized as a core-plus-tail model, with 90% of GRBs having a
3.7 deg error and a small tail suffering a larger than 10 deg systematic error.
[Connaughton et al. 2015, ApJS, 216, 32] ).

The angle from the Fermi LAT boresight at the GBM trigger time is 71 degrees.

The GBM light curve consists of single burst
with a duration (T90) of about 0.25 s (50-300 keV).
The time-averaged spectrum from T0-0.064 s to T0+0.256 s is
best fit by a power law function with an exponential
high-energy cutoff.  The power law index is -0.22 +/- 0.08 and
the cutoff energy, parameterized as Epeak, is 838 +/- 71 keV.

The event fluence (10-1000 keV) in this time interval is
(2.51 +/- 0.08)E-06 erg/cm^2. The 64 msec peak photon flux measured
starting from T0+0.13 s in the 10-1000 keV band
is 31 +/- 1 ph/s/cm^2.

A Band function fits the spectrum equally well with Epeak = 836 +/- 75 keV,
alpha = -0.22 +/- 0.08, and beta = -3.9+/- 1.9.

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/"