TITLE:   GCN CIRCULAR
NUMBER:  23263
SUBJECT: GRB 180923B: Fermi GBM detection
DATE:    18/09/24 19:51:56 GMT
FROM:    Oliver J Roberts at USRA/NASA  <oliver.roberts@nasa.gov>

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

"At 22:20:30.53 UT on 23 September 2018, the Fermi 
Gamma-Ray Burst Monitor triggered and located 
GRB 180923B (trigger 559434035 / 180923931).
The on-ground calculated location, using the GBM trigger
data, is

RA, Dec = 310.6, -0.7 (J2000 degrees)

with an uncertainty of 1.0 degree (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 initial angle from the Fermi LAT boresight to
the best location is 86 degrees.

The GBM light curve shows/consists of two bright peaks
with a duration (T90) of about 8.2 s (50-300 keV).
The time-averaged spectrum from T0+0 s to T0+8.9 s is
best fit by a Band function with Epeak = 44.5 +/- 0.5,
alpha index = -0.77 +/- 0.03 and beta index = -3.02 +/- 0.05.

The event fluence (10-1000 keV) in this time interval is
(1.880 +/- 0.003)E-05 erg/cm^2. The 1s peak photon flux 
measured starting from T0+4.0 s in the 10-1000 keV band
is 53 +/- 1 ph/s/cm^2.

The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."

[GCN OPS NOTE(24sep18): Based on CIrcular 23265, the name
of the GRB used in this circular (23263) was changed
from "180923A" to "180923B".]