[vsnet-grb-info 20024] GRB 171108A: Fermi GBM detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22112
SUBJECT: GRB 171108A: Fermi GBM detection
DATE:    17/11/09 05:41:13 GMT
FROM:    Rachel Hamburg at UAH  <rkh0007 at uah.edu>

R. Hamburg (UAH) and C. Meegan (UAH)
report on behalf of the Fermi GBM Team:

"At 15:44:46.50 UT on 8 November 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 171108A (trigger 531848691 / 171108656).
The on-ground calculated location, using the GBM trigger
data, is RA = 115.0, DEC = +39.8 (J2000 degrees,
equivalent to 07 h 40 m, 39 d 48'), with an uncertainty
of 4.7 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]).

Although this GRB is soft in nature, the trigger resulted in an
Autonomous Repoint Request (ARR) by the GBM Flight Software
owing to high peak flux in the background before trigger time.
This ARR was accepted and the spacecraft slewed to the GBM in-flight
location. The initial angle from the Fermi LAT boresight to
the best GBM ground location is 84 degrees.

The GBM light curve shows a bright peak
with a duration (T90) of about 0.03 s (50-300 keV).
The time-averaged spectrum from T0-0.064 s to T0+0.064 s is
adequately fit by a power law function with an exponential
high-energy cutoff and a blackbody. The power law index is -1.6 +/- 0.4,
the cutoff energy, parameterized as Epeak, is 47 +/- 28 keV,
and the temperature is 18.0 +/- 3.0 kT.

The event fluence (10-1000 keV) in this time interval is
(1.0 +/- 0.1)E-05 erg/cm^2. The 0.064-sec peak photon flux measured
starting from T0-0.064 s in the 10-1000 keV band
is 687 +/- 166 ph/s/cm^2.

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