[vsnet-grb-info 19763] GRB 170831A: Fermi GBM detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  21777
SUBJECT: GRB 170831A: Fermi GBM detection
DATE:    17/08/31 17:05:23 GMT
FROM:    Elisabetta Bissaldi at U.Innsbruk/IAPP  <Elisabetta.Bissaldi at uibk.ac.at>

E. Bissaldi (Politecnico & INFN Bari)
reports on behalf of the Fermi GBM Team:


"At 04:18:11.13 UT on 31 August 2017, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 170831A (trigger 525845896 / 170831179).

The on-ground calculated location, using the GBM trigger
data, is

RA = 158.7, DEC = +49.8

(J2000 degrees, equivalent to 10h 34m, 49d 48'), with an uncertainty
of 1.6 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 trigger resulted in an Autonomous Repoint Request (ARR)
by the GBM Flight Software owing to the high peak flux
of the GRB. This ARR was accepted and the spacecraft slewed to the GBM in-flight
location. The initial angle from the Fermi LAT boresight to
the GBM ground location is 43 degrees.

The GBM light curve consists of multiple peaks
with a duration (T90) of about 60 s (50-300 keV).
The time-averaged spectrum from T0-1.5 s to T0+55 s is
adequately fit by a Band function with Epeak = 90 +/- 5 keV,
alpha = -1.32 +/- 0.03, and beta = -2.52 +/- 0.12.

The event fluence (10-1000 keV) in this time interval is
(2.03 +/- 0.05)E-05 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+44 s in the 10-1000 keV band
is 23.8 +/- 0.4 ph/s/cm^2.


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