TITLE:   GCN CIRCULAR
NUMBER:  21528
SUBJECT: LIGO/Virgo G298048 - Update on Fermi/GBM GRB 170817A Analysis
DATE:    17/08/18 00:36:12 GMT
FROM:    Adam Goldstein at Fermi/GBM  <adam.michael.goldstein@gmail.com>

A. Goldstein (USRA), P. Veres (UAH), and A. von Kienlin (MPE) report on
behalf of the GBM-LIGO Group:
L. Blackburn (CfA), M. S. Briggs (UAH), J. Broida (Carleton College), E.
Burns (NASA/GSFC), J. Camp (NASA/GSFC), T. Dal Canton (NASA/GSFC), N.
Christensen (Carleton College), V. Connaughton (USRA), R. Hamburg (UAH), C.
M. Hui (NASA/MSFC), P. Jenke (UAH), D. Kocevski (NASA/MSFC), N. Leroy
(LAL), T. Littenberg (NASA/MSFC), J. McEnery (NASA/GSFC), R. Preece (UAH),
J. Racusin (NASA/GSFC), P. Shawhan (UMD), K. Siellez (GATech), L. Singer
(NASA/GSFC), J. Veitch (Glasgow), C. Wilson-Hodge (NASA/MSFC)


The GBM trigger (Connaughton et al., LVC GCN 21506), which we determine to
be a short GRB, occurred about 2 minutes before Fermi entered the South
Atlantic Anomaly. To estimate the False Alarm Rate (FAR) of a chance
coincident short GRB without using spatial information, we estimate the
rate of short GRBs (defined as t90 <= 2 s) triggered by GBM.  GBM has a
total of 351 triggered short GRBs over an estimated triggering livetime of
3.23e8 s. This results in a FAR of one per 10.7 days. A simple estimate of
the False Alarm Probability using only the temporal information of the two
signals is 2.2e-6 (~4.5 sigma).

The initial public report of the GRB (von Kienlin et al., GCN 21520)
contained the spectral information using the maximum of the GBM
localization posterior. Using the maximum of the updated LIGO/Virgo
localization posterior (LVC, LVC GCN 21513), which is consistent with the
GBM localization, we report the updated preliminary spectral information.

The estimated GRB duration is ~ 2 s and is best fit by a power-law function
with an exponential high-energy cutoff from T0-0.75 to T0+1.25 s.  The
power-law index is -0.88 +/- 0.44 and the cutoff energy, parametrized as
Epeak, is 128.0 +/- 48.7 keV. The event fluence (10-1000 keV) in this time
interval is (2.2 +/- 0.5)e-7 erg/cm^2, which we estimate to be near the
58th percentile of GBM-triggered short GRB fluences. The 64 ms peak photon
flux estimated starting at T0 is 3.6 +/- 1.1 ph/s/cm^2 (10-1000 keV).  The
corresponding spectrum over this peak flux interval is fit by an
exponential high-energy power law with index = -0.29 +/- 1.01 and Epeak =
124.2 +/- 52.6 keV.  The event peak energy flux (10-1000 keV) is (7.3 +/-
2.4)e-7 erg/s/cm^2.

Using this spectral information and the distance information provided by
the LVC (40 +/- 8 Mpc) we estimate that the isotropic energy release in
gamma-rays, Eiso ~ 4.3e46 erg in the 1 keV-10 MeV energy range and the
isotropic peak luminosity, Liso ~1.4e47 erg/s.

These results are preliminary and may be updated with further analysis.