[vsnet-grb-info 14158] GRB 131108A: Fermi-LAT refined analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  15472
SUBJECT: GRB 131108A: Fermi-LAT refined analysis
DATE:    13/11/09 11:04:27 GMT
FROM:    Giacomo Vianello at SLAC  <giacomov at slac.stanford.edu>

G. Vianello (Stanford), N.Omodei (Stanford), J.Racusin (NASA/GSFC) and M.
Ohno (Hiroshima) report on behalf of the Fermi LAT team:

We have analyzed 1.5 ks of data for GRB 131108A which triggered onboard
Fermi/LAT (Racusin et al, GCN 15464) as well as Fermi/GBM.

We confirm the detection of the GRB in our standard analysis at the
position of the afterglow candidate (Stroh and Kennea, GCN 15468; Gorosabel
et al., GCN 15469; de Ugarte Postigo et al., GCN 15470), with a very large
significance (>> 10 sigma).

As seen by Fermi/LAT the burst is bright, but relatively soft. In this
preliminary analysis the light curve appears well correlated with the
emission at lower energy, as seen by Fermi/GBM.
More than 118 photons above 100 MeV and more than 3 photons above 1 GeV are
observed within 1500 seconds. The highest energy photon is a 1.5 GeV event
which is observed 66 seconds after the GBM trigger, which at redshift 2.4
(GCN 15470) corresponds to 5 GeV rest-frame. The time-integrated spectrum
in the same time interval can be well described by a power law with photon
index -2.66 +/- 0.12, with a mean flux of 1.0e-08 +/- 1.3e-09 erg/cm2/s
(100 MeV - 100 GeV).

 Fermi-LAT pointed observations are ongoing.

The Fermi LAT point of contact for this burst is Masanori Ohno (
ohno at hep01.hepl.hiroshima-u.ac.jp).

The Fermi LAT is a pair conversion telescope designed to cover the energy
band from 20 MeV to greater than 300 GeV. It is the product of an
international collaboration between NASA and DOE in the U.S. and many
scientific institutions across France, Italy, Japan and Sweden.