[vsnet-grb-info 10766] GRB 110721A: Fermi-LAT detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  12188
SUBJECT: GRB 110721A: Fermi-LAT detection
DATE:    11/07/21 15:09:33 GMT
FROM:    Vlasios Vasileiou at LUPM/Fermi-LAT  <Vlasios.Vasileiou at lupm.in2p3.fr>

V. Vasileiou (CNRS/IN2P3/LUPM), F. Piron (CNRS/IN2P3/LUPM), D. Tierney (UCD), A. 
von Kienlin (MPE), S. Guiriec (UAH), and J. L. Racusin (NASA/GSFC) report on 
behalf of the Fermi-LAT team:

At 04:47:45 on July 21, 2011, Fermi-LAT detected high energy
emission from GRB 110721A, which was also detected by Fermi-GBM (trigger 
332916465, Tierney et al. (GCN 12187)), and Integral (SPI ACS
trigger 6326).

The best preliminary LAT on-ground location is found to be (RA, DEC)=
(333.4,-39.0) (J2000; deg) with an error radius of 0.75 (0.51) deg
at 90% (68%) containment (statistical error only), which was 40 deg from the LAT 
boresight at the time of the trigger. We further report that
the Fermi Observatory executed an autonomous repoint maneuver
to follow this trigger for the next 2.5 hours, subject to
Earth-angle constraints.

The data from the Fermi-LAT show a significant increase in the
event rate that is spatially and temporally correlated with the
GBM emission with high significance. This detection was independently
confirmed by the blind on-ground GRB search of the LAT data. More
than 20 (1) photons above 100 MeV (1 GeV) using a standard event
selection and about 1000 photons using a non-standard event
selection sensitive mostly to tens-of-MeV energies are observed
within 20 sec. The lightcurve at tens-of-MeV energies has a Fast Rise 
Exponential Decay (FRED) profile with an ~16 sec duration
similar to the GBM results. The highest energy event has an 1.7 GeV
energy, was observed about 0.7 sec after the trigger, and was
associated to the GRB with a high (>0.9) probability.

Preliminary spectral analyses on the LAT data taken during the
first 20 sec after the trigger resulted in a -2.9+-0.4 spectral index
and a 3.3+-0.8 x 10^-4 ph/cm^2/sec average flux integrated over E>100 MeV. This
spectral index is considerably softer than the extrapolation of the GBM spectral 
index at E>100 MeV energies (Tierney et al. (GCN 12187)), which suggests a 
spectral cutoff or softening of the spectrum.

A Swift Target of Opportunity observation has been requested and tiling 
observations are ongoing.

The Fermi LAT point of contact for this burst is Vlasios Vasileiou
(vlasios.vasileiou at lupm.in2p3.fr).

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.