[vsnet-grb-info 11594] GRB 120226A: Fermi-LAT detection of a burst

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  12989
SUBJECT: GRB 120226A: Fermi-LAT detection of a burst
DATE:    12/02/27 13:53:07 GMT
FROM:    Vlasios Vasileiou at LUPM/Fermi-LAT  <Vlasios.Vasileiou at lupm.in2p3.fr>

V. Vasileiou (CNRS/IN2P3/LUPM) and J. L. Racusin (NASA/GSFC) report on behalf of
the Fermi-LAT team:

Fermi-LAT has detected high energy emission from GRB 120226A in ground analysis.
The GRB was triggered on by Fermi-GBM at 20:54:17.03 on Febuary 26, 2012
(trigger 351982459).

The GRB was significantly detected during the prompt phase only using a
non-standard LAT data selection most sensitive in the tens-of-MeV energy range,
with which over ~50 counts above background were detected within a 60 s interval
coinciding with the time of the GBM emission. This data selection has
insufficient spatial resolution to provide a reliable LAT localization.

However, the GRB became detectable with the standard LAT analysis techniques
only after data of longer time scales were accumulated. A preliminary
maximum-likelihood analysis of the E>100MeV P7SOURCE_V6 LAT data generated
during the first 1.9 ks after the GBM trigger (until the GRB became occulted by
the Earth) revealed a source with a ~4 sigma statistical significance,
corresponding to ~14 excess events. The source spectrum is best fit with a power
law of index -1.6+-0.3. Using this analysis, we obtained the best LAT on-ground
localization of:
RA(J2000) = 305.30 deg
Dec(J2000) = 40.75 deg
with an error radius 0.36 deg (90% containment, statistical error only), which
is 4 deg from the best GBM localization and ~37 deg with respect to the LAT
boresight at the time of the trigger. The GRB triggered an autonomous repoint of
the Fermi spacecraft bringing our best localization to ~12 deg from the LAT
boresight for 1.9 ks after the trigger. The burst position is close to the
Galactic plane at (l,b) = 78.5, 2.3 deg.

A Swift TOO has been requested.

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.