[vsnet-grb-info 10398] GRB 110328B: Fermi-LAT Detection

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  11835
SUBJECT: GRB 110328B: Fermi-LAT Detection
DATE:    11/03/29 16:58:08 GMT
FROM:    Vlasios Vasileiou at LUPM/Fermi-LAT  <Vlasios.Vasileiou at lupm.in2p3.fr>

V. Vasileiou (CNRS/IN2P3/LUPM), N. Omodei (Stanford), J. Chiang (SLAC),
G. Vianello (SLAC), D. Kocevski (SLAC), and J. Racusin (NASA/GSFC)
report on behalf of the Fermi-LAT collaboration.

The Fermi-Large Area Telescope (LAT) detected GRB 110328B on 28 March 2011
at 12:29:31 UT, using an automated on-ground analysis. This GRB was also
independently detected by the GBM (von Kienlin, GCN 11831) and INTEGRAL SPI-ACS.

The source is not detectable using our standard 0.1 - 300 GeV dataset.
However, it is detectable if the lower energy threshold is reduced to 50 MeV. Using
this dataset, we obtain a localization of RA, DEC (J2000 deg) = 117.6, 43.2
(07h 50m 24s, 43d 12' 00"), with an error of 1.7 deg (68% CL; statistical),
compatible with the GBM localization. At the trigger time, the localization
was at angle of ~33 degrees from the LAT boresight and ~45 degrees from the Earth's
limb.

Using a non-standard data selection that increases the low energy tens-of-MeV
acceptance at the expense of a greater background, we find that the detected
emission consists of a single pulse starting approximately at the time of the
GBM trigger and lasting ~40 sec. According to preliminary spectral fits
using this non-standard dataset, the spectral index of the detected
emission is -3.31 +-  0.21 (68% CL), steeper than the Band-function beta
reported by the GBM (-1.94 (+0.10/-0.22)). This suggests the presence of
a spectral break between few MeV and few tens-of-MeV energies.

Further analysis is 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.