GRB 110328B

A. von Kienlin (MPE) reports on behalf of the Fermi GBM Team: 

"At 12:29:19.19 UT on 28 March 2011, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 110328B (trigger 323008161 / 110328520).

The on-ground calculated location, using the GBM trigger 
data, is RA = 121.1, DEC = 45.8 (J2000 degrees, 
equivalent to 8h 4m, 45d 48'), with an uncertainty 
of 1.2 degrees (radius, 1-sigma containment, 
statistical only; there is additionally a systematic
error which is currently estimated to be 2 to 3 degrees).
 
The angle from the Fermi LAT boresight is 31 degrees.

This burst was also independently detected by INTEGRAL SPI-ACS.

The GBM light curve consists of a single pulse
with a duration (T90) of about 120 s (50-300 keV). 
The time-averaged spectrum from T0+0.003 s to T0+122.882 s is
well fit by a power law function with an exponential
high-energy cutoff.  The power law index is -1.17 (+0.04/-0.04) and
the cutoff energy, parameterized as Epeak, is 538 (+96/-70) keV.

The event fluence (10-1000 keV) in this time interval is 
(2.6 +/- 0.1)E-05 erg/cm^2. The 1.024-sec peak photon flux measured 
starting from T0+4.93  s in the 10-1000 keV band 
is 5.8 +/- 0.3 ph/s/cm^2.

A Band function fits the spectrum equally well with 
Epeak = 369 (+115/-69) keV, alpha = -1.11 +/-0.06
and beta =  -1.94 (+0.10/-0.22).

The spectral analysis results presented above are preliminary; 
final results will be published in the GBM GRB Catalog."

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
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 ) 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@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.