Publications (2)0 Total impact
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ABSTRACT: Launched on June 11, 2008, the LAT instrument onboard the $Fermi$ Gamma-ray
Space Telescope has provided a rare opportunity to study high energy photon
emission from gamma-ray bursts. Although the majority of such events (27) have
been iden tified by the Fermi LAT Collaboration, four were uncovered by using
more sensiti ve statistical techniques (Akerlof et al 2010, Akerlof et al 2011,
Zheng et al 2 012). In this paper, we continue our earlier work by finding
three more GRBs ass ociated with high energy photon emission, GRB 110709A,
111117A and 120107A. To s ystematize our matched filter approach, a pipeline
has been developed to identif y these objects in near real time. GRB 120107A is
the first product of this anal ysis procedure. Despite the reduced threshold
for identification, the number of GRB events has not increased significantly.
This relative dearth of events with low photon number prompted a study of the
apparent photon number distribution. W e find an extremely good fit to a simple
power-law with an exponent of -1.8 $\pm $ 0.3 for the differential
distribution. As might be expected, there is a substa ntial correlation between
the number of lower energy photons detected by the GBM and the number observed
by the LAT. Thus, high energy photon emission is associ ated with some but not
all of the brighter GBM events. Deeper studies of the pro perties of the small
population of high energy emitting bursts may eventually yi eld a better
understanding of these entire phenomena.
03/2012;
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ABSTRACT: Since the launch of the Fermi gamma - ray Space Telescope on June 11, 2008,
the LAT instrument has solidly detected more than 20 GRBs with high energy
photon emission above 100 MeV. Using the matched filter technique, 3 more GRBs
have also shown evidence of correlation with high energy photon emission as
demonstrated by Akerlof et al. In this paper, we present another GRB
unambiguously detected by the matched filter technique, GRB 081006A. This event
is associated with more than 13 high energy photons above 100 MeV. The
likelihood analysis code provided by the $Fermi$ Science Support Center (FSSC)
generated an independent verification of this detection by comparison of the
Test Statistics (TS) value with similar calculations for random LAT data
fields. We have performed detailed temporal and spectral analysis of photons
from 8 keV up to 0.8 GeV from the GBM and the LAT. The properties of GRB
081006A can be compared to the other two long duration GRBs detected at similar
significance, GRB 080825C and GRB 090217A. We find that GRB 081006A is more
similar to GRB 080825C with comparable appearances of late high energy photon
emission. As demonstrated previously, there appears to be a surprising dearth
of faint LAT GRBs, with only one additional GRB identified in a sample of 74
GRBs. In this unique period when both $Swift$ and $Fermi$ are operational,
there is some urgency to explore this aspect of GRBs as fully as possible.
01/2012;
