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GCN Circular 12100

Subject
GRB 110625A: Fermi GBM and LAT observations
Date
2011-06-27T21:24:51Z (13 years ago)
From
David Gruber at MPE <dgruber@mpe.mpg.de>
David Gruber (MPE), Nicola Omodei (Stanford U.), Vandiver Chaplin (UAH), 
J. Chiang (KIPAC/SLAC), J. McEnery (NASA/GSFC), J. L. Racusin (NASA/GSFC) 
report on behalf of the Fermi GBM and LAT Teams: 

At 21:08:18.24 UT on 25 June 2011, the Fermi Gamma-Ray Burst Monitor 
triggered and located GRB 110625A (trigger 330728900 / 110625881), which 
was also detected by the Swift/BAT (Page et al. 2011, GCN 12088). The GBM 
on-ground location is consistent with the Swift position.

The angle from the Fermi LAT boresight is 88 degrees at the trigger time. 
Moreover, this burst was bright enough to result in a Fermi spacecraft 
autonomous rapid repoint (ARR) maneuver.

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

The GBM light curve shows several pulses with a duration (T90) of about 
27.7 +/- 1.4 s (50-300 keV). The time-averaged spectrum from T0-2.048 s 
to T0+60.417 s is best fit by a power law function with an exponential 
high-energy cutoff.  The power law index is -0.95 +/- 0.01 and the cutoff 
energy, parameterized as Epeak, is 209 +/- 3 keV.

The event fluence (10-1000 keV) in this time interval is 
(6.71 +/- 0.02)E-05 erg/cm^2. The 1-sec peak photon flux measured starting 
from T0+23.9 s in the 10-1000 keV band is 77.3 +/- 0.7 ph/s/cm^2.

A Band function fits the spectrum equally well with Epeak= 175 +/- 5 keV, 
alpha = -0.85 +/- 0.02 and beta = -2.37 +/- 0.05. 

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

The ARR maneuver put the GRB inside the Fermi/LAT field of view 
for several hundred seconds (from ~100-600 seconds after the GBM trigger).

The Spacecraft continued its maneuver toward the Fermi GBM flight software 
reconstructed position, that was off by 68 degrees from the Enhanced Swift/XRT 
position (GCN 12092), providing non-optimal exposure for LAT follow-up 
observations.

We analyzed the LAT data and we see significant high-energy emission from a 
point source coincident with the XRT position, confirming the already 
reported results by P.H.T. Tam and A.K.H. Kong (GCN 12097). 

We model the source spectrum with a power law model N(e) = N0 e^{\beta}, and 
the background including both the contribution from the Galactic diffuse 
emission and the isotropic contribution due to the residual contamination of 
charged particles.

We detect the GRB in two distinct time periods. In the first time bin (from 
237.1 seconds to 316.2 seconds after the GBM trigger), we measure a flux 
(100 MeV-10 GeV) of (4.9 +/- 1.9) x 1e-05 ph/cm^2/s and a spectral index beta 
of -2.1 +/- 0.3, with a Test Statistic TS=31.

In the second time bin, from 421.7 to 562.3 seconds after the GBM trigger, 
the measured flux is (3.7 +/- 1.5) x 1e-5  ph/cm^2/s with spectral index beta 
of -3.6 +/- 0.8 and a TS=19 (stat errors only).

The Fermi LAT point of contact for this burst is Nicola Omodei (nicola.omodei@gmail.com).

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.
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