GCN Circular 14515

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F. Verrecchia, C. Pittori (ASDC and INAF/OAR), A. Giuliani (INAF/IASF-Mi),
M. Marisaldi (INAF/IASF-Bo), F. Longo (University of Trieste and INFN
Trieste), F. Lucarelli (ASDC and INAF/OAR), E. Del Monte, F. Lazzarotto, I.
Donnarumma, Y. Evangelista, M. Feroci, L. Pacciani, P. Soffitta, E. Costa,
I. Lapshov, M. Rapisarda (INAF/IAPS Rome), G. Barbiellini, (INFN Trieste),
A. Bulgarelli, F. Gianotti, M. Trifoglio, G. Di Cocco, C. Labanti, V.
Fioretti, F. Fuschino, M. Galli (INAF/IASF-Bo), A. Chen, S. Mereghetti,
F. Perotti, P. Caraveo (INAF/IASF-Mi), M. Cardillo, E. Striani, M. Tavani
(INAF/IAPS Rome, and Univ. Roma Tor Vergata), A. Argan, G. Piano, S.
Sabatini, V. Vittorini (INAF/IAPS Rome), G. Pucella (ENEA Frascati), A.
Pellizzoni, A. Trois (INAF/OA Cagliari), M. Pilia (ASTRON), S. Vercellone
(INAF/IASF-Pa), P. W. Cattaneo, A. Rappoldi (INFN Pavia), A. Morselli, P.
Picozza (INFN Roma-2), M. Prest, E. Vallazza (Universita` 
dell'Insubria), P.
Lipari, D. Zanello (INFN Roma-1), P. Giommi (ASI), and G. Valentini (ASI),
on behalf of the AGILE Team, report:

The AGILE Gamma Ray Imaging Detector (GRID) detected high energy emission
from GRB 130427A (A. Maselli et al., GCN 14448
Not found
 
 
), also reported
by Fermi-LAT (S. Zhu et al., GCN 14471
Not found
 
 
).
A preliminary analysis of the AGILE-GRID data in temporal coincidence with
the GRB shows a significant excess of gamma-ray photons above 50 MeV at the
location of the event. The emission detected by the AGILE-GRID mostly
occurred between ~ t0 + 180 sec and t0 + 700 sec where t0 is 27 April 2013
at 07:47:15 UT. During this interval, the burst position was inside the
instrument FOV.

A maximum likelihood analysis of the AGILE-GRID data integrating over 12
hours, from 2013-04-27 05:00 UT to 2013-04-27 17:00 UT, using the standard
parameters used by AGILE quick look to detect persistent gamma-ray sources,
yields a detection at a significance level larger than 6 sigma, and a mean
flux F = (8.0+/-2.7) 10-6 ph/cm2/s (E > 100 MeV).
The preliminary photon spectral index obtained with this integration is
1.55 +/- 0.30.
Due to the exceptionally high fluence above 100 MeV of this burst, it is
possible for the first time to derive its properties using the maximum
likelihood techniques routinely used in the standard data analysis of
AGILE-GRID point sources.

The GRB also triggered the AGILE Minicalorimeter (MCAL), sensitive to
gamma-rays above 350 keV, at the time 07:47:06 UT.
According to the MCAL light curve, the emission lasts for about 20 s 
divided
into three main episodes. Although the large initial off-axis angle (more
than 120 degrees) prevents an accurate spectral analysis of MCAL data,
significant emission above 15 MeV is detected.

This measurement was obtained with AGILE observing a large portion of the
sky in spinning mode.

This message may be cited.
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