Upper limits on the high-energy emission from gamma-ray bursts observed by AGILE-GRID^⋆

¹ Dip. di Fisica, Univ. di Trieste, via Valerio 2, 34127 Trieste, Italy
e-mail: francesco.longo@ts.infn.it
² INFN–Trieste, via Valerio 2, 34127 Trieste, Italy
³ CIFS–Torino, c/o Dipartimento di Fisica, Univ. di Torino, via Pietro Giuria 1, 10125 Torino, Italy
⁴ Royal Institute of Technology (KTH), Stockholm, Sweden
⁵ The Oskar Klein Centre for Cosmoparticle Physics, Stockholm, Sweden
⁶ APC, Univ Paris Diderot, CNRS/IN2P3, CEA/Irfu, Obs de Paris, Sorbonne Paris Cité, 10 rue Domon et Duquet, 75205 Paris, France
⁷ Dept. of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁸ Fakultät für Physik und Astronomie, Ruhr-Universität Bochum, 44780 Bochum, Germany
⁹ INAF/IAPS, via del Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁰ INFN–Pavia, via Bassi 6, 27100 Pavia, Italy
¹¹ INAF/IASF–Bologna, via Gobetti 101, 40129 Bologna, Italy
¹² INAF/IASF–Milano, via E. Bassini 15, 20133 Milano, Italy
¹³ ASI Science Data Center (ASDC), ESRIN, 00044 Frascati ( RM), Italy
¹⁴ INAF – Osservatorio Astron. di Roma, via di Frascati 33, 00040, Monte Porzio Catone, Italy
¹⁵ Dip. di Fisica, Univ. “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁶ Dip. di Fisica, Univ. di Perugia, via Alessandro Pascoli, 06123 Perugia, Italia
¹⁷ Dip. di Fisica Generale “A. Avogadro”, via Pietro Giuria 1, Univ. di Torino, 10125 Torino, Italy
¹⁸ ENEA, via Martiri di Monte Sole 4, 40129 Bologna, Italy
¹⁹ INFN–Roma “La Sapienza”, Piazzale A. Moro 2, 00185 Roma, Italy
²⁰ INFN–Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
²¹ INAF – Osservatorio Astronomico di Cagliari, localita’ Poggio dei Pini, strada 54, 09012 Capoterra, Italy
²² NASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
²³ Dip. di Fisica, Univ. dell’Insubria, via Valleggio 11, 22100 Como, Italy
²⁴ INFN–Milano Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
²⁵ ENEA–Frascati, via E. Fermi 45, 00044 Frascati ( Roma), Italy
²⁶ INAF-IASF-Palermo, via U. La Malfa 153, 90146 Palermo, Italy
²⁷ Wits University, 1 Jan Smuts Avenue Braamfontein 2000, Johannesburg, South Africa
²⁸ ASI, Viale Liegi 26, 00198 Roma, Italy

Received: 30 November 2010
Accepted: 25 August 2012

Abstract

Context. The detection and the characterization of the highenergy emission component from individual gamma-ray bursts (GRBs) is one of the key science objectives of the currently operating gamma-ray satellite AGILE, launched in April 2007. In its first two years of operation AGILE detected three GRBs with photons of energy larger than 30 MeV. One more GRB was detected in AGILE third operation year, while operating in spinning mode.

Aims. For the 64 other GRBs localized during the period July 2007 to October 2009 in the field of view of the AGILE Gamma-Ray Imaging Detector (GRID), but not detected by this instrument, we estimate the count and flux upper limits on the GRB high energy emission in the AGILE-GRID energy band (30 MeV−3 GeV).

Methods. To calculate the count upper limits, we adopted a Bayesian approach. The flux upper limits are derived using several assumptions on the high-energy spectral behavior. For 28 GRBs with available prompt spectral information, a flux upper limit and the comparison with the expected flux estimated from spectral extrapolation of the Band spectrum to the 30 MeV−3 GeV band are provided. Moreover, upper limits on the flux under the assumption of an extra power law component dominating the 30 MeV−3 GeV band are calculated for all GRBs and considering four different values for the spectral photon index. Finally, we performed a likelihood upper limit on the possible delayed emission up to 1 h after the GRB.

Results. The estimated flux upper limits range between 1  ×  10^-4 and  ~2  ×  10^-2 photons cm^-2 s^-1 and generally lie above the flux estimated from the extrapolation of the prompt emission in the 30 MeV−3 GeV band. A notable case is GRB 080721, where the AGILE-GRID upper limit suggests a steeper spectral index or the presence of a cut-off in the high energy part of the Band prompt spectrum. The four GRBs detected by AGILE-GRID show high-energy (30 MeV−3 GeV) to low-energy (1 keV−10 MeV) fluence ratios similar to those estimated in this paper for the 64 GRBs without GRID detection, favoring the possibility that AGILE-GRID detected only high-fluence, hard GRBs. From the flux upper limits derived in this work we put some constraint on high-energy radiation from the afterglow emission and from synchrotron self Compton emission in internal shocks.