The AGILE observations of the hard and bright GRB 100724B

¹ INAF IASF Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy
e-mail: ettore.delmonte@iasf-roma.inaf.it
² INFN Trieste, Padriciano 99, 34012 Trieste, Italy
³ Dip. di Fisica, Università di Trieste, via Valerio 2, 34127 Trieste, Italy
⁴ INAF IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
⁵ INAF IASF Milano, via E. Bassini 15, 20133 Milano, Italy
⁶ ENEA C. R. Frascati, via E. Fermi 45, 00044 Frascati (Rm), Italy
⁷ Dip. di Fisica, Università degli Studi di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
⁸ INFN Pavia, via Bassi 6, 27100 Pavia, Italy
⁹ INAF IASF Palermo, via U. La Malfa 153, 90146 Palermo, Italy
¹⁰ ENEA C.R. “E. Clementel”, via Martiri di Monte Sole 4, 40129 Bologna, Italy
¹¹ Dip. di Fisica, Università degli Studi di Roma “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy
¹² INAF Osservatorio Astronomico di Cagliari, loc. Poggio dei Pini, strada 54, 09012 Capoterra (Ca), Italy
¹³ Dip. di Fisica e Matematica, Università dell’Insubria, via Valleggio 11, 20100 Como, Italy
¹⁴ ASI Science Data Center, via G. Galilei, 00044 Frascati (Rm), Italy
¹⁵ INAF Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone (Rm), Italy
¹⁶ INAF staff resident at ASI Science Data Center  
¹⁷ Agenzia Spaziale Italiana, Unità Osservazione dell’Universo, Viale Liegi 26, 00198 Roma, Italy

Received: 8 April 2011
Accepted: 8 September 2011

Abstract

The observation of gamma ray bursts (GRBs) in the gamma ray band has been advanced by the AGILE and Fermi satellites after the era of the Compton Gamma-Ray Observatory. AGILE and Fermi are showing that the GeV-bright GRBs share a set of common features, particularly the high fluence from the keV up to the GeV energy bands, the high value of the minimum Lorentz factor, an extended emission of gamma rays, which is often delayed with respect to lower energies, and finally the possible multiple spectral components. GRB 100724B, localised in a joint effort by Fermi and the InterPlanetary Newtork, is the brightest burst detected in gamma rays so far by AGILE. Characteristic features of GRB 100724B are the simultaneous emissions at MeV and GeV, without delayed onset or any time lag as shown by the analysis of the cross correlation function, and the significant spectral evolution in hard X-rays over the event duration. In this paper we show the analysis of the AGILE data of GRB 100724B and discuss its features in the context of the bursts observed so far in gamma rays and the recently proposed models.