A double component in GRB 090618: a proto-black hole and a genuinely long gamma-ray burst

¹ Dip. di Fisica and ICRA, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Rome, Italy
e-mail: Ruffini@icra.it; luca.izzo@icra.it; ana.penacchioni@icra.it; bianco@icra.it; letizia.caito@icra.it; jorge.rueda@icra.it
² ICRANet, Piazza della Repubblica 10, 65122 Pescara, Italy
e-mail: barbara.patricelli@icranet.org
³ S. N. Bose National Center for Basic Sciences, Salt Lake, 700098 Kolkata, India
e-mail: chakraba@bose.res.in
⁴ Indian Center for Space Physics, Garia, 700084 Kolkata, India
⁵ Universite de Nice Sophia Antipolis, CEDEX 2, Grand Chateau Parc Valrose, Nice, France

Received: 7 June 2011
Accepted: 3 April 2012

Abstract

Context. The joint X-ray and gamma-ray observations of GRB 090618 by very many satellites offer an unprecedented possibility of testing crucial aspects of theoretical models. In particular, they allow us to test (a) in the process of gravitational collapse, the formation of an optically thick e⁺e⁻-baryon plasma self-accelerating to Lorentz factors in the range 200 < Γ < 3000; (b) its transparency condition with the emission of a component of 10⁵³⁻⁵⁴ baryons in the TeV region and (c) the collision of these baryons with the circumburst medium (CBM) clouds, characterized by dimensions of 10¹⁵⁻¹⁶ cm. In addition, these observations offer the possibility of testing a new understanding of the thermal and power-law components in the early phase of this GRB.

Aims. We test the fireshell model of GRBs in one of the closest (z = 0.54) and most energetic (E_iso = 2.90 × 10⁵³ erg) GRBs, namely GRB 090618. It was observed at ideal conditions by several satellites, namely Fermi, Swift, Konus-WIND, AGILE, RT-2, and Suzaku, as well as from on-ground optical observatories.

Methods. We analyzed the emission from GRB 090618 using several spectral models, with special attention to the thermal and power-law components. We determined the fundamental parameters of a canonical GRB within the context of the fireshell model, including the identification of the total energy of the e⁺e⁻ plasma, E_tot^e+e−, the proper GRB (P-GRB), the baryon load, the density and structure of the CBM.

Results. We find evidence of the existence of two different episodes in GRB 090618. The first episode lasts 50 s and is characterized by a spectrum consisting of a thermal component, which evolves between kT = 54 keV and kT = 12 keV, and a power law with an average index γ = 1.75 ± 0.04. The second episode, which lasts for  ~100 s, behaves as a canonical long GRB with a Lorentz gamma factor at transparency of Γ = 495, a temperature at transparency of 29.22 keV and with a characteristic size of the surrounding clouds of R_cl ~ 10¹⁵⁻¹⁶ cm and masses of  ~10²²⁻²⁴ g.

Conclusions. We support the recently proposed two-component nature of GRB 090618, namely, episode 1 and episode 2, with a specific theoretical analysis. We furthermore illustrate that episode 1 cannot be considered to be either a GRB or a part of a GRB event, but it appears to be related to the progenitor of the collapsing bare core, leading to the formation of the black hole, which we call a “proto-black hole”. Thus, for the first time, we are witnessing the process of formation of a black hole from the phases just preceding the gravitational collapse all the way up to the GRB emission.