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

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 10 53−54 baryons in the TeV region and (c) the collision of these baryons with the circumburst medium (CBM) clouds, characterized by dimensions of 10 15−16 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 × 10 53 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 powerlaw 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 e + e − tot , 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 ∼ 10 15−16 cm and masses of ∼10 22−24 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.

GRB 110709B in the induced gravitational collapse paradigm

“…The broken PL is indeed a constant function plus a simple power-law function. This is the same behavior as observed in the previously analyzed GRB 090618 (Izzo et al 2012a) and GRB 101023 (Penacchioni et al 2012). However, the temperatures for this GRB are lower.…”

“…The redshift of GRB 110709B is unknown, therefore we used in Sect. 4 four phenomenological methods to constrain it; i.e., Grupe (Grupe et al 2007), Amati (Amati 2006), Yonetoku (Yonetoku 2004) and the scaling of the X-ray afterglow (Izzo et al 2012a;Penacchioni et al 2012). The first method gives an upper limit of z < 1.35.…”

“…The prototype for the IGC paradigm has recently been given in the analysis of GRB 090618 (Izzo et al 2012a), following the works of Rueda & Ruffini (2012) and Izzo et al (2012b). In this work we follow the same line and identify four different Article published by EDP Sciences A133, page 1 of 12 episodes in GRB 110709B.…”

“…For this, the induced gravitational collapse (IGC) paradigm was first introduced by Ruffini et al (2001b) and was subsequently analyzed in Ruffini et al (2007Ruffini et al ( , 2008; Rueda & Ruffini (2012), and Izzo et al (2012b). Recently, it has been shown that this process can indeed explain the coincidence between the SN and GRB emission, both from an observational and a theoretical point of view (Izzo et al 2012a;Penacchioni et al 2012).…”

What can we learn from GRBs?

“…The large majority of long bursts is related to SNe and are spatially correlated with bright star-forming regions in their host galaxies [31,32] with a typical CBM density of ∼ 1 cm −3 [33,34]. However, the above single progenitor model contrasts with -X-ray flashes (XRFs) with E iso 10 52 erg and restframe spectral peak energy E p,i 200 keV.…”