Volume 538, February 2012
|Number of page(s)||11|
|Section||Stellar structure and evolution|
|Published online||01 February 2012|
Evidence for a proto-black hole and a double astrophysical component in GRB 101023
Dip. di Fisica, Sapienza Università di Roma and ICRA,
Piazzale Aldo Moro 5,
e-mail: firstname.lastname@example.org; email@example.com; firstname.lastname@example.org; email@example.com; firstname.lastname@example.org; email@example.com
2 ICRANet, Piazzale della Repubblica 10, 65122 Pescara, Italy
3 Université de Nice Sophia Antipolis, Nice, Cedex 2, Grand Chateau Parc Valrose, France
4 Italian National Institute for Astrophysics (INAF) – IASF Bologna, via P. Gobetti 101, 40129 Bologna, Italy
Accepted: 5 December 2011
Context. It has been recently shown that GRB 090618, observed by AGILE, Coronas Photon, Fermi, Konus, Suzaku, and Swift, is composed of two very different components: episode 1, lasting 50 s, shows a thermal plus power-law spectrum with a characteristic temperature evolving in time as a power law; episode 2 (the remaining 100 s) is a canonical long GRB. We have associated episode 1 to the progenitor of a collapsing bare core leading to the formation of a black hole: what was defined as a “proto black hole”
Aims. In precise analogy with GRB 090618 we aim to analyze the 89 s of the emission of GRB 101023, observed by Fermi, Gemini, Konus and Swift, to see if there are two different episodes: the first one presenting a characteristic black-body temperature evolving in time as a broken power law, and the second one consistent with a canonical GRB.
Methods. To obtain information on the spectra, we analyzed the data provided by the GBM detector onboard the Fermi satellite, and we used the heasoft package XSPEC and RMFIT to obtain their spectral distribution. We also used the numerical code GRBsim to simulate the emission in the context of the fireshell scenario for episode 2.
Results. We confirm that the first episode can be well fit by a black body plus power-law spectral model. The temperature changes with time following a broken power law, and the photon index of the power-law component presents a soft-to-hard evolution. We estimate that the radius of this source increases with time with a velocity of 1.5 × 104 km s-1. The second episode appears to be a canonical GRB. By using the Amati and the Atteia relations, we determined the cosmological redshift, z ~ 0.9 ± 0.084(stat.) ± 0.2(sys.). The results of GRB 090618 are compared and contrasted with the results of GRB 101023. Particularly striking is the scaling law of the soft X-ray component of the afterglow.
Conclusions. We identify GRB 090618 and GRB 101023 with a new family of GRBs related to a single core collapse and presenting two astrophysical components: a first one related to the proto-black hole prior to the process of gravitational collapse (episode 1), and a second one, which is the canonical GRB (episode 2) emitted during the formation of the black hole. For the first time we are witnessing the process of a black hole formation from the instants preceding the gravitational collapse up to the GRB emission. This analysis indicates progress towards developing a GRB distance indicator based on understanding the P-GRB and the prompt emission, as well as the soft X-ray behavior of the late afterglow.
Key words: black hole physics / gamma-ray burst: individual: GRB 101023
© ESO, 2012
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