GRB 090618: a possible case of multiple GRB?

R. RUFFINI,; L. IZZO; A.V. PENACCHIONI; C.L. BIANCO; L. CAITO; S.K. CHAKRABARTI; A. NANDI

Heidelberg

Congreso; 25th Texas Symposium on Relativistic Astrophysics; 2010

GRB 090618 is one of the nearest and most energetic GRB, at a redshift of z=0.54, and with an isotropic energy of Eiso = 2.76 × 1053 ergs. We studied its temporal evolution from the data by the GBM detector on board Fermi satellite (in the energy range between 8KeV and 10 MeV) and two detectors on board Swift satellite: BAT [9] (15- 150 KeV) and XRT [10] (0.3- 10 KeV), and data from the Indian payloada RT-2/S and RT-2/G on board the Russian satellite Coronas-Photon [20]. The light curve shows a complex behavior, with several peaks. We have recently shown [22] that it is not possible to simulate the event as a single GRB in the context of the fireshell scenario [21]. We here analyze the data assuming as an ansatz that it is composed of two GRBs and we analyze them within the fireshell scenario [21]. The first, named Episode 1, lasts around 50 s, with a dyadosphere energy of Edya = 3.87 × 1052 erg. The second, named Episode 2, is compatible with a GRB of Edya = 5.0 × 1053 erg. The burst is itself followed by a third signal detected in the soft X-ray energy range. In order to perform this analysis, we have provided a detailed study of the spectral emission of all components. The results of this analysis are clearly significant for making assesment within the fireshell scenario. They are of course of general validity and can be used in order to test any possible alternative interpretation. We conclude, from the analysis of the Episode 1, that its interpretion as a separate GRB can be excluded: the interpretation of such an episode as a GRB would imply either to a P-GRB with a correct intensity but spectral properties in contrast with the observations or, alternatively, to a GRB whose spectral distribution is acceptable but with a P-GRB intensity not seen during the observation. The interpretation of the second episode as a GRB is confirmed and enriched with the identification of the P-GRB. We find very peculiar similarities between the first and the last two pulses in this second episode with respect to the relative intensity of these spikes. In conclusion, the multi component structure of GRB 090618 is confirmed. This result opens the way to a more detailed analysis for the identification of the episode 1.