GRB 110709B in the induced gravitational collapse paradigm

Context. Gamma-ray burst (GRB) 110709B is the first source for which Swift-BAT was triggered twice, with a time separation of ∼10 min. The first emission (called here episode 1) lasted from 40 s before the first trigger time until 60 s after it. The second emission (hereafter episode 2) lasted from 35 s before the second trigger time until 100 s after it. These features reproduce those of GRB 090618, which has recently been interpreted within the induced gravitational collapse (IGC) paradigm. In line with this paradigm, we assume the progenitor to be a close binary system composed of the core of an evolved star and a neutron star (NS). The evolved star explodes as a supernova (SN) and ejects material that is partially accreted by the NS. We identify this process with episode 1. The accretion process accumulates more than the critical mass of the NS, which gravitationally collapses into a black hole (BH). This process leads to the GRB emission, episode 2. The two trigger episodes have for the first time provided the possibility to cover the X-ray emission observed by XRT both prior to and during the prompt phase of GRB 110709B. Aims. We analyze the spectra and time variability of episodes 1 and 2 and compute the relevant parameters of the binary progenitor, as well as the astrophysical parameters both in the SN and the GRB phase in the IGC paradigm. Methods. We performed a time-resolved spectral analysis of episode 1 by fitting the spectrum with a blackbody (BB) plus a power-law (PL) spectral model. From the BB fluxes and temperatures of episode 1 and the luminosity distance dL, we evaluated the evolution with time of the radius of the BB emitter, associated here to the evolution of the SN ejecta. We analyzed episode 2 within the fireshell model, identifying the proper GRB (P-GRB) and simulating the light curve and spectrum. We established the redshift to be z = 0.75, following the phenomenological methods described in the literature, and our analysis of the late X-ray afterglow. It is most remarkable that the determination of the cosmological redshift on the basis of scaling the late X-ray afterglow, which was already verified in GRB 090618 and GRB 101023, is again verified by this analysis. Results. We find for episode 1 a temperature of the BB component that evolves with time following a broken PL, with the slope of the PL at early times α = 0 (constant function) and the slope of the PL at late times β = −4 ± 2. The break occurs at t = 41.21 s. The total energy of episode 1 is E (1) = 1.42 × 10 53 erg. The total energy of episode 2 is E (2) = 2.43 × 10 52 erg. We find at transparency a Lorentz factor Γ ∼ 1.73 × 10 2 , laboratory radius of 6.04 × 10 13 cm, P-GRB observed temperature kTP−GRB = 12.36 keV, baryon load B = 5.7 × 10 −3 and P-GRB energy of EP−GRB = 3.44 × 10 50 erg. We find a remarkable coincidence of the cosmological redshift by scaling the XRT data and with three other phenomenological methods. Conclusions. We interpret GRB 110709B as a member of the IGC sources, together with GRB 970828, GRB 090618, and GRB 101023. The existence of the XRT data during the prompt phase of the emission of GRB 110709B (episode 2) offers an unprecedented tool for improving the diagnostic of GRBs emission.