The stellar-wind envelope around the supernova XRF/GRB060218/SN2006aj massive progenitor star

Abstract

In BTA spectra of the supernova SN2006aj, identified with the X-ray flash (XRF) and gamma-ray burst XRF/GRB060218/SN2006aj, we detected details interpreted as hydrogen lines, which is a sign of stellar-wind envelope around a massive progenitor star of the gamma-ray burst. Results of modeling two early spectra obtained with the BTA in 2.55 and 3.55 days after the explosion of Type Ic supernova SN2006aj (z=0.0331) are presented. The spectra were modeled in the Sobolev approximation with the SYNOW code (Branch et al. 2001; Elmhamdi et al. 2006). In the spectra of the optical afterglow of the X-ray flash XRF/GRB060218 we detected spectral features interpreted as (1) the H_alpha PCyg profile for the velocity 33000 km s$^{-1}$ -- a wide and almost unnoticeable deformation of continuum in the range of $\simeq5600 - 6600Å$ for the first epoch (2.55 days) and (2) a part (``remnant'') of the H_alpha PCyg profile in absorption blueshifted by 24000 km s$^{-1}$ -- a wide spectral feature with a minimum at $\simeq6100Å$ (the rest wavelength) for the second epoch (3.55 days). Taking into consideration early BTA observations and spectra obtained with other telescopes (ESO Lick, ESO VLT, NOT) before 2006 Feb. 23 UT, it can be said that we observe evolution of optical spectra of Type Ic core-collapse supernova SN 2006aj during {\it transition} from the short phase related to the shock breakout to outer layers of the stellar-wind envelope to spectra of the phase of increasing brightness corresponding to radioactive heating. Signs of hydrogen in spectra of the gamma-ray burst afterglow were detected for the first time.