Towards the properties of long gamma-ray burst progenitors with Swift data

Abstract

We investigate the properties of both the prompt and X-ray afterglows of gamma-ray bursts (GRBs) in the burst frame with a sample of 33 Swift GRBs. Assuming that the steep decay segment in the canonical X-ray afterglow light curves is due to the curvature effect, we fit the light curves with a broken power law to derive the zero time of the last emission epoch of the prompt emission (t1) and the beginning as well as the end time of the shallow decay segment (t2 and t3). We show that both the isotropic peak gamma-ray luminosity (Lpeak,γ) and gamma-ray energy (Eiso,γ) are correlated with the isotropic X-ray energy (Eiso,X) of the shallow decay phase and the isotropic X-ray luminosity at). We infer the properties of the progenitor stars based on a model proposed by Kumar et al. who suggested that both the prompt gamma-rays and the X-ray afterglows are due to the accretions of different layers of materials of the GRB progenitor star by a central black hole (BH). We find that most of the derived masses of the core layers are Mc = 0.1 ∼ 5 M ⊙, and their average accretion rates in the prompt gamma-ray phase are, with a radius of rc = 108 ∼ 1010 cm. The rotation parameter is correlated with the burst duration, being consistent with the expectation of collapsar models. The estimated radii and the masses of the fall-back materials for the envelope layers are re = 1010 ∼ 1012 cm and Me = 10-3 ∼ 1 M⊙, respectively. The average accretion rates in the shallow decay phase are correlated with those in the prompt gamma-ray phase, but they are much lower, i.e. The re values are smaller than the photospheric radii of Wolf-Rayet stars. In our calculation, we assume a uniform mass of the central BH (MBH = 10 M⊙). Therefore, we may compare our results with simulation results. It is interesting that the assembled mass density profile for the bursts in our sample is well consistent with the simulation for a pre-supernova star with mass M = 25 M ⊙. © 2009 RAS.