Towards the Properties of Long Gamma-Ray Burst Progenitors with Swift Data

TL;DR

This study analyzes Swift GRB data to explore the properties of long gamma-ray burst progenitors, linking observed afterglow features to progenitor star characteristics and supporting collapsar models.

Contribution

It introduces a method to infer progenitor star properties from X-ray afterglow data and correlates these with gamma-ray emission features.

Findings

Progenitor core masses are 0.1-5 solar masses with radii of 10^8-10^10 cm.

Rotation parameter correlates with burst duration, supporting collapsar models.

Envelope layer radii are 10^10-10^12 cm, with fall-back masses 10^-3 to 1 solar mass.

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 lightcurves is due to the curvature effect, we fit the lightcurves 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 and gamma-ray energy are correlated with the isotropic X-ray energy of the shallow decay phase and the isotropic X-ray luminosity at t2. 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 0.1-5 solar mass with a radius of 10^8-10^10 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 10^10-10^12 cm and 10^-3~1 solar mass, 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. The derived radii of the envelope are smaller than the photospheric radii of Wolf-Rayet (WR) stars. It is interesting that the assembled mass density profile for the bursts in our sample is also well consistent with the simulation for a pre-supernova star with 25 solar mass.