The rest-frame ultraviolet spectra of GRBs from massive rapidly-rotating stellar progenitors

TL;DR

This study models the ultraviolet spectra of GRBs from massive, rapidly rotating progenitors, revealing that early-time spectral lines are transient and unlikely to provide routine insights into progenitor winds.

Contribution

It provides a detailed, time-dependent spectral analysis of GRBs in the circumstellar medium shaped by rapidly rotating massive stars, considering dust depletion effects.

Findings

High-velocity low-ionization lines are short-lived and unlikely to be observed routinely.

Rest-frame lines from the termination shock are consistently visible.

Early-time spectroscopy is ineffective for studying progenitor wind properties.

Abstract

The properties of a massive star prior to its final explosion are imprinted in the circumstellar medium (CSM) created by its wind and termination shock. We perform a detailed, comprehensive calculation of the time-variable and angle-dependent transmission spectra of an average-luminosity Gamma-Ray Burst (GRB) which explodes in the CSM structure produced by the collapse of a 20 Msun, rapidly rotating, Z=0.001 progenitor star. We study both the case in which metals are initially in the gaseous phase, as well as the situation in which they are heavily depleted into dust. We find that high-velocity lines from low-ionization states of silicon, carbon, and iron are initially present in the spectrum only if the metals are heavily depleted into dust prior to the GRB explosion. However, such lines disappear on timescales of a fraction of a second for a burst observed on-axis, and of a few seconds for a burst seen at high-latitude, making their observation virtually impossible. Rest-frame lines produced in the termination shock are instead clearly visible in all conditions. We conclude that time-resolved, early-time spectroscopy is not a promising way in which the properties of the GRB progenitor wind can be routinely studied. Previous detections of high velocity features in GRB UV spectra must have been due either due to a superposition of a physically unrelated absorber or to a progenitor star with very unusual properties.