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

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.

Published in MNRAS; higher resolution figures in published version.

References in corpus (14)

• Binary star progenitors of long gamma-ray bursts
• A Comprehensive Analysis of Swift/XRT Data: III. Jet Break Candidates in X-ray and Optical Afterglow Lightcurves
• High-Ion Absorption in Seven GRB Host Galaxies at z=2-4: Evidence for both Circumburst Plasma and Outflowing Interstellar Gas
• Absorption Systems in the Spectrum of GRB 021004
• A Survey for NV Absorption at z~z_GRB in GRB Afterglow Spectra: Clues to Gas Near the Progenitor Star
• The prompt, high resolution spectroscopic view of the "naked-eye" GRB080319B
• Swift GRB Afterglows and the Forward-Shock Model
• The circumstellar medium around a rapidly rotating, chemically homogeneously evolving, possible gamma-ray burst progenitor
• The Molecular Hydrogen Deficit in Gamma-Ray Burst Afterglows
• The host of GRB 060206: kinematics of a distant galaxy
• UVES/VLT high resolution absorption spectroscopy of the GRB080330 afterglow: a study of the GRB host galaxy and intervening absorbers
• Outliers from the mainstream: how a massive star can produce a gamma-ray burst
• Time resolved spectroscopy of GRB 021004 reveals a clumpy extended wind
• The nature of HHL 73 from optical imaging and Integral Field Spectroscopy

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