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

Mon. Not. R. Astron. Soc. 000, (0000) Printed 10 September 2021 (MN LATEX style file v2.2) The rest-frame ultraviolet spectra of GRBs from massive rapidly-rotating stellar progenitors Peter B. Robinson1, Rosalba Perna 1, Davide Lazzati2, and Allard J. van Marle3,4. 1JILA,University of Colorado, 440 UCB, Boulder, CO 80309-0440 2Department of Physics, NC State University, Campus Box 8202, Raleigh, NC 27695-8202 3Bartol Research Institute, University of Delaware, 102 Sharp Laboratory, Newark, 19716 DE, USA 4Presently residing at: Centre for Plasma Astrophysics, K.U. Leuven (Leuven Mathematical Modeling and Computational Science Center), Celestijnenlaan 200B, 3001 Heverlee, Belgium 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 trans- mission spectra of an average-luminosity Gamma-Ray Burst (GRB) which explodes in the CSM structure produced by the collapse of a 20 M⊙, 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 ini- tially 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 ve- locity 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. 1 INTRODUCTION For a few hours after their onset, the afterglows of Gamma- Ray Bursts (GRBs) are the brightest sources in the far Uni- verse. Their high luminosity, together with their powerlaw, featureless spectrum, make them ideal sources to probe their surrounding environment through the absorption lines im- printed in their spectra. Theoretical studies have suggested that long Gamma Ray Bursts (GRBs) are produced by the collapse of rapidly- rotating, chemically homogeneous, massive stars (e.g. Mac- fadyen & Woosley 1999). The association between GRBs and massive stars, which has been observationally supported (Stanek et al. 2003; Hjorth et. al 2003), makes absorption studies potentially useful as a new way to probe star-forming regions at intermediate and high redshifts, and/or the last hundreds of years of the progenitor evolution. Absorption lines imprinted by the material ejected by the star prior to its explosion allow one to probe the velocity structure and metal content of the ejecta. High-resolution spectroscopy of GRB afterglows has been performed in a number of cases (e.g. M¨oller et al. (2002); Matheson et al. (2003); Mirabal et al. (2003); Schaefer et al. (2003); Starling et al. (2005); Fiore et al. (2005); D’Elia et al. (2009); Chen et al. (2008); Prochanska et al. (2008a), (2008b); Fox et al. (2008); Thoene et al. (2008); see also Whalen et al. 2008 for an extended dis- cussion on absorbers in GRBs), yielding constraints on the nature of the absorbing medium. An especially well studied burst was GRB 021004, whose high-resolution spectroscopy revealed a complex velocity structure of the absorbing ma- terial, with velocities up to ≳3000 km/s. The interpreta- tion of these features has been controversial. Initial studies claimed that the high velocity lines were a direct proof of the association of GRBs with WR stars. Starling et al. (2005) argued that the lines must be produced in a fossil stellar wind with hydrogen enrichment from a companion. Mirabal et al. (2003) and Schaefer et al. (2003) interpreted the lines as the result of shells of material which are present around the progenitor. More recent studies have however cast doubts on the initial interpretation. Lazzati et al. (2006) performed a de- tailed time dependent analysis, taking into account the burst flash ionization. Even though they still considered the wind of the WR progenitor star as the best absorber candidate, they pointed out that such an interpretation would require a termination shock at a distance of at least 100 pc. Such a large radius of the termination shock is somewhat at odds with current wind models and could be accounted for only if the progenitor were an extremely massive star evolving in arXiv:1003.3840v1 [astro-ph.HE] 19 Mar 2010 2 Peter B. Robinson, Rosalba Perna, Davide Lazzati, and Allard J. van Marle a fairl

…(Full text truncated)…

This content is AI-processed based on ArXiv data.