Magnetic flux of progenitor stars sets gamma-ray burst luminosity and variability

Alexander Tchekhovskoy*, Dimitrios Giannios

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

30 Scopus citations

Abstract

Long-duration gamma-ray bursts (GRBs) are thought to come from the core collapse of Wolf-Rayet stars. Whereas their stellarmasses M* have a rather narrowdistribution, the population of GRBs is very diverse, with gamma-ray luminosities Lγ spanning several orders of magnitude. This suggests the existence of a 'hidden' stellar variable whose burst-to-burst variation leads to a spread in Lγ. Whatever this hidden variable is, its variation should not noticeably affect the shape of GRB light curves, which display a constant luminosity (in a time-average sense) followed by a sharp drop at the end of the burst seen with Swift/XRT. We argue that such a hidden variable is progenitor star's large-scale magnetic flux. Shortly after the core collapse, most of stellar magnetic flux accumulates near the black hole (BH) and remains there. The flux extracts BH rotational energy and powers jets of roughly a constant luminosity, Lj. However, once BH mass accretion rate M˙ falls below ~Lj/c2, the flux becomes dynamically important and diffuses outwards, with the jet luminosity set by the rapidly declining mass accretion rate, Lj ~ M˙c2. This provides a potential explanation for the sharp end of GRBs and the universal shape of their light curves. During the GRB, gas infall translates spatial variation of stellar magnetic flux into temporal variation of Lj. We make use of the deviations from constancy in Lj to perform stellar magnetic flux 'tomography'. Using this method, we infer the presence of magnetized tori in the outer layers of progenitor stars for GRB 920513 and GRB 940210.

Original languageEnglish (US)
Pages (from-to)327-344
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Volume447
Issue number1
DOIs
StatePublished - Feb 11 2015

Keywords

  • Gamma-Rays: stars
  • MHD
  • Methods: numerical
  • methods: analytical
  • stars: magnetic field

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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