Forming a constant density medium close to long gamma-ray bursts

Abstract

The progenitor stars of long Gamma-Ray Bursts (GRBs) are thought to be Wolf-Rayet stars, which generate a massive and energetic wind. Nevertheless, about 25 percent of all GRB afterglows light curves indicate a constant density medium close to the exploding star. We explore various ways to produce this, by creating situations where the wind termination shock arrives very close to the star, as the shocked wind material has a nearly constant density. Typically, the distance between a Wolf-Rayet star and the wind termination shock is too large to allow afterglow formation in the shocked wind material. Here, we investigate possible causes allowing for a smaller distance: A high density or a high pressure in the surrounding interstellar medium (ISM), a weak Wolf-Rayet star wind, the presence of a binary companion, and fast motion of the Wolf-Rayet star relative to the ISM. We find that all four scenarios are possible in a limited parameter space, but that none of them is by itself likely to explain the large fraction of constant density afterglows. A low GRB progenitor metallicity, and a high GRB energy make the occurrence of a GRB afterglow in a constant density medium more likely. This may be consistent with constant densities beingpreferentially found for energetic, high redshift GRBs.

13 pages, 13 figures, new version: as accepted by Astronomy & Astrophysics

References in corpus (1)

• Optical Photometry of GRB 021004: The First Month

Cited by in corpus (13)

• Binary star progenitors of long gamma-ray bursts
• A Survey for NV Absorption at z~z_GRB in GRB Afterglow Spectra: Clues to Gas Near the Progenitor Star
• Gamma-Ray Burst Afterglows as Probes of Environment and Blastwave Physics II: the Distribution of p and Structure of the Circumburst Medium
• AMRVAC and Relativistic Hydrodynamic simulations for GRB afterglow phases
• The circumstellar medium around a rapidly rotating, chemically homogeneously evolving, possible gamma-ray burst progenitor
• Constraints on gamma-ray burst and supernova progenitors through circumstellar absorption lines. (II): Post-LBV Wolf-Rayet stars
• No visible optical variability from a relativistic blast wave encountering a wind-termination shock
• Observations of the Optical Afterglow of GRB 050319 : Wind to ISM transition in view
• The circumburst environment of a FRED GRB: study of the prompt emission and X-ray/optical afterglow of GRB 051111
• Asymmetric Wolf-Rayet winds: implications for GRB afterglows
• GRB blastwaves through wind-shaped circumburst media
• Cosmic ray protons in the energy range $10^{16}-10^{18.5}$ eV: stochastic gyroresonant acceleration in hypernova shocks?
• The circumstellar environment of rotating Wolf-Rayet Stars and the implications for GRB afterglows