Towards an understanding of long gamma-ray burst environments through circumstellar medium population synthesis predictions

Chrimes, A A and Gompertz, B P and Kann, D A and van Marle, A J and Eldridge, J J and Groot, P J and Laskar, T and Levan, A J and Nicholl, M and Stanway, E R and Wiersema, K (2022) Towards an understanding of long gamma-ray burst environments through circumstellar medium population synthesis predictions. Monthly Notices of the Royal Astronomical Society. ISSN 0035-8711

Text (GRB_progenitor_winds)
GRB_progenitor_winds.pdf - Accepted Version
Available under License Creative Commons Attribution.
Download (1MB)

Abstract

The temporal and spectral evolution of gamma-ray burst (GRB) afterglows can be used to infer the density and density profile of the medium through which the shock is propagating. In long-duration (core-collapse) GRBs, the circumstellar medium (CSM) is expected to resemble a wind-blown bubble, with a termination shock separating the stellar wind and the interstellar medium (ISM). A long standing problem is that flat density profiles, indicative of the ISM, are often found at lower radii than expected for a massive star progenitor. Furthermore, the presence of both wind-like environments at high radii and ISM-like environments at low radii remains a mystery. In this paper, we perform a ‘CSM population synthesis’ with long GRB progenitor stellar evolution models. Analytic results for the evolution of wind blown bubbles are adjusted through comparison with a grid of 2D hydrodynamical simulations. Predictions for the emission radii, ratio of ISM to wind-like environments, wind and ISM densities are compared with the largest sample of afterglow-derived parameters yet compiled, which we make available for the community. We find that high ISM densities of n ∼ 1000 cm−3 best reproduce observations. If long GRBs instead occur in typical ISM densities of n ∼ 1 cm−3, then the discrepancy between theory and observations is shown to persist at a population level. We discuss possible explanations for the origin of variety in long GRB afterglows, and for the overall trend of CSM modelling to over-predict the termination shock radius.