Dust in the wind: the role of recent mass loss in long gamma-ray bursts

TL;DR

This study links the late-time super-soft X-ray emission in certain long gamma-ray bursts to the turbulent mass-loss history of their progenitors, revealing a connection between environmental properties and burst duration.

Contribution

It introduces a new interpretation of super-soft X-ray afterglows as reprocessed radiation from progenitor mass loss, connecting environmental factors with burst characteristics.

Findings

Identification of a population with super-soft X-ray emission and long-duration gamma-ray bursts.

Correlation between intrinsic absorption, super-soft X-ray emission, and burst duration.

Proposed link between progenitor mass loss history and observed burst properties.

Abstract

We study the late-time (t>0.5 days) X-ray afterglows of nearby (z<0.5) long Gamma-Ray Bursts (GRB) with Swift and identify a population of explosions with slowly decaying, super-soft (photon index Gamma_x>3) X-ray emission that is inconsistent with forward shock synchrotron radiation associated with the afterglow. These explosions also show larger-than-average intrinsic absorption (NH_x,i >6d21 cm-2) and prompt gamma-ray emission with extremely long duration (T_90>1000 s). Chance association of these three rare properties (i.e. large NH_x,i, super-soft Gamma_x and extreme duration) in the same class of explosions is statistically unlikely. We associate these properties with the turbulent mass-loss history of the progenitor star that enriched and shaped the circum-burst medium. We identify a natural connection between NH_x,i Gamma_x and T_90 in these sources by suggesting that the late-time super-soft X-rays originate from radiation reprocessed by material lost to the environment by the stellar progenitor before exploding, (either in the form of a dust echo or as reprocessed radiation from a long-lived GRB remnant), and that the interaction of the explosion's shock/jet with the complex medium is the source of the extremely long prompt emission. However, current observations do not allow us to exclude the possibility that super-soft X-ray emitters originate from peculiar stellar progenitors with large radii that only form in very dusty environments.