DUST IN THE WIND: THE ROLE OF RECENT MASS LOSS IN LONG GAMMA-RAY BURSTS
We study the late-time ( t > 0.5 ?> days) X-ray afterglows of nearby ( z < 0.5 ?> ) long gamma-ray bursts (GRBs) with Swift and identify a population of explosions with slowly decaying, super-soft (photon index Γ 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 > 6 × 10 21 cm − 2 ?> ) and prompt γ-ray emission with extremely long duration ( T 90 > 1000 ?> s). The chance association of these three rare properties (i.e., large NH x , i ?> , super-soft Γ 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 circumburst medium. We identify a natural connection between NH x , i ?> , Γ 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.