Long-Duration, Low-Luminosity Gamma-Ray Bursts: Towards a Comprehensive Model of the Weakest Engine-Driven Explosions

Irwin, Christopher, Astronomy - Graduate School of Arts and Sciences, University of Virginia
Chevalier, Roger, Department of Astronomy, University of Virginia

Long-duration, low-luminosity gamma-ray bursts (LLGRBs) are a peculiar subclass of gamma-ray bursts (GRBs) that are up to 5 orders of magnitude less luminous than typical bursts. The observation of Type Ic supernovae (SNe) accompanying LLGRBs supports the notion that, like typical GRBs, they are powered by the gravitational collapse of a massive star's core, but beyond this little is known about their origin, and the differences between the progenitors of LLGRBs and GRBs are unclear. In this dissertation, I present analytical and numerical modeling that aims to improve our understanding of the physical conditions leading to LLGRBs.

We consider first the unusual GRB 060218, a well-studied event that is prototypical of the long-duration LLGRB class. We present a model for this burst that can account for the prompt X-ray emission lasting for thousands of seconds, the early optical peak lasting for hours, and the X-ray and radio afterglow lasting for several days. The basic ingredients of the model are a long-lived jet with an unusually low luminosity, a progenitor star surrounded by an extended, optically thick, low-mass envelope, and a modest amount of dust in the interstellar environment.

We fit the observed thermal component of the prompt X-ray emission with a simple photospheric model, and then calculate the nonthermal emission due to inverse Compton (IC) scattering of thermal photons from the external shocks of the jet. We find that this model can fit the observed nonthermal emission, if the conditions are such that the reverse shock dominates the emission. We show that the low-power jet can successfully traverse the progenitor star and extended envelope, and that it can also produce the observed radio emission via external shock synchrotron emission at late times. In the meantime, fast ejecta from the associated SN 2006aj shock the extended envelope, which emits a burst of optical emission as it expands and cools, consistent with the early optical peak. We interpret the anomalous X-ray afterglow in GRB 060218 as a light echo from dust situated tens of parsecs from the burst. Our results suggest that LLGRB progenitors are shrouded in dense circumstellar envelopes, and that they require both a jet and a supernova. Support for this view comes from Nakar (2015), who considered a different model for LLGRBs involving choked jets, but arrived at the same main conclusions.

Motivated by these results, we also perform numerical simulations of jets with standard GRB properties in stars with extended envelopes. We present analytical calculations that predict whether or not the jet escapes successfully from the envelope, and apply them to interpret the numerical results. We find that the jet-driven outflow is more well-confined by the envelope than previous analytical results suggested, leading to a flow that is far from spherically symmetric when it breaks out of the envelope. This is generally true for a wide range of physical parameters. Even though the outflow fails to sphericize in the envelope, the jet still pumps most of its energy into a hot, mildly relativistic cocoon that expands after breakout to produce a roughly spherical explosion outside the envelope. The dynamics are thus very different from the standard case of a bare WR star progenitor, where the jet does become spherical if choked, and the outflow remains beamed for a long time post-breakout. Our results have far-reaching implications for LLGRBs, suggesting a different event rate and different behavior of the breakout radiation compared to past analytical work that assumed spherical symmetry is achieved in the envelope.

PHD (Doctor of Philosophy)
supernovae, gamma-ray bursts, hydrodynamics
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