The Energetics and Environments of Swift Gamma-Ray Bursts

Citation

Cenko, Stephen Bradley (2009) The Energetics and Environments of Swift Gamma-Ray Bursts. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/SNRD-BR66. https://resolver.caltech.edu/CaltechETD:etd-06272008-153145

Abstract

For their short durations, gamma-ray bursts (GRBs) are the most electromagnetically luminous objects in the universe. In this thesis, I uses the fascinating objects, both as signposts, indicating the presence of an ultra-relativistic (Lorentz factor ~ 100-1000) outflow from a newly born stellar mass black hole, and as lighthouses, illuminating the circumburst (r ~ 1 pc) and inter-stellar (r ~ 1 kpc) media along the line of sight.

In Part I, I describe my efforts to automate the Palomar 60 inch telescope (P60), the primary instrument on which much of this thesis is based. Designed to capitalize on NASA's recently launched Swift Gamma-Ray Burst Explorer, P60 now routinely provides moderately fast (t <~ 3 min) and sustained (R <~ 23 mag) observations of GRB afterglows and other optical transients.

Part II focuses on the geometry and energetics of some of the best-sampled events in the Swift era. I find both GRB050820A and GRB060418 are an order of magnitude more energetic than pre-Swift events, with a total energy release in excess of 10^52 erg. Both GRBs are therefore members of an emerging class of "hyper-energetic" GRBs, suggesting a much broader energy distribution than previously thought and challenging current massive star progenitor models.

Finally, in Part III I study the environments of long-duration GRBs. The massive star progenitors should leave an imprint on the GRB environment, both on the pc scale as a wind from mass loss of the outer envelope, and on the kpc scale from the dense, dusty disk where massive stars form. Interestingly, I demonstrate that GRB070125 exploded in a halo environment, suggesting at least some massive stars form far away from the disk of their host. By comparing the X-ray and optical light curves from a sample of 29 GRBs, I find nearly half show evidence for suppressed optical emission. I attribute this to dust absorption in the host galaxy, consistent with a massive star origin for long-duration GRBs.

Thesis Files