The Many Facets of Cosmic Explosions

by Alicia Margarita Soderberg

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Synopsis

Over the past few years, long-duration gamma-ray bursts (GRBs), including the subclass of X-ray flashes (XRFs), have been revealed to be a rare variety of Type Ibc supernova (SN Ibc). While all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary Type Ibc SNe by many orders of magnitude. The observed diversity of stellar death corresponds to large variations in the energy, velocity, and geometry of the explosion ejecta. Using multi-wavelength (radio, optical, X-ray) observations of the nearest GRBs, XRFs, and SNe Ibc, I show that GRBs and XRFs couple at least 1048 erg to relativistic material while SNe Ibc typically couple less than 1048 erg to their fastest (albeit non-relativistic) outflows. Specifically, I find that less than 3 percent of local SNe Ibc show any evidence for association with a GRB or XRF. Interestingly, this dichotomy is not echoed by the properties of their optical SN emission, dominated by the radioactive decay of Nickel-56; I find that GRBs, XRFs, and SNe Ibc show significant overlap in their optical peak luminosity and photospheric velocities. Recently, I identified a new class of GRBs and XRFs that are under-luminous in comparison with the statistical sample of GRBs. Owing to their faint high-energy emission, these sub-energetic bursts are only detectable nearby (z < 0.1) and are likely 10 times more common than cosmological GRBs. In comparison with local SNe Ibc and typical GRBs/XRFs, these explosions are intermediate in terms of both volumetric rate and energetics. Yet the essential physical process that causes a dying star to produce a GRB, XRF, or sub-energetic burst, and not just a SN, remains a crucial open question. Progress requires a detailed understanding of ordinary SNe Ibc which will be facilitated with the launch of wide-field optical surveys in the near future.

About The Author

Alicia Margarita Soderberg received her Bachelor of Science degree in Physics and Mathematics (double major) at Bates College in 2000. Her senior thesis focused on detection rate efficiency of optical supernova searches. As a National Science Foundation Graduate Research Fellow at Churchill College, she completed Part Three of the Mathematical Tripos at Cambridge University (2001) and received a Certificate of Advanced Study in Applied Mathematics. Working with Professor Shrinivas R. Kulkarni on the observed diversity of cosmic explosions, the author received a Ph.D. in Astrophysics at the California Institute of Technology in 2007 where she was a NASA Graduate Research Fellow. She currently holds a Hubble Postdoctoral Fellowship with a joint appointment at Princeton University and the Observatories of the Carnegie Institution of Washington.