The various possibilities for the origin ("progenitors") of
gamma-ray bursts (GRBs) manifest in differing observable properties.
Through deep spectroscopic and high-resolution imaging observations of
some GRB hosts, I demonstrate that well-localized long-duration GRBs
are connected with otherwise normal star-forming galaxies at moderate
redshifts of order unity. Using high-mass binary stellar population
synthesis models, I quantify the expected spatial extent around
galaxies of coalescing neutron stars, one of the leading contenders
for GRB progenitors. I then test this scenario by examining the offset
distribution of GRBs about their apparent hosts making extensive use
of ground-based optical data from Keck and Palomar and space-based
imaging from the Hubble Space Telescope. The offset distribution
appears to be inconsistent with the coalescing neutron star binary
hypothesis (and, similarly, black-hole--neutron star coalescences);
instead, the distribution is statistically consistent with a
population of progenitors that closely traces the ultra-violet light
of galaxies. This is naturally explained by bursts which originate
from the collapse of massive stars ("collapsars"). This claim is
further supported by the unambiguous detections of intermediate-time
(approximately three weeks after the bursts) emission "bumps" which
appear substantially more red than the afterglows themselves. I claim
that these bumps could originate from supernovae that occur at
approximately the same time as the associated GRB; if true, GRB 980326
and GRB 011121 provide strong observational evidence connecting
cosmological GRBs to high-redshift supernovae and implicate massive
stars as the progenitors of at least some long-duration
GRBs. Regardless of the true physical origin of these bumps, it
appears that all viable alternative models of these bumps (such as
dust scattering of the afterglow light) require a substantial amount
of circumburst matter that is distributed as a wind-stratified medium;
this too, implicates massive stars. Also suggested herein are some
future observations which could further solidify or refute the
supernova claim. In addition to the observational and modeling work,
I also constructed the Jacobs Camera (JCAM), a dual-beam optical
camera for the Palomar 200--inch Telescope designed to follow-up rapid
GRB localizations.
Joshua Simon Bloom attended Harvard College
(1992-1996) for his undergraduate degree in Physics and
Astrophysics. On a Herschel Smith Harvard Fellowship, he then obtained
a Masters of Philosophy degree at Cambridge University (1997) under
the tutelage of Professor Martin Rees. With Professor Shrinivas
R. Kulkarni as adviser, the author obtained a Ph.D. in Astrophysics at
the California Institute of Technology (Caltech) in Pasadena (2002),
where he was a Fannie and John Hertz Foundation Scholar. The author
is currently a Junior Fellow to the Harvard Society of Fellows. He
resides in Cambridge, Massachusetts.
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