Mathematics – Logic
Scientific paper
Dec 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt.........4p&link_type=abstract
Thesis (PhD). HARVARD UNIVERSITY, Source DAI-B 60/06, p. 2749, Dec 1999, 122 pages.
Mathematics
Logic
Afterglows, Absorption Spectrum, Microlensing
Scientific paper
Gamma-Ray Bursts (GRBs) have challenged astrophysicists ever since their discovery almost thirty years ago. With an energy release of 1051-53 ergs, GRBs are the most electromagnetically luminous objects in the universe. One of the many unanswered questions about GRBs is the nature of the source that powers them. This thesis studies the interactions of GRBs and their afterglows with their surrounding environment, and shows how the signatures left by this interaction can yield valuable information on the properties of the GRB sources. We show that the passage of the afterglow through its surrounding medium produces time-dependent lines in its absorption spectrum, whose identification could serve to precisely determine the GRB redshift and tightly constrain the density of the GRB environment. The ionized medium eventually cools and recombines. We study the spectral features expected from a cooling GRB remnant and identify signatures in the line diagnostics which are peculiar to GRB remnants. Identification of such remnants in nearby galaxies will allow direct observations of GRB sites and degree of beaming. Besides photoionizing the medium, GRB explosions also have dynamical effects: they result in expanding blast waves, some of which should still be visible in our galaxy and nearby ones. We suggest that they might have already been observed under the form of expanding HI supershells. GRB energetics and rates are crucially dependent on the beaming fraction. We use the data from radio surveys to place the constraint q>~6 ° on the beaming angle. Since GRBs are cosmological sources at high redshift, they can be used as tools to explore the universe. We study the observable signatures that result when a GRB afterglow is microlensed by an intervening star. We show how a microlensing event could be used to study size and superluminal expansion rate of the source, and the structure of the magnetic field on the afterglow photosphere. At the same time, the fraction of microlensed afterglows can be used to calibrate the density parameter of compact objects in the universe.
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