For centuries, we have known that our dynamic universe is adorned by cosmic fireworks: energetic and ephemeral beacons of light from a single star that are a million (nova) to a billion (supernova) times brighter than our sun. However, it had been an age-old conundrum that the brightest nova is approximately 1000 times fainter than than the faintest supernova; why should nature leave such a wide \"gap\"?
\r\n\r\nIn search of an answer, I undertook three systematic surveys for my thesis. Since I was looking for transients fainter, faster and rarer than supernovae, I focused my search on galaxies in the local universe. We now have convincing evidence of multiple, distinct populations of rare transients bridging this \"gap\". Perhaps, we are witnessing new stellar physics --- shell detonations in ultra-compact white dwarf binaries, electron-capture supernovae, white dwarfs collapsing into neutron stars and birth of black-holes.
\r\n\r\nA small number of intensively followed-up discoveries of elusive transients sets the stage for population studies with the upcoming \"Large Synoptic Survey Telescope\". This effort works towards building a complete inventory of transients in the local universe (d < 200 Mpc). It better prepares us for the search for potential electromagnetic counterparts to events in the emerging fields of gravitational wave, neutrino and ultra high energy cosmic ray astronomy as these experiments are also limited to the local universe.
", "doi": "10.7907/EA7Q-WX87", "publication_date": "2011", "thesis_type": "phd", "thesis_year": "2011" }, { "id": "thesis:1892", "collection": "thesis", "collection_id": "1892", "cite_using_url": "https://resolver.caltech.edu/CaltechETD:etd-05202004-165422", "primary_object_url": { "basename": "book_main.pdf", "content": "final", "filesize": 6429477, "license": "other", "mime_type": "application/pdf", "url": "/1892/1/book_main.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Cosmic Explosions: The Beasts and Their Lair", "author": [ { "family_name": "Berger", "given_name": "Edo", "orcid": "0000-0002-9392-9681", "clpid": "Berger-Edo" } ], "thesis_advisor": [ { "family_name": "Kulkarni", "given_name": "Shrinivas R.", "clpid": "Kulkarni-S-R" } ], "thesis_committee": [ { "family_name": "Harrison", "given_name": "Fiona A.", "clpid": "Harrison-F-A" }, { "family_name": "Frail", "given_name": "Dale A.", "clpid": "Frail-D-A" }, { "family_name": "Phinney", "given_name": "E. Sterl", "clpid": "Phinney-E-S" }, { "family_name": "Kulkarni", "given_name": "Shrinivas R.", "clpid": "Kulkarni-S-R" }, { "family_name": "Golwala", "given_name": "Sunil", "clpid": "Golwala-S-R" } ], "local_group": [ { "literal": "Astronomy Department" }, { "literal": "div_pma" } ], "abstract": "The diversity of stellar death is revealed in the energy, velocity and geometry of the explosion debris (\"ejecta\"). Using multi-wavelength observations of gamma-ray burst (GRB) afterglows I show that GRBs, arising from the death of massive stars, are marked by relativistic, collimated ejecta (\"jets\") with a wide range of opening angles. I further show that the jet opening angles are strongly correlated with the isotropic-equivalent kinetic energies, such that the true relativistic energy of GRBs is nearly standard, with a value of few times 10^51 erg. A geometry-independent analysis which relies on the simple non-relativistic dynamics of GRBs at late time confirms these inferences. Still, the energy in the highest velocity ejecta, which give rise to the prompt gamma-ray emission, is highly variable. These results suggest that various cosmic explosions are powered by a common energy source, an \"engine\" (possibly an accreting stellar-mass black hole), with their diverse appearances determined solely by the variable high velocity output. On the other hand, using radio observations I show that local type Ibc core-collapse supernovae generally lack relativistic ejecta and are therefore not powered by engines. Instead, the highest velocity debris in these sources, typically with a velocity lower than 100,000 km/sec, are produced in the (effectively) spherical ejection of the stellar envelope. The relative rates of engine- and collapse-powered explosions suggest that the former account for only a small fraction of the stellar death rate. Motivated by the connection of GRBs to massive stars, and by their ability to overcome the biases inhenert in current galaxy surveys, I investigate the relation between GRB hosts and the underlying population of star-forming galaxies. Using the first radio and submillimeter observations of GRB hosts, I show that some are extreme starburst galaxies with the bursts directly associated with the regions of most intense star formation. I suggest, by comparison to other well-studied samples, that GRBs preferentially occur in sub-luminous, low mass galaxies, undergoing the early stages of a starburst process. If confirmed with future observations, this trend will place GRBs in the forefront of star formation and galaxy evolution studies.", "doi": "10.7907/QV7S-4S81", "publication_date": "2004", "thesis_type": "phd", "thesis_year": "2004" } ]