A thesis for the degree of Doctor of Philosophy defended June, 2018.
The PhD School of Science, Faculty of Science, DARK, Niels Bohr Institute, University of Copenhagen
Lise Bech Christensen
Illuminating the Dark with cosmic explosions and their afterglows
The large majority of our Universe is dark, its vast expanse only sparsely permeated by a thin web of matter. Inside this web, gravity has worked to form islands of light made up of immense collections of stars. These conglomerations of matter, what we have come to know as galaxies, carry with them the story of the life they have experienced in the form of their chemical composi- tions. At any time, this composition is set by the seed gas from which they were formed, but also the inevitable enrichment carried by the stellar appetite for turning light elements heavier. The cataclysmic explosions that either mark the end of stellar lives or are the results of mergers of stellar remnants, scatter enriched material throughout galaxies in which they live, providing the sustenance of a new generation of stars, richer and more complex in their compositions. In order for us to understand our place as humans in this universe, we need to investigate the cosmic history carried by the elements from which we are formed. This thesis investigates the most powerful explosions in the universe, gamma-ray bursts, how these can be used to probe the dark universe intersected by their light, and what consequence these events have for the universe that harbors them. I here present a homogeneously selected sample of gamma-ray burst (GRB) afterglows that are the results of more than eight years of sustained effort. This dataset is a unique resource that can be used to harvest the potential of GRB afterglows in illuminating the universe. These spectra contain rich information about the gas, dust, metals, molecules surrounding the GRB explosions, spanning a wide range of redshifts, thus providing unique insights in the star formation and chemical history of the universe. This treasure trove allowed me to discover especially significant events and in particular, I was able to single out GRB 111117A as the most distant short GRB ever discovered. The study of this single event allowed to me to constrain the intrinsic redshift distribution of short GRBs and the conditions of the progenitor system of short GRBs. I also present the observations of an electromagnetic counterpart to a simultaneous gravitational wave signal and short GRB, which is caused by the merger of two neutron stars. These data hold the secret to understanding the physical mechanisms behind this enigmatic phenomenon that has now come to be named a kilonova. Lastly, in these spectroscopic data of the kilonova, the spectral signatures of light r-process elements are identified, providing the first compelling evidence for the site of at least the light r-process elements.