Constraining the progenitors of short gamma-ray bursts
Tunnicliffe, Rachel L.
arxiv: Astrophysics::Galaxy Astrophysics | Astrophysics::High Energy Astrophysical Phenomena | Astrophysics::Cosmology and Extragalactic Astrophysics
So far the progenitors of short γ-ray bursts (SGRBs) have proved elusive.\ud Their presence within both old and young environments and their bias against starforming regions provide tantalising evidence of a neutron star binary or neutron star - black hole merger origin. Within this thesis we study an array of characteristics of the population of SGRBs focussing in particular on their host environments and afterglow properties in the optical and X-ray bands.\ud In particular we consider a set of SGRBs with no detectable host galaxy to deep limits and no clear host in the field from probabilistic arguments. These GRBs either represent a population at high redshift or with high offsets from low redshift hosts. Comparing the offsets of these GRBs from their potential hosts with random positions on the sky we find they are somewhat closer than expected, suggesting these GRBs are more likely to have been kicked from relatively local hosts. \ud We also consider the issue of classification, given suggestions that the often used two second duration divide for SGRBs may produce a sample with a high contamination from collapsar objects or potentially a suggested third class of intermediate objects. We look at a sample of optically-detected SGRBs below the nominal two second divide and go on to consider properties of a larger sample of GRBs comparing varying duration bins. From constructed optical lightcurves and SEDs, we constrain the presence of extinction, jet breaks, supernovae and kilonovae.\ud Though there is a suggestion that such a sample would be 40% contaminated from collapsar objects we find, from supernova constraints combined with duration and spectral hardness fits from Bromberg et al. (2013) that only 22% of objects in our sample could have been collapsars.\ud The optical constraints placed on a kilonova (an r-process transient associated with neutron star mergers), suggest this transient is fainter than has sometimes been predicted but is consistent when considering additional opacities from the rprocess material which could cause strong reddening to the infra-red.\ud Finally, we do not find evidence for a distinct class of intermediate GRBs, though there are likely additional progenitors which create GRB-like objects. At the intermediate duration we do find two unusual individual events not typical of LGRBs: GRBs 100816A and 060505. We find that GRB100816A is most likely a mis-classified SGRB, from its position within its host and the constraint on any associated supernova.
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