Different pathways for the formation of GRB progenitors through binary stellar evolution
(Thesis in collaboration with the University of Geneva)
Most massive stars are formed in binaries. Stellar evolution in a binary differs from that of isolated stars, because it can be affected by transfer of mass and angular momentum between the binary components, leading to a wide variety of different possible evolution pathways. Some of these may endow the star with properties that, in the moments that follow the core collapse, enable the launch of a relativistic jet and hence the emission of a gamma-ray burst. The aim of this thesis project is to analyze a suite of detailed binary stellar evolution simulations produced with POSYDON in order to identify the potential GRB progenitors and their properties. This population of simulated progenitors can be used to explore the predicted properties of the population of GRBs, the link with the progenitor properties, and the rate of these events.
Impact of relativistic aberration effects on the photospheric spectrum of kilonovae
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| Spectrum of AT2017gfo, from Sneppen 2023 |
Kilonovae are rapidly evolving infrared-optical-ultraviolet transients produced by the expanding ejecta of binary neutron star mergers, powered by the decay of unstable heavy elements produced by rapid-neutron-capture nucleosynthesis in the first instants following the merger. Exquisite spectra of kilonova AT2017gfo, associated to the binary neutron star merger GW170817 detected through gravitational waves by the LIGO-Virgo interferometer network in 2017, have been obtained with the X-Shooter spectrograph at the Very Large Telescope. These spectra reveal many broad absorption features, but the underlying continuum is very well consistent with a blackbody. This is somewhat at odds with the mildly relativistic expansion velocities of the ejecta, which should produce deviations from a blackbody spectral shape owing to relativistic light aberration. This thesis project will focus on improving the understanding of the impact of relativistic aberration on these spectra, in order to derive constraints on the ejecta structure from the observed spectral shape.