Master Thesis defense by Albert Bjerregård Sneppen

Title: The Spherical Symmetry of the Kilonova AT2017gfo

Abstract: The geometry and composition of the astrophysical transients called kilonovae is set by extreme and observationally ill-constrained physics. It requires an understanding of the physics of neutron stars with their incredible gravity, high-density and strong electromagnetic fields. Furthermore, it depends on the the nature of neutrino trans port and by the extreme energetics of a putative, potentially powerful and short-lived massive remnant. Additionally, an understanding of kilonovae ejecta is essential in revealing the formation of all the heaviest elements of the periodic table. Despite the unique physical processes worth probing and a multitude of prior analysis, there exists as of yet no tight observational constraints on either the geometry or composition of these extreme explosions.

In this thesis, we determine the first precision measurements of geometry for the only well-studied kilonova to date, AT2017gfo. We will exploit the blackbody nature of the early kilonova spectra and the strong Sr+ P Cygni absorption-emission feature, which in conjunction constrain the sphericity at multiple epochs by comparing its radial line velocity to its tangential emitting area. Contrary to the typically aspherical ejecta of current hydrodynamical merger models, we show that the kilonova is highly spherical in early epochs. Line shape analysis combined with the known inclination angle of the source independently validate the same sphericity. The highly spherical ejecta not only challenges and constrains future hydrodynamical merger modelling, but it may also imply that kilonovae such as AT2017gfo are precise cosmological probes of distance.

Supervisor:

• Darach Watson, University of Copenhagen, Niels Bohr Institute

Censor:

• Allan Hornstrup, DTU Space