Master Thesis defense by Katrine Høier Frantzen
Title: Spectral analysis of the kilonova AT2017gfo using TARDIS and X-shooter data
Abstract: On August 17 2017 the Advanced LIGO and Virgo interferometers measured the first gravitational waves signal from the merger of two neutron stars. The detection was followed by measurements of the associated electromagnetic radiation and thus became the first measurement of a so-called the kilonova. The kilonova was later found to originate from the coalescence of two neutron stars. These are thought to be a site of synthesis of heavy elements through rapid neutron capture, and for this
reason this rare type of events has received increasing interest among astronomers during the last decades. In this thesis we present a spectral analysis of the kilonova AT2017gfo based on synthetic spectra created using the radiative transfer code tardis and observed spectra measured by the X-shooter spectrograph. We use the Markov chain Monte Carlo method to compare the synthetic and observed spectra in order to place constrains on the distance, luminosity, density and the velocities of the photosphere and the outer edge. Of these parameters the distance is of great interest. We model the spectra measured during the first four epochs after the explosion and find that the first epoch spectrum gives the best and most reliable fit. As such, we find a best distance estimate to the transient of 38.58 +- 0.02 Mpc, which is consistent with distances found in the literature. We use the inferred distance estimates to calculate the Hubble constant and find a value between 71 and 77 km s-1 Mpc-1.
This is consistent with previous estimates based on measurements from the late universe, and this proves that the emission from neutron star mergers may be a useful tool for cosmology. Furthermore, we investigate the evolution of strontium
(Sr) abundance and find that it decreases over time. Despite leaving behind a number of improvements to be implemented in future work, the consistency between the distance estimate and previously determined estimates is striking considering the assumptions that were made throughout the analysis. This is promising for future work and proves that the method may
become useful for spectral analysis of AT2017gfo and future kilonovae.
Supervisor
Darach Watson, University of Copenhagen, Niels Bohr Insitute
Censor:
Jérôme Chenevez, DTU Space
------------------------------------------------------------------------------------------------------------------------------------------
ZOOM-link for safety regarding COVID-19:
https://ucph-ku.zoom.us/j/69659698376?pwd=OElrajFkL1hsRERjQWtvNGRqYzJ2UT09
Meeting ID: 696 5969 8376
Passcode: 939721