Master defense by Charlotte Cramer Kristensen

Title: Investigation of the existence and expected mass of a companion black hole for Sagittarius A*

Abstract: Binary systems is a well known phenomenon in the Universe. This thesis has investigated the possibility of the supermassive black hole, Sagittarius A*, in the center of the Milky Way being part of a binary system with a second black hole. In order to obtain knowledge of the companion black hole, it has been assumed that the two black holes are part of a triple body system with a star as the third body. Using this assumption, the Konzai-Lidov mechanism has been implemented in order to obtain an upper limit for the mass of the companion black hole. Previous work by Naoz et al. (2020) presented the opportunity of a companion black hole with a mass that does not exceed $10^5$ $\Msun$ with a semi-major axis greater than 170 AU for a circular orbit. This maximum mass has been reduced by using further stellar orbital properties of 12 different stars orbiting Sgr. A*. \\
The estimated maximum masses are $6.5 \cdot 10^4$ $\Msun$ with a semi-major axis of 150 AU for a circular orbit and $3.1 \cdot 10^4$ $\Msun$ with a semi-major axis of 115 AU for a highly eccentric orbit. This maximum mass of the companion black hole corresponds to a intermediate black hole, which still is a type of black hole under investigation of its origin. The confirmation of a companion black hole will be a possibility in the coming decade of the 2030's with LISA, which can detect gravitational wave radiation that the black hole will - if existing.