Date of Award
12-2025
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Physics and Astronomy
Committee Chair/Advisor
Marco Ajello
Committee Member
Bradley Meyer
Committee Member
Jonathan J. Zrake
Committee Member
Stephen R. Kaeppler
Abstract
Galaxies are vast cosmic islands of stars, dust, and gas. They come in various shapes and sizes. They might look static for the timescales we are used to, but they are dynamic and collisional systems that can merge with other galaxies to make bigger galaxies. Most galaxies, including the Milky Way, host a central supermassive black hole (SMBH) with a mass greater than a million (sometimes a billion) times the mass of the Sun. When galaxies merge, their SMBHs form a binary system before ultimately merging. These cosmic duets are of great interest to astronomers and astrophysicists as they are prime candidates for gravitational wave (GW) signals. Discovering such binary systems is important for current and upcoming GW detectors and understanding galaxies’ mergers and evolution.
Binary stars or smaller mass black hole binaries can be identified through dynamical analysis or direct imaging, but these SMBH binary systems are typically too far away for direct imaging; thus, we look for their secondary signatures. One of the most promising signatures of binary SMBHs is a periodic variation in the emission. When the two SMBH are dancing around each other, the relativistic jet from one of the black holes gets periodically disturbed by the motion of the other one. My research in this thesis focuses on analyzing the flux from blazars, a class of galaxies whose jets are pointing towards us. I analyzed multiple decades of data from various optical and gamma-ray telescopes, along with a century’s worth of historical optical data, to search for periodic variability in the emission of blazars.
The first part of my research focused on a well-known candidate of a binary SMBH system called PG 1553+113 because of the system’s stable and prominent 2.2-year periodic flux variation. I analyzed about 100 years of historical optical data for any secondary periodicity and discovered a second oscillation of ∼22-year along with the faster 2.2-year one. The dual periodicity happening by chance was calculated to be extremely low. I ran simulations of the binary system in our University’s ii supercomputer to test the possibility of the existence of the 10:1 dual periods. I found that the double periodicity can be explained by a binary SMBH, reinforcing the candidacy of PG 1553+113 as a binary SMBH system. I also made a testable prediction that the next emission peak of the 22-year oscillation occurs around July 2025.
The second project expanded this study to all bright blazars recorded in the 2FHL FermiLAT catalog, a catalog for the brightest objects in the γ-ray sky. I found ten more candidates in the catalog for the analysis. Further rigorous investigation led me to consider four objects, and the signals in three likely showed periodic variability due to the nature of the data. After more statistical analysis, none were promising candidates for binary systems. This project highlights the need for robust statistical testing of periodicity and the need to be careful when using archival data.
The research works presented in this thesis present strong evidence for a binary SMBH in the PG 1553+113 system and also highlight the necessity of robust statistical methods while looking for the secondary signatures of these elusive cosmic duets.
Recommended Citation
Adhikari, Sagar, "Cosmic Duets: A Search for Binary Supermassive Black Holes in Merging Galaxies" (2025). All Dissertations. 4196.
https://open.clemson.edu/all_dissertations/4196
Author ORCID Identifier
0009-0006-1029-1026
Included in
Cosmology, Relativity, and Gravity Commons, External Galaxies Commons, Instrumentation Commons, Physical Processes Commons