School of Physics Thesis Proposal

Presenter:          Julie Malewicz

Title:                    Massive Black Hole Binaries as Multimessenger Sources: Modeling the X-Ray Counterparts to Gravitational Wave Emission

Date:                   Wednesday, April 26, 2023

Time:                   10:00 a.m.

Place:                  Boggs Viz-Lab

Committee:       Dr. Tamara Bogdanovic, School of Physics, Georgia Institute of Technology (advisor)

Dr. David Ballantyne, School of Physics, Georgia Institute of Technology

Dr. Dimitrios Psaltis, School of Physics, Georgia Institute of Technology

Dr. Laura Brenneman, Center for Astrophysics, Harvard University

Abstract:

Following momentous strides in multimessenger observations of both black hole binaries (LIGO Scientific Collaboration and Virgo Collaboration, 2016, e.g.) and lone supermassive black holes (The Event Horizon Telescope Collaboration, 2019, e.g.), all eyes are now turned to supermassive black hole binaries (SMBHBs1 ) as the next milestone in our quest to reach the most fascinating, yet elusive objects in our Universe. Motivated by the anticipated multimessenger detections of SMBHBs in the near future, I propose to model the X-ray spectra and light curves from inspiraling SMBHBs that are prime gravitational wave (GW) sources for the Laser Interferometer Space Antenna (LISA) and the Pulsar Timing Arrays (PTAs). The current and future X-ray missions (like Strobe-X, Athena, or XRISM) are also promising unparalleled energy resolution and sensitivity in the keV range. It is believed that SMBHs reside at the center of most galaxies, with the most luminous among them earning the moniker of Active Galactic Nuclei (or AGN). Dual AGNs may however only be powering a small fraction (< 10−3 ) of those galaxies within redshift z < 0.7 (Volonteri and Natarajan, 2009). This underlines the urgency of a robust framework to detect and properly classify these evasive objects. SMBHBs are thought to primarily, if not exclusively, arise from galactic mergers. Such a pair therefore carries the imprint of the evolution of two initial galaxies, and of the mechanisms that led to their union. Indeed, consistently measuring black hole spin across mass ranges and redshift would allow us to discriminate between different evolutionary pathway scenarios. As such, building a census for supermassive binary candidates among current and future sources would help shed light on galaxy dynamics. Studies of a singular stand-out feature in SMBH spectra, known as the FeKα line, has been used since the 1990s to produce spin measurements in single black hole engines. The rest-frame energy of this component is at 6.4 keV, placing it in the X-ray band. I propose to center that feature in a systematic study of inspiraling supermassive black hole binaries for the first time, using state-of-the-art algorithm relxill to model not only the iron K line, but the full X-ray reflection spectrum that forms the continuum emission around it in the presence of strong relativistic effects. The goals are to build a library of models from a number of time-dependent binary configurations and to subsequently develop the pipeline for parameter estimation on real data. This approach would allow us to gain insights into the characteristic ways binary signals might differ from AGNs powered by single black holes.