What happens when you throw something into a black hole?
As an undergraduate physics student, I was initially drawn to the field of astrophysics because of the notion of the Universe as a Laboratory, where we can probe physics at its most extreme. Yet, we are limited in our ability to perform controlled experiments, to understand the causal connection between the input and output of our experiments. My overarching research goal is to use transient and time-variable events to make 'controlled experiments' of one of the most extreme phenomena in the Universe: supermassive black holes. Astronomers have known for decades that the growth of black holes through the inflow of gas releases copious amounts of energy. What we do not yet understand is how gas inflow leads to the production of the relativistic jets, massive outflows and intense radiation that can affect the host galaxy on scales that are billions of times larger than the black hole event horizon. Transient and time-variable accretion events can solve this problem because they allow us to ask the colloquial question: what happens when you throw something into a black hole? Described below are some of the probes of the causal connection between black hole accretion and ejection that we are pursuing in my group at MIT.
In recent years, we have made important progress in our understanding of black hole X-ray binaries (see Kalemci, Kara & Tomsick 2022 for a review). This breakthrough was enabled by X-ray Reverberation Mapping, a technique I have worked to advanced over the past decade, where X-rays produced close to the black hole reverberate off inflowing gas. This allows us to map scales close to the event horizon—well beyond the resolution of our telescopes.
Thanks to these new techniques and the launch of the NICER Observatory with its unprecedented throughput and time resolution, we find that by using both spectral and timing information, we can understand the accretion flow geometry well enough to accurately measure spins in stellar-mass black holes. This is especially important now that LIGO regularly detects binary black holes. Using mass and spin distributions of these different populations is essential for constraining binary evolution scenarios.
Schematic of the accreting supermassive black hole Mrk 817 during the AGN STORM 2 Campaign, Kara et al., 2021