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.


Probing the disk-jet connection in black holes with X-ray Reverberation Mapping.

Aurora Simmonet 

Cover Art for Kara et al., 2019

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.

Multi-wavelength Reverberation Mapping in Active Galactic Nuclei

In the canonical Active Galactic Nuclei reverberation mapping paradigm, the multi-wavelength variability we observe is driven by intrinsic, rapid variability in the X-ray corona. The corona irradiates other gas flows, and those light echoes across different wavebands allow us to map scales from the inner disk (timescales of minutes) to the outer disk (timescales of days) to the broad line region or BLR (timescales of weeks). See Cackett, Bentz & Kara 2021 for a review on different reverberation sub-fields, which have largely developed independently. Now, we aim to bridge the gap between ‘traditional’ optical/BLR reverberation lags and our newer X-ray measurements, and to this end, we are executing large multi-wavelength programs. The largest such campaign to date is the AGN STORM 2 campaign on Mrk 817 (Kara et al., 2021, see Figure). 

Schematic of the accreting supermassive black hole  Mrk 817 during the AGN STORM 2 Campaign, Kara et al., 2021

Serendipity favors the prepared mind: Discovery and follow-up of exotic transients

Credit: DESY, Science Communication Lab

Recent time-domain surveys reveal transient accretion phenomena that defy all predictions. We witness stars torn apart by black holes (known as Tidal Disruption Events) and Active Galactic Nuclei that accrete material much faster than disk theory predicts. My group uses optical, IR and X-ray surveys to find new transients and follow them up.

I am also working to develop future high-energy Time Domain surveys and follow-up facilities. I am the Deputy Principal Investigator of the AXIS Observatory, a NASA Probe Mission Concept that will have fast slew capabilities and a robust community ToO program to catch X-ray counterparts to rare and exotic (but likely faint) transients soon to be revealed by Rubin, Roman, LIGO and LISA. I also co-lead the Science Team of the STAR-X Observatory, a finalist for the next NASA Medium Explorer Mission, that will perform X-ray and UV Time Domain surveys to discover new Tidal Disruption Events and other extreme transients.