Research

How do supermassive black holes grow and shape their environments?

How do the complex stellar and gaseous ecosystems of galactic nuclei regulate that growth?

We are living a renaissance in black hole accretion physics thanks to new time domain facilities that are up-ending what we thought we knew about how supermassive black holes (SMBHs) grow via accretion and mergers. We are witnessing previously quiescent black holes suddenly ‘turn on’ due to the tidal disruption of unfortunate stars on close-approach to black holes; we regularly discover AGN that accrete material on timescales orders of magnitude faster than we thought theoretically possible; we are seeing exotic quasi-periodic X-ray flaring in disks around SMBHs, that are posited to be due to the impact of a stellar mass object with the accretion disk, and may lead to joint EM-GW discoveries with LISA. We hope to soon discover other LISA sources, too, with the robust discovery

of SMBH binaries at milli-pc separations.

In addition to these seemingly rare, transient events, we know that SMBHs sit at the centers of nearly all massive galaxies and all have undergone some period of growth via accretion. In my research I try to bridge the gap between “standard accretion” as seen in Active Galactic Nuclei (AGN) that persists for many millions of year, and transient phenomena in Tidal Disruption Events (TDEs) and Changing-Look AGN (CLAGN), where we observe an impulse of accretion over just several weeks to months. I believe the only way to truly understand these extreme events is by also understanding the physics of accretion in nearby, more typical AGN.

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.

Using Tidal Disruption Events as probes of galactic nuclei and extreme accretion

Tidal Disruption Events occur when a star gets so close to a black hole that its gravitational tidal forces rip the star apart. Theorized back in the 1980s, these events are now observed regularly thanks all-sky surveys, monitoring the night sky across optical, IR and X-ray wavelengths, searching for new transient phenomena. I'm particularly interested in the X-ray counterparts to TDEs because they anchor our interpretation of these events: when we see X-rays, we know that the stellar debris has formed an accretion disk around the black hole. Then we can use these events to understand what happens next: we observe the formation of hot X-ray coronae, we see relativistic outflows launching. TDEs gives us a new window into understanding the initial conditions required to launch jets and outflows in all types of accreting systems.

Hydro simulation of a TDE by James Guillochon

Quasi Periodic Eruptions: EM signatures of Extreme Mass Ratio Inspirals?

Quasi-Periodic Eruptions are a new class of extragalactic transients that were discovered serendipitously in 2019. They are regular, high-amplitude soft X-ray flashes that occur every few hours to days from supermassive black holes. Up to this point, Active Galactic Nuclei were mostly only known to vary stochastically, and so this discovery was a complete shock to many of us, including myself. Since then, we have been searching for more, and working to characterize these systems, by mapping their timing and spectral signatures. While we still do not understand their origin, one of the most compelling interpretations is that they arise from stellar mass objects orbiting supermassive black holes. If correct, we can use QPEs to constrain the population of stars and compact objects in the nuclear star clusters around supermassive black holes. QPEs may also be detected concurrently in low-frequency gravitational waves with the LISA Observatory in the next decade.

Figure credit: Ricardo Arcodia

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: Mission Development

Credit: DESY, Science Communication Lab

I also work on developing future high-energy Time Domain surveys and follow-up facilities. I was the Deputy Principal Investigator of the AXIS Observatory, a NASA Probe Mission Concept, and am a NASA Participating Scientist for the ULTRASAT mission, and a member of the TDE and AGN working groups. I am also a member of the LISA Science Team, with the aim of uncovering Multi Messenger transients, such as binary supermassive black hole mergers, in both electromagnetic and gravitational radiation.

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