In this series of posts, we sit down with a few of the keynote speakers of the 241st AAS meeting to learn more about them and their research. You can see a full schedule of their talks here, and read our other interviews here!
Professor Erin Kara thinks about all things black holes all the time. Well, not quite — she thinks about all things black hole accretion most of the time (she does have a work-life balance!). Though she is a junior professor at MIT, in her young career thus far, she has already made a name for herself as a pioneer in the study of accretion around black holes large and small. Indeed, at this year’s AAS meeting, she is being honored with the 2022 Newton Lacy Pierce Prize in Astronomy for her innovative and sustained contributions to high-energy astrophysics.
Back to the Beginning
Kara wasn’t always passionate about astronomy — in fact, she started her undergraduate degree at Barnard College thinking that she was going to be a (medical) doctor. Coming into undergrad, “I knew that I liked science, and knew that I liked math and my only model for somebody who like[d] science [was] a medical doctor,” she said. However, all that changed when she took a mechanics class in her first semester taught by Professor Reshmi Mukherjee, who happened to be an astrophysicist. Noticing Kara’s excitement for physics, Professor Mukherjee invited her to join their group for a research project that summer. Soon, she was hooked with the realization that “what scientists are actually doing is not like problems out of a textbook.”
Then, more convinced that she was going to pursue astronomy, Kara followed that first summer of research up with two REUs (Research Experiences for Undergraduates; see this Astrobite for more details about the program), one at Marshall Space Flight Center working on gamma ray bursts, and another with the LIGO group at Caltech. During the latter of these she recalled learning an important lesson about discovering the work environment that worked for her — she found that she “preferred working in smaller groups of around ten or so people,” rather than large collaborations. As her senior year rolled around, she was enjoying studying astronomy and doing research, so she decided to continue on as a Masters’ and later PhD student at the University of Cambridge in the UK, working on X-ray observations of active galactic nuclei (AGN). Though she was pursuing graduate study, at this point, she still didn’t know if she was going to pursue academia. She recounts, “I hated public speaking as an undergrad and the beginning of my PhD, so I thought I could never be a teacher.”
However, by the end of her degree, she had given enough presentations that it actually grew to become something that she enjoyed, and she loved doing research, so continuing with academia was the natural move. She continued from her PhD as a postdoc at NASA’s Goddard Space Flight Center and later as a faculty member at MIT.
Let’s Get Down to Business
Over the course of her career so far, Kara has been tackling the gamut of open questions in black hole accretion and will be discussing some of this work in her plenary. Broadly, she is currently thinking about questions such as “Why are there black holes that are a million or a billion times the mass of the Sun in all massive galaxies? How did they get so big in the first place? And then, what effect does that accretion process have on their environments and on the galaxies in which they reside?”. When we observe supermassive black holes in other galaxies, we find that they are thousands of times less massive than their host galaxies, so in turn, the black hole’s gravitational sphere of influence is miniscule compared to the size of the galaxy. Despite this separation of scales, we also find that the black holes puzzlingly appear to have a hand in things happening on galactic scales (see e.g., this astrobite). To begin to answer this question, Kara (and her group) are “prob[ing] as close as possible to the event horizon of highly accreting black holes.”
To describe this work, Kara drew from the example of the recent Event Horizon Telescope images of two black holes, the 2019 images of M87’s supermassive black hole and the images of our Milky Way’s black hole, Sagittarius A*, from earlier this year. These images, while incredibly difficult to make in their own right, were only possible because the chosen targets, M87* and Sgr A*, are relatively ‘quiet’ black holes, accreting material at a relatively slow rate, so simulating the underlying physics and observing closer to event horizon scales is simpler. Kara’s group focuses on black holes that are many times more active (read: the accretion is stronger). Their challenge, she said, involves “reconstruct[ing] an image even though we don’t have any spatial information about the event horizon scales [by] measuring light echoes, basically measuring X-ray light bouncing off gas flows that are… about to accrete into the black hole.” This is a technique known as reverberation mapping, and is commonly used to measure the mass of black holes. Kara’s group uses these observations in conjunction with their simulations and models for how light interacts with the accretion disk to measure the geometry of these AGN systems. If the models are robust enough, this allows them to start answering questions, for example, about the black hole’s superheated corona, and sheds light onto some of the fundamental properties of the black hole!
In recent years, Kara’s group has extended this sort of analysis beyond just the most massive black holes and has analyzed the geometry of the regions around stellar mass black holes, which we observe in X-ray binary systems, using observations from telescopes such as XMM-Newton and NICER. This work gives an unprecedented view into some fundamental properties of black holes in these systems and can be combined with gravitational wave observations from LIGO to better understand the different populations of black holes that may form in the universe. Moving forward, Kara is also interested in thinking about systems that fall outside our standard paradigm of black hole accretion because of transient events, such as during tidal disruption events (TDEs). She is also interested in looking back in time and trying to identify the seeds of the very first black holes that formed in the first billion years of our universe’s history.
Reflecting on her career thus far, Kara noted that one of the reasons she loves astronomy is that “there are so many open questions that even a first year undergrad could make an impact,” and is thrilled to be able to give such opportunities to undergraduates now as an advisor. However, this abundance of opportunity is a double edged sword. “There are so many fun opportunities and you’ll want to say yes to everything, but then you’ll realize that there are only so many hours in the day,” so it is important to learn to say no and know your limits, a lesson she continues to learn every day. She also emphasized that she came to astronomy at what she felt was relatively late in her academic career, and noted that you don’t necessarily need to have studied astronomy all your life to be a successful astronomer. In kind, she also highlighted the importance of breaking down the “lone genius stereotype” — astronomy is a deeply collaborative field and it “takes all different types of people” with different backgrounds and skills to tackle the sorts of problems she works on every day and loves it for that.
Come hear more about black hole accretion at Prof. Kara’s AAS 241 talk, currently scheduled for Monday, January 9th, 4:40-5:30 pm PT.
Astrobite edited by: Sumeet Kulkarni
Featured image credit: American Astronomical Society