Peering Into The Depths Of The Event Horizon

17:20 minutes

A star being sucked into a small black hole
A stellar-mass black hole. Credit: NASA/CXC/M. Weiss

Discovered only decades ago, black holes remain one of the universe’s most mysterious objects, with such a strong gravitational pull that light—and even data—can’t escape. Oftentimes researchers can only observe black holes indirectly, like from blasts of energy that come from when the massive bodies “feed” on nearby objects.

hot gas orbiting a black hole
This artist’s impression shows hot gas orbiting a rapidly-spinning disk. Click through for the full image. Credit: NASA/CXC/M. Weiss

But where is that energy generated, and how does that eating process actually progress through the geometry of the black hole? Erin Kara, a postdoctoral fellow at the University of Maryland and NASA Goddard Space Flight Center, describes new research published in Nature into how echoes of X-rays in small, stellar-mass black holes can point the way.

At the other end of the spectrum, supermassive black holes billions of times the mass of our Sun are believed to dwell at the hearts of galaxies. Many are active, drawing in nearby gas and dust and emitting energy in response, but others are dormant, with nothing close to feed on. MIT postdoctoral fellow Dheeraj Pasham talks about what happens when these dormant black holes suddenly encounter and tear apart a star—and how the fallout can shed light on how these black holes spin. His research appeared in Science this week.

The researchers also discuss how black holes could lead the way to understanding how galaxies evolve, and other black hole mysteries.

Further Reading

Segment Guests

Erin Kara

Erin Kara is a postdoctoral assistant in Astronomy, at the University of Maryland and NASA Goddard Space Flight Center in College Park, Maryland.

Dheeraj Pasham

Dheeraj Pasham is a postdoctoral fellow in Astronomy at the MIT Kavli Institute for Astrophysics and Space Research in Cambridge, Massachusetts.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Black holes live eventful lives. It makes sense, right? You have voracious gravitational fields gobbling up any matter or energy that gets too close. These feats of matter often turn to bursts of energy, which is some of the few ways we can actually observe a black hole because it is a black object. But we have a lot of questions still about the structures around black holes and how the black holes evolve as they absorb and emit energy over time. Where does that energy actually come from? 

And black holes spin but we can’t actually measure how fast they do it all the time. Well, new research in two journals this week reveals some surprising answers, including a black hole spinning at half the speed of light. We’re going to talk about that with my next guests. Dheeraj Pasham is MIT’s Kavli Institute in Cambridge, Massachusetts. Welcome to Science Friday. 

DHEERAJ PASHAM: Hi, happy new year. 

IRA FLATOW: And Erin Kara of NASA’s Goddard Space Flight Center and the University of Maryland in College Park. Welcome to Science Friday. 

ERIN KARA: Thank you, great to be here. 

IRA FLATOW: Let me ask both of you. This idea of a black hole spinning at half the speed of light– it sounds crazy! I mean, we’re going to get back to that in a moment. But first, I want you to help me visualize the structures we’re talking about. A black hole isn’t just a hole. It’s got stuff around it, right, Dheeraj? 

DHEERAJ PASHAM: Yes, that is correct. 

IRA FLATOW: Tell us about that. 

DHEERAJ PASHAM: So most astronomers think that the way to think about structures around the black hole is you have the black hole and there is some sort of an accretion flow around the black hole. It’s debated what the structure of this accretion flow is, but it’s most likely in some sort of disk-like. 

IRA FLATOW: Why would it be formed into a disk? I’m thinking of the rings of Saturn. Is it spinning? Is that what makes it form into a disk? 

DHEERAJ PASHAM: So the reason you have disk is to conserve angular momentum. So when [INAUDIBLE] starts off from far distance, as it gets closer, it has to still conserve angular momentum. In order to do so, it forms sort of a ring-like structure around the black hole. 

IRA FLATOW: Erin, you observed something that might explain the inner workings of small black holes. What was going on in this black hole you observed? 

ERIN KARA: Yeah, that’s right. So in addition to this accretion disk that DJ was talking about, we also see a lot of emission coming from this very mysterious part close to the black hole. It’s a region of subatomic particles– electrons, positrons, protons– that are moving very quickly, and are very hot, about a billion degrees Celsius. And we call this region the corona and it emits in high-energy X-ray light. And the thing is, we know that this emission is coming somewhere from close to the black hole, but we don’t understand a lot about what this structure of the corona is. How is it so hot in the first place? And so that’s part of the work that we did for this paper. 

IRA FLATOW: It sounds familiar because we really don’t understand the corona around the sun. 

ERIN KARA: Exactly. Yeah, and it’s akin to that. You can make clear analogies. Corona in Latin means crown, so the corona of the sun is something that’s around the sun. It’s a hot plasma around the sun and, similarly, there’s this corona around a black hole. It’s this hot plasma that kind of is a halo around this accretion disk. 

IRA FLATOW: Let’s talk about this accretion disk. What happens to the energy that comes out of the black hole? I understand there’s like an echo in the black hole. Explain that please. 

ERIN KARA: Yeah, that’s right. So we want to understand more about what this corona looks like, what the structure of the accretion disk is, and yet these systems are incredibly small and they’re very far away. So this particular stellar-mass black hole system that I was observing is 10,000 light years away. And so we can’t really ever spatially resolve these regions close to the black hole. And so we need to rely on other properties of the light to kind of infer what’s going on. 

And so what we were able to measure in this observation, thanks to an incredibly bright outburst and a new telescope called NICER on the International Space Station, is that we were measuring light echoes. Basically, this corona, this hot, hot plasma that emits in high-energy X-rays, it will shine in all directions, and some of those photons will come to us directly. And some of those photons will hit the accretion disk, causing the disk to heat up again and readmit light. 

And there will be some time delay between that reprocessed light from the accretion disk and the corona light. And that time delay is due to the light travel time between the accretion disk and the corona. So kind of just like how bats use echolocation to map out a dark cave that they can’t see with their eyes, we’re using these light echoes to map out what it must look like around a black hole that we can’t spatially resolve with our telescopes. 

IRA FLATOW: So you’re saying by measuring the difference in time it takes for the light to get bouncing indirectly from the black hole, you know how big that accretion disk might be? 

ERIN KARA: Yeah. So in this particular discovery, we were able to measure light echoes that were smaller than ever before possible in one of these stellar-mass black hole systems. We’re talking about time delays of half of a millisecond, and this is closer to the black hole than ever before possible. And it helped us understand the structure of the corona, actually. 

What we discovered was that as we watched the system evolve over several weeks, these light echoes that already started out incredibly small, got shorter and shorter by an order of magnitude. So the light that got closer together suggested that the corona was actually becoming more compact over time. 

IRA FLATOW: Wow. It’s getting smaller and smaller. I’m thinking the corona is surrounding it, so it starts shrinking? 

ERIN KARA: Yeah, exactly. So it goes from something kind of vertically extended and then compacts into something extremely close and spherical to the black hole. And this vertical structure, now that we can constrain that it has this vertical structure, it may be hinting that the corona is related to the base of this relativistic jet that you see coming out of a lot of black hole systems. And so it could be that the corona is really the birthplace of jets. 

IRA FLATOW: Wow. There’s jets coming out the top and the bottom of the– 

ERIN KARA: That’s right. 

IRA FLATOW: –black hole. Why does a black hole have a top and a bottom? 


ERIN KARA: Well, you have material that’s kind of orbiting around this black hole in this accretion disk and it has a direction to it. And then energy just has to come out somewhere and it comes out, basically, along the rotation axis of this system. And basically, the jets themselves and other kinds of outflows just take, in some ways, the path of least resistance, kind of perpendicular to this accretion flow. 

IRA FLATOW: One question. In the introduction, I was talking about reading the research. It’s saying that the black hole is spinning at half the speed of light? 

ERIN KARA: Yeah. So a lot of these black holes that we measure, both the stellar-mass and the supermassive black hole systems, we can measure how fast they’re rotating. And we can make these kinds of observations in systems that have strong accretion disks, like the one that I studied. And we do find that they are spinning extremely fast, most of them. And in DJ’s work, he was able to put these new constraints on how fast the black hole is rotating in a system that doesn’t normally have an accretion disk. And this is one of the first constraints that we have on measuring the spin, the rotation in a “dormant” black hole. 

IRA FLATOW: Dheeraj, how confident are you that it’s spinning at such a high rate? And is that rate surprising? 

DHEERAJ PASHAM: So this is a different kind of a black hole, compared to what Erin was studying. This is a supermassive black hole that is sitting in the center of a galaxy that’s 290 million light years away. And before November of 2014, there has not been any emission from near the black hole. But around November of 2014, a star passed close enough and it got shredded and that provided a soup of gas for the black hole to feed on and that is what we observed. 

We observed the X-ray emission from this shredded material as it approached the event horizon of the black hole. And by essentially tracking the motion of this material, we were able to figure out what is the last orbit of this material. And the last orbit the material is a strong function of the spin on the black hole and that’s how we are able to constrain the spin. 

IRA FLATOW: Is that sort of like the Doppler shift that you measure or some other way? 

DHEERAJ PASHAM: We actually measure the regular changes in X-ray emission, X-ray radiation from the source, and there was a periodicity associated with that. And if you assume that this periodicity is associated with a blob just about to fall into the black hole, then you have the size of the orbit of that blob. And that size is directly proportional to the spin of the black hole, so that’s how we were able to set a constrain a lower limit on the spin of the black hole. 

IRA FLATOW: We have a tweet from Darrell who says, “a black hole spinning at half the speed. Is that the surface of the black hole or the event horizon?” 

DHEERAJ PASHAM: So I think the best way to think about it is in terms of test particles, so imagine a test particle in the equatorial plane of the black hole. It’s the change in the azimuthal position of the test particle as a function of time. So the black hole itself is not rotating, it’s the space that’s around it that’s actually getting dragged because of the angular momentum of the black hole. 

ERIN KARA: So as these black holes spin faster and faster, it changes the spacetime. And it means that, basically, the inner edge of that accretion disk gets moved in closer and closer to the black hole as you spin it up faster and faster. So by measuring basically the material moving in that last orbit, you can constrain what that last orbit is. And then that will give you an estimate of what the spin must be. 

DHEERAJ PASHAM: Yeah, so that’s what we did. The periodicity that we observed, we associate that with the last orbit of the material. 

IRA FLATOW: Wow. You know, I’m trying to think of what the spacetime around a black hole spinning that quickly must be like. Is it imaginable? Is it dragging the universe with it as it spins like that? You know, because we’ve heard from studying gravity waves that the universe is really a gravity, it’s just a good field of gravity dragging all of that stuff with it. 

DHEERAJ PASHAM: I mean, I wouldn’t say all of it. Yeah, it’s definitely dragging the space right next to it. So right next to the black hole, the space is just being dragged. 

IRA FLATOW: Does it look like a whirlpool? 

DHEERAJ PASHAM: Yeah, it’s exactly like a whirlpool, if you want compare. 

IRA FLATOW: My mind is sufficiently blown, to give the idea. I’m Ira Flatow. This is Science Friday from WNYC Studios. So around a spinning black hole– how fast and how many times a second would it be spinning? 

DHEERAJ PASHAM: So you’d be spinning once every two minutes. 

IRA FLATOW: Once every two minutes at half the speed of light. That is a big black hole. 

DHEERAJ PASHAM: Yeah. So if you want to compare, Earth takes 24 hours to rotate, right? And this black hole is roughly 300 times bigger than Earth and it’s spinning 700 times faster than Earth. So imagine an object that is 300 times bigger than Earth rotating 700 times faster than Earth. 

IRA FLATOW: I can’t. 


IRA FLATOW: I’ll try. So this is just great useful information. It’s wonderful to hear it. But is there anything useful about knowing how fast the spin is? What do we do with that information? 

DHEERAJ PASHAM: Yeah. So spin contains information about how the black hole grew in the first place. So supermassive black holes– we currently don’t know how supermassive black holes grew since the last couple of billions of years. There’s two channels, or two major channels, people think. One is supermassive black holes can grow by mergers with other supermassive black holes. So they start off small, but they can merge with other black holes and then get to the weight that they are now. Or the other way is they could be embedded in a big soup of gas that can constantly be dumped out of the black hole and that’s another way to grow the black hole. 

So in the first case, when you have mergers of just black holes with random spin orientations with respect to each other, in the end, after several, several mergers, you would expect the spin to be very low because you’re just canceling out the angular momentum as you’re merging in random orientations. But in the second case, when you’re constantly dumping material in the same direction, what you’re doing is you’re spinning up the black hole. So in the second case, you end up with the highly spinning black hole. 

So by just knowing this one number, the spin of the black hole, you are able to tell the channel through which it grew all the way from a couple of years after the Big Bang until now. 

IRA FLATOW: I saw some news this morning. Researchers were saying they saw a flare last June that may have been our first look at the birth of a black hole. Erin and Dheeraj, is it too early to say? 

ERIN KARA: Yes, so this was an exciting discussion at our recent American Astronomical Society meeting this past week. This is a transient called AT2018cow, and that’s just because it was assigned to those numbers and those letters as this new transient was found. But it just happened to spell out cow, so we call it the cow, and it was an extremely bright flare in another galaxy. Not in the center of the galaxy, but kind of in the spiral arms of this galaxy. 

And it was a very fast evolving flare and, after the initial discovery, it was announced to the community. Many, many telescopes from all different wavelengths of across the electromagnetic spectrum started taking observations of this object and we still don’t understand what it is. Whatever it is, it’s a completely unprecedented finding which actually happens quite a lot in astronomy. We’re often surprised, but it could be that it’s some new kind of fast evolving supernova. This is the death of a massive star and collapse into a black hole. 

Or the other competing interpretation is that it is sort of like a system that DJ was telling us about, that it’s a black hole and a star– in this case, it would have to be a white dwarf star– gets too close to that black hole and it gets ripped apart. And what we’re watching in real time is the feeding of that white dwarf as it falls into that black hole. And again, in this case it would also launch these relativistic jets and fast outflows. 

IRA FLATOW: Exciting stuff, exciting stuff. Thank you both for taking time to be with us today. 

ERIN KARA: Thank you so much. 

IRA FLATOW: You’re welcome. Dheeraj Pasham, postdoctoral fellow at MIT’s Kavli Institude in Cambridge, and Erin Kara, postdoctoral assistant at NASA’s Goddard Space Flight Center and University of Maryland in College Park, Maryland. Thanks again, and have a great weekend. 

DHEERAJ PASHAM: Thanks for having us. 

ERIN KARA: Thank you. 

IRA FLATOW: That’s about all the time we have. BJ Leiderman composed our theme music. And if I missed any part of the program, of course you’ve got our podcast and you can play our podcast on your smart speaker now. Just ask it to play and you get the latest episode of Science Friday. And we’re active all day on our website and on all social media. Have a great weekend. I’m Ira Flatow in New York.

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