IRA FLATOW, host:

You're listening to SCIENCE FRIDAY from NPR News. I'm Ira Flatow. Here's something new to talk about this evening. If you want something new to talk about, how about black holes? And there's some interesting news about black holes. You know, they don't just come in one size. It's just like the three bears, black holes can be really big or really small, and it's that middle-sized one that has eluded astronomers until now.

Tucked into the reams of data from the European spacecraft XMM-Newton that has beamed back to Earth all that data is evidence of a new type of black hole, the size of which falls somewhere between the really big ones called, appropriately, super-massive black holes, and the much smaller stellar-mass black holes. And joining me now to talk about the evidence for this new kind of black hole is Sean Farrell. He is the XMM-Newton catalog scientist in the department of physics and astronomy at the University of Leicester in the U.K. Welcome to SCIENCE FRIDAY.

Dr. SEAN FARRELL (Catalog Scientist, XMM-Newton, Department of Physics and Astronomy, University of Leicester): Well, thank you very much, Ira.

FLATOW: Tell us about this new kind of black hole.

Dr. FARRELL: Well, for a long time, astronomers have theorized that there might be something in between the small ones, as you put it, and the super-massive ones. Now there have been lots and lots of claims put forward of discoveries of ones in this intermediate-mass range, these middle-sized ones, but until this discovery, we haven't really had any strong evidence. And it's not really been well-accepted by the other astronomers.

FLATOW: Tell us about the spacecraft, the XMM-Newton. What does - what is its mission?

Dr. FARRELL: Well, XMM-Newton is the European Space Agency's premier space telescope. It's an enormous space telescope that observes in X-rays, and it's actually in many ways similar and yet complementary to NASA's Chandra space telescope. And it carries two main instruments in X-rays, and one of them which is an imaging instrument, the EPIC camera, and also an optical monitor, an optical telescope. And we use the data from the EPIC X-ray camera in a catalog that we actually helped build called the 2XMM Serendipitous Source Catalogue. And we were looking for white dwarves and neutron stars, and that's when we found, accidentally, this black hole.

FLATOW: Now is it important that you found this middle-sized black hole?

Dr. FARRELL: Well, yes, I think it is. The issue for quite a while has been, why don't we have something between the stellar-mass and the super-massive black holes? You know, if we have a small one and we have a really big one, why don't we have one in the middle? And up until recently, there's been a lot of conjecture by the astronomers and the theorists as to how you would get one in between. And I think that providing strong evidence that they actually exist, whether or not we can explain how they form, you know, actually finding one is going to sort of tip the balance on that one.

And the most important conclusion from this also is, we don't really know how the super-massive black holes form, but a very popular theory is that you may form a super-massive black hole in the middle of a galaxy from mergers of smaller black holes. So you start with the stellar-mass black holes, which then merge with another black hole. And they grow, and then they grow through an intermediate-mass phase, and then they arrive at a super-massive range, millions to billions of times the mass of the sun.

FLATOW: But you had to find that intermediate range to support the theory?

Dr. FARRELL: Yes, and as I said, there have been lots of claims in the past of good candidates, primarily from this popular of what we call the ultra-luminous X-ray sources. These are objects which are found to be residing in distant galaxies that are very, very bright in X-rays, far brighter than you'd expect to produce from a small black hole. The X-rays comes from gas and dust that swirls around the black hole and heats up to very high temperatures and then emits X-rays.

Now the illuminating brightness for these black holes is set by the mass of the black hole. So small black holes can glow at a certain brightness. Super-massive black holes can glow a lot brighter than that, but we found all these objects - or have been found in the past to glow in the range between the two. So that indicated that there may well be intermediate mass, but there have been a lot of very clever theories that can explain these luminosities, these brightnesses, through other means.

So, up until now, there's always been some doubt as to whether these ultra-luminous X-ray sources are actually intermediate-mass black holes, and I suspect that many of them are not, but some of them might be. Where this one stands out is it's actually 10 times brighter than the previous record-holder. So while it was getting difficult to explain the really bright ones previously, now we've got something that's even 10 times brighter than that. It becomes very difficult to explain it without having quite a massive black hole.

FLATOW: 1-800-989-8255, talking about this discovery of this intermediate-sized black hole, talking with Sean Farrell from the University of Leicester in the U.K. And did you say that you found it by accident, you weren't looking for it?

Dr. FARRELL: Yes. It was complete providence. We were actually exploiting the 2XMM catalog, looking for what we call soft-spectrum X-ray sources. So the brightest in the low-energy realm of the X-ray spectrum. And we were particularly looking for isolated neutron stars and white dwarves, which are cousins, if you like, of black holes, which are interesting in their own. But we were looking through this catalog for objects like this, and we found something that obviously wasn't one of those objects, but it was very, very luminous, very bright, and it was - appeared to reside in a distant galaxy. And that's when we worked out that we had something quite unique and very interesting.

FLATOW: If a black hole is black, how do you detect that it's there?

Dr. FARRELL: Well, there are a couple of ways of doing it. One way is that because they're so dense, they actually distort space and time around them. So if you have a black hole that's sitting by itself somewhere in the galaxy, and there's a star or a galaxy behind it, the light from that galaxy or star can actually be distorted by the intense gravitational pull of the black hole.

So we can actually see the presence of them by how they affect the light from distant objects. But the way we found this one was simply based on the X-ray emission coming from a disk of material that was swirling around the black hole, a bit like water going down a plug hole. And as I said, when you get close to a black hole, it's very, very extreme conditions, and the gas heats up to very high temperatures. And so they glow very brightly in X-rays, and that's how we could actually see this one.

FLATOW: So as it swirls down the drain of the black hole, it heats up and gives off all these X-rays.

Dr. FARRELL: That's pretty much it, yeah.

FLATOW: Yeah, and do we think that black holes are at the center of many galaxies, including our Milky Way, now?

Dr. FARRELL: Yes, I think that there's a lot of very strong evidence to indicate that the objects at the middle of these galaxies, and perhaps all galaxies, perhaps not all galaxies, but certainly in the case of many of them, we can actually measure the velocity of stars around these galaxy centers. And from that you can work out the mass of the central object. And in many cases, this comes out to be millions or billions of times the mass of the sun, which is far too massive to explain for any object other than a black hole.

FLATOW: Is there a limiting factor on how massive a black hole can be?

Dr. FARRELL: I suppose there probably is, but I'm not sure we've actually reached it yet. I think that the mass that a black hole can reach is going to be pretty much set by the amount of material that surrounds it. So for the case of a stellar-mass black hole, which is formed when a very massive star explodes, its initial mass is set by the mass range you can have for stars.

You could have stars that maybe weigh up to, you know, 50 to 100 times the mass of the sun. And if you go back to the very early range of stars, the first stars to form after the universe was created, the Big Bang, then you can get even more massive stars. But the mass that the black hole can attain is really limited by how much material it can swallow, which will swell its mass.

FLATOW: Now I understand that you're getting watching time on the Chandra tomorrow. Is that correct?

Dr. FARRELL: That is correct, yes. We've been very lucky to be granted time on the Chandra space telescope by NASA to observe this, to really lock down its location so that we can then follow it up with a range of other ground-based and space-based telescopes.

FLATOW: I guess everybody's taking off on the Fourth, and you Brits get some time on it.

(Soundbite of laughter)

Dr. FARRELL: It certainly seems to have stirred quite a bit of interest both in the media and in the astronomical community.

FLATOW: Can you look at it with the Hubble? Can you tell anything from the Hubble space telescope?

Dr. FARRELL: Well, we're hoping we can, actually. It's funny you should ask that. The thing about this object, it's so far away. It's 290-million light years away. So we can see the galaxy quite clearly in a relatively, with a relatively large telescope, but we don't really expect to detect any stars that are around this particular object. They're going to be too faint in optical wavelengths.

So if it is actually in, say, a binary system with another star, then we won't see that other star. But it must be feeding somehow because we can see it very brightly in X-rays. So the question is, if it's feeding, from all the objects that we've seen, from galaxies to stellar-mass black holes, they always form disks around them, almost always, I should say.

So you get this accretion disk forming around it, and that disk, as I said, gets very, very hot. And it also emits a lot of energy in other wavelengths, including ultraviolet. So Hubble would potentially be very, very good to look at this and see if we can see the signature of the accretion disk and see the near ultraviolet wavelengths.

FLATOW: We're talking about future telescopes like the Webb telescope that's going to follow the Hubble. Would that be able to see any better, see these kinds of objects?

Dr. FARRELL: Well, to be honest, I don't know much about the James Webb telescope, but I do understand it's a significant advance from Hubble. So I'm certain that if we expect to be able to study it well with Hubble, we'll certainly be able to do better with the Webb telescope.

FLATOW: And what more do you need to more about this one, no matter how you look at it?

Dr. FARRELL: Well, I think detecting the accretion disk is probably the first step for us. That's where we're going now. That will allow us to characterize what's actually going on and actually put sort of more of a final nail in the coffin to prove that it is really is a black hole in the intermediate-mass range. But there are other studies we can do, but we're going to be limited somewhat by the current range of instrumentation that's available.

One test that would be really good to do would be to study emission lines in the X-ray spectrum. These are typically seen in sort of stellar-mass and supper-massive black-hole spectra at an energy range that is well-suited to the instrumentation we have. But we don't have a telescope that's sensitive enough to actually detect this at this point.

Now these emission lines, particularly due to heavily ionized iron gas that's very close to the black hole, they show up as a certain signature. Now if it's close to the black hole, it can be strongly affected by the gravity of the black hole, and it actually skews it a bit. It makes it distort the signal. So if we can study that and see that, that's again, you know, very strong evidence that we've got a very massive black hole, but currently we can't do that with XMM-Newton. (unintelligible) which is the most sensitive X-ray telescope we have.

FLATOW: And we'll have to wait. We've run out of time, Doctor.

Dr. FARRELL: Oh, sorry about that.

FLATOW: No, no, that's okay. I wish we had more time to talk. We'll follow you in your work, though, and we'll be checking back with you. I want to thank you for taking time to be with us.

Dr. FARRELL: It was my pleasure.

FLATOW: And have a good weekend.

Dr. FARRELL: Yeah, you, too.

FLATOW: Sean Farrell is the XMM-Newton catalog scientist in the department of physics and astronomy in the University of Leicester in the U.K. Stay with us, we'll come back, talk about homemade fireworks. I'm Ira Flatow. This is SCIENCE FRIDAY from NPR News.

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