Shine Brightly, Little Neutron Star

IRA FLATOW: For decades, astronomers have been puzzling over the mystery of ULXs. Those are Ultra Luminous X-ray sources– of course you knew that– often perched in the edges of galaxies, not quite strong enough to be super-massive black holes, but too bright to be other kinds of known objects. Well, four years ago, the mystery seemed solved.

The fledgling NuSTAR telescope detected pulsations that proved at least a few of them were neutron stars. Neutron stars– tiny dense stars that pack the mass of several suns into the space of just a few kilometers. But that just created a new problem, because these neutron stars were far brighter than they should be– given what we know about the limitations of mass and luminosity in stars, hundreds of times too bright, the kind of a problem astronomers love to have.

Well, now new research might have an answer. These neutron stars might have an intensely-powerful magnetic field, 10 billion times the strongest magnetic field we’ve ever generated on Earth. The research is published in Nature Astronomy this week. And my guest, Dr. Matt Middleton, a lecturer in physics and astronomy at the University of Southampton in UK is one of the co-authors. Welcome, Dr. Middleton, to Science Friday.

MATT MIDDLETON: Hi, there. Thank you for having me on.

IRA FLATOW: So what made these X-ray sources so mysterious?

MATT MIDDLETON: Right. So the mystery really goes back to the 1970s, when people first started looking out into the universe with X-ray telescopes. Because remember, you can’t actually see X-ray bright objects on the Earth, because– thankfully– our atmosphere protects us from such harmful radiation. And when people looked out, they found these things that were really, really bright. They were far too bright than they had any right to be, really.

They weren’t associated with the centers of galaxies, where we know material falls onto these super-massive black holes, where the mass is millions of billions times the mass of the sun. But they were still way too bright to be what we normally see in X-ray binaries, where we have a black hole which is, say, 10 times the mass of the sun, or a neutron star, which is around 1 to 1.4 time the mass of the sun, or even white dwarfs, which can be even more tiny. But they’re really, really gravitationally-compact objects. And material falls onto them. And they get very bright.

But these ULXs, they seem to be too bright for that. So for a long time, astronomers were sort of scratching their heads and going, well, what could this be? And people first thought, OK, well, maybe these are a new type of gravitationally-compact objects. And maybe these are what we’ll call intermediate mass black holes, which is a great idea, right? Because we know that super-massive black holes in the centers of basically all galaxies had to form somehow. So they had to build themselves up from very, very small, possibly primordial black holes.

And somewhere along that line, there may have been intermediate mass black holes, around 100, 1,000, 10,000 times the mass of the sun. So maybe that’s what they were. But we can’t just go and weight these things. It’s very, very difficult. So we needed indirect evidence to work out what they were.

And so for a very long time this argument in the community raged, on what are they? Do we think they’re normal, stellar mass compact objects? Or do we think they’re intermediate mass black holes? So that was really the root of all these arguments.

IRA FLATOW: So voila– you look at your data. And you say, they’re not black holes. They are a different kind of neutron star that we’ve not seen before?

MATT MIDDLETON: Well, so it’s slightly more complicated than that. So we looked at them, and the evidence wasn’t clear cut. So we looked at what we call the X-ray spectra and their variability. So the spectrum is where you break your light into various different channels. And you look at the shape of that. And you can compare that to objects that we know and study really well.

And for a long time we looked at them and we thought, well, we don’t really know what these look like. They could be intermediate mass black holes. Or they could be stellar mass black holes. Maybe they could be neutron stars. We don’t really know. We all had to come up with indirect lines of thought to get there.

And then all of a sudden, things got much simpler, because a colleague of ours, Matteo Bachetti, discovered with the NuSTAR telescope– which is led by Fiona Harrison, who I know you’ve had on the show before– they discovered these pulsations. And these positions indicated that this thing had to have a surface. And it was spinning really quickly. So all of a sudden they thought, well, it can’t be a black hole, because black holes don’t have surfaces.

IRA FLATOW: Yeah.

MATT MIDDLETON: Although hypersurface is a different topic altogether. So it had to be neutron star. And then the question is, how can these neutron stars be so bright?

IRA FLATOW: And then your answer was, well, we give them this giant magnetic field.

MATT MIDDLETON: So there’s two ways to do it two ways to skin this cat. You can either put a lot of material at very, very rapid rates towards a neutron star. And then what happens– at a certain radius, in what we call an accretion disk, so imagine a pancake. And in the middle of that pancake you’ve got a neutron star. At some radius, or some distance from the neutron star, the disk puffs up. And then it tapers down to the neutron star, so in a bit of a cone, right?

IRA FLATOW: Right.

MATT MIDDLETON: And then all that radiation that’s produced within that disk or from that neutron star gets trapped within that cone. And it gets what we call geometrically beamed towards us. So it’s like having a flashlight. It’s mirrored behind it, so that it reflects that illumination that’s coming from the bulb back out towards you. That’s why it looks so much brighter than it would do if you just had the bulb on it’s own. So that’s one way you can do it.

The other way you can do it is by having, as you said, a really, really strong magnetic field. And it’s possible the truth lies somewhere in between. The importance of the magnetic field in this picture is that it reduces what we call the cross-section, or the scattering cross-section to electrons.

And that’s horribly complicated. So let me just say what I mean there. What we define as the Eddington limit is the point at which radiation– so the push by light is balanced by gravity. And that push by light is related to what those photons in that light can scatter off. It’s called a cross-section, OK?

So that cross-section changes. If it becomes smaller, then you can put more material onto your neutron star or your black hole. And so you can essentially generate more luminosity that way. So there are two ways to do it. You either have geometrical beaming or you have a very, very strong magnetic field. And those are really the two options that people have been investigating.

IRA FLATOW: Mm-hmm. This is Science Friday from PRI, Public Radio International. Talking with Matt Middleton, lecturer in physics and astronomy at the University of Southampton in the UK. You know, black holes get so much attention, don’t they?

MATT MIDDLETON: They do.

IRA FLATOW: Do you neutron star guys get a little jealous of not giving enough neutron star love out there to these stars?

MATT MIDDLETON: Oh, there’s so much neutron star love. There’s so much neutron star love. You know, a big shout-out to all my people who work on ultra-dense matter. I think the thing is that black holes are fundamentally very sexy. You know, look at films like Interstellar. And if you’re old enough, you might remember a Disney film called Black Hole.

It’s very easy– because they’re quite scary things and they’re quite cool. They’re beyond what we can conceptualize. Neutron stars are very difficult to conceptualize. But they are essentially a star. So you can picture that in your head. A black hole is shrunk down beyond the [INAUDIBLE]. It’s so small you have to invoke quantum gravity and all sorts of weird effects.

But the neutron star community is so alive and effervescent. And they’re working all sorts of fantastic things, including ultra-dense matter, which is something we can never really probe on Earth. So the physics that they’re doing, you can’t really approach through any other means. So I don’t think the neutron star guys are upset by black hole astrophysicists and the results that we get out.

I think maybe black hole guys sometimes wish there was a surface, because those have their own very special effects. And sometimes neutron star guys probably think, wow, I really wish this thing didn’t have a surface. It’d make things in my life a lot easier.

IRA FLATOW: Right.

MATT MIDDLETON: But they’re different bits of science, and both of them extremely interesting.

IRA FLATOW: Of course, last year’s big gravitational wave news was that LIGO and Virgo had detected these colliding neutron stars–

MATT MIDDLETON: Yeah, yeah.

IRA FLATOW: Something we’ve never observed before. Is there something else like that we might soon have a chance to observe?

MATT MIDDLETON: Yeah, sure. Well, I mean, so the very first detection was two black holes merging together.

IRA FLATOW: Right.

MATT MIDDLETON: And then neutron stars have also been found, which were long expected to be found. And it’s a really interesting question. So the way that you detect gravitational waves is that– well, not the way you detect them, but the way you produce them is having what we call a gravitational quadrupole moment. And lots of things can have gravitational waves. You or I could be producing gravitational waves, just so small we could never detect them.

So who knows what we’re going to find out there. We could be seeing super-massive black holes that are spinning together and merging. We could be looking back at the early times in the universe and looking at the gravitational waves coming from those black holes merging, and then the galaxies being formed and developed around them. We could be finding neutron stars, [? coming out of ?] black holes, or white dwarfs [? coming ?] [? out of ?] neutron stars, all sorts of things. So it’s going to be an extremely interesting and important window we’ve opened to the universe. And I personally feel very, very honored to be part of astronomy in this exciting time, because it’s going to get really interesting.

IRA FLATOW: Because I talk to astronomers and physicists all the time. And what really excites them is that they don’t know something.

MATT MIDDLETON: Yeah, absolutely.

IRA FLATOW: It’s the hunt that they like.

MATT MIDDLETON: Well, of course. I mean, we can’t write papers on what people already know. We’re in it for the mystery, right? And I love astronomy. You know, I grew up watching Star Trek– I mean, obviously, the Patrick Stewart version, you know?

IRA FLATOW: Yeah.

MATT MIDDLETON: But those sort of programs really touched me as a child. And of course I got interested in astronomy and the mysteries that are out there. And yeah, OK, I don’t fly around in a starship. But I can close my eyes and pretend.

IRA FLATOW: Yeah.

MATT MIDDLETON: But I get to work on these amazing objects. And I’m just really fortunate to get to do that.

IRA FLATOW: Well, I’m glad you “engage” in your work.

Thank you very much–

MATT MIDDLETON: Hell of a pun.

IRA FLATOW: Yeah I know. That’s me. Matt Middleton lectures in physics and astronomy, University of Southampton, and co-author of the new neutron star research. Glad to see you have so much fun with this. Thank you for taking time to be with us today.

MATT MIDDLETON: My pleasure, anytime.

IRA FLATOW: You’re welcome.

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