Artwork depicting a millisecond pulsar, sped up to phenomenal rotation rates by material it's eating from its companion star. Credit: NASA / GSFC SVS / Dana Berry
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Artwork depicting a millisecond pulsar, sped up to phenomenal rotation rates by material it's eating from its companion star. Credit: NASA / GSFC SVS / Dana Berry

The most compact accreting binary X-ray millisecond pulsar ever!

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Jun 20, 2019

Via the wonderful site Astrobites I learned about the discovery of an amazing astronomical object: IGR J17062−6143, the most compact binary system accreting X-ray millisecond pulsar ever found!

Yeah, so OK. That's a lot of jargon. Tell you what: Let's tackle it right to left. First, what's a pulsar?

A pulsar is special kind of neutron star. OK, what's a neutron star? It's the über-dense ball of neutrons left over when a massive star explodes. The outer layers blow outwards, but the core of the star collapses, smashing together protons and electrons to form neutrons (it's actually a wee bit more complicated than that, but that's the gist). It has about the mass of the Sun, but it's only a few kilometers across! So it's unbelievably dense. I once calculated that a cubic centimeter of neutronium (a bit smaller than a sugar cube) would weigh as much as all the cars on Earth. This slightly tongue-in-cheek article I wrote should help you out.

Neutron stars have ferocious magnetic fields, and tend to spin pretty rapidly, sometimes in under a second. Sometimes even faster (ooo, foreshadowing). Like the Earth, they have a roughly bipolar magnetic field, like a bar magnet, with a north and south magnetic pole. Beams of radiation emerge from the location of the poles on the surface, incredibly energetic beams. Also like the Earth, the magnetic poles aren't necessarily aligned with the spin poles, so as the neutron star spins these beams sweep out like lighthouse beams across space. When one of the beams sweeps over us we see a little blip of energy, pulsing in time with the spin. Hence the term pulsars.

OK, so millisecond?

Some pulsars spin much faster than others. The fastest one known spins over 700 times per second! That is almost one rotation per millisecond, so there you go.

Right. Now, X-rays.

Wait! That comes more naturally after accretion.

That's a term astronomers use for an object that is accumulating mass from somewhere. For example, if a pulsar has a companion star, the ridiculously fierce gravity can pull matter off that star and draw it in to the pulsar. It accelerates this material to something like 2/3rds the speed of light! When this material hits the surface of the neutron star it releases so much energy that the light emitted is in the X-ray part of the electromagnetic spectrum — you need temperatures of millions of degrees to do that, so yeah, this is quite a thing. Also, complicated physics can occur as this stuff interacts with the magnetic field, which can then emit X-rays as well.

Going back a bit, millisecond pulsars are made from normal pulsars as material from a companion star is drawn in by the neutron star's gravity. The material spirals in, adding its angular momentum to the star. Over time, this spins the pulsar up, making it rotate extremely rapidly.

Still with me? OK, so now binary system.

Well, like I said, you need another star to make a millisecond pulsar, so there's a second star orbiting the pulsar. That makes this a binary.

And finally, compact.

This part gets terrifying. For this, we have to actually talk about IGR J17062−6143, the topic of this article. Let's call it J17062 to save my fingers a bit.

This pulsar was first detected when it had an outburst. That means for some reason it got fantastically brighter for a short time. For normal pulsars, this is usually due to eating an unusually large gulp of its companion. They get bright, then fade right after. However, J17062 just kept on going. It's been consistently blasting out high-energy radiation for years.

A lot of energy. It's emitting 150 times the Sun's total energy output… and it's doing that in just X-rays. This thing is a beast.

To get a better look at it, astronomers used a very cool instrument called the Neutron Star Interior Composition Explorer (or NICER). This is actually installed on board the International Space Station, where it has a stable platform and plenty of power. It was used to look at J17062 in August of 2017.

The data confirmed it's a pulsar, and they nailed its rotation at 6.11 milliseconds, 163.656 rotations per second. But they also found an odd periodic change in its rotation rate, which must be due to the mutual orbital motion of J17062 and an unseen companion. The orbital period they found was a staggering 38 minutes. That's incredibly short, and that means its companion has to be very close to the pulsar… and hey, that means the system is compact. So now we've covered all the terms.

But there's more. The companion can't be a regular star like the Sun. The orbital period implies a distance between the two of about 300,000 kilometers — less than the distance from the Earth to the Moon!

First off, the Sun is a lot bigger than that. At 1.4 million kilometers across, a star like the Sun wouldn't even fit. Even a red dwarf won't work, either: The gravity of the neutron star is so fierce that the tides would rip any normal star to shreds.

That means the companion star must be very small. In fact, it's a white dwarf: The leftover core of a normal star like the Sun after it runs out of nuclear fuel, blows off its outer layers, and leaves behind a dense (though not neutronium dense) ball about the size of the Earth.

The surface gravity of a white dwarf is immense, hundreds of thousands of times Earth's gravity. Yet the gravity of the neutron star is so much more powerful — millions or billions of times Earth's gravity —that it can tear stuff from the white dwarf's surface, lift it off into space, and let it slam down on the neutron star's surface!

J17062 has been doing this for a long time, too. Normally a white dwarf has about half the mass of the Sun, but the one in this system has a mass probably less than 1% of the Sun! The pulsar has been eating so much of it there's hardly anything left.

Everything about this system is extreme. But it's the stars themselves that get to me. Think of it this way: The Earth's gravity is strong enough to keep the Moon going around it with an orbital period of a little over 27 days. At roughly the same distance, J17062's gravity whips its companion around over 1,000 times faster.

And its companion is another star.

Things like this make the hair on the back of my neck stand up. The Universe is incredible.

It's odd to think that some billions of years ago, J17062 was a binary star system like many others. Both stars likely were more massive than the Sun. One died first, expanding into a red giant, and dumping a lot of its mass onto the other star. When that was done, what was left was a white dwarf.

The other star got enough mass that it could explode as a supernova, however. When that was over with it left behind the neutron star, the system we see more or less the same today. Again, I'm leaving a few steps out (this is actually a pretty complicated process) but the point is the most bizarre things in the Universe sometimes start off pretty normal. When you look up at the night sky and see all those stars, don't forget that there are a lot of weird things going on out there in the dark. It's one of the many, many reasons I love astronomy.

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