Bad astronomy | Black Widow’s Pulsar zaps and eats its mate

Astronomers have discovered a new example of a kind of creepy dead star that shoots death rays so powerful they cook up their binary companion star and eat its remains.

No, I’m not kidding. And this discovery is interesting because the two stars are closer to each other than any other object in its class, swinging around each other in an astonishing 62 minutes. This makes it a weird example of what a weird object class is in the first place.

To search for such objects, a team of astronomers used archived observations from the Zwicky Transient Facility – a sky survey that takes huge images of the sky every night – looking for things that move or change brightness: asteroids, supernovae, variable stars, black holes with bad table manners, etc.

They searched the data for periodic short-term brightness changes in 20 million objects that were fainter than would be expected for a star of this type. It might sound pretty specific, but they had a specific type of object in mind: Black Widow pulsars.

It’s an apt name for something that, well, shoots out a death ray, cooks its mate, and eats it.

I have written several times about pulsars:

Pulsars are neutron stars, the incredibly dense corpse of the core of a massive star that exploded in a supernova. The outer layers of the star are destroyed, but the core itself collapses. If the nucleus has less than about three times the mass of the Sun, it will become a ball of neutrons about 20 kilometers wide. This makes it ultra-dense; one cubic centimeter of it – the size of a six-sided die – will have a mass of about 100 million tons, about the same as if you took all the cars in the United States and crushed them until that they are all together the size of a sugar cube.

Neutron stars tend to spin quickly and have strong magnetic fields that can be billions of times stronger than Earth’s. This sets up a lot of different phenomena; the first is that it powers incredibly powerful beams of radiation that shoot away from the star’s magnetic poles, which sweep across the sky due to the neutron star’s rotation like a pair of beacon beams. When these beams pass over the Earth, we see a flash of light, a pulse, hence the term pulsar.

Some pulsars are solitary, and others have “normal” stars like the Sun orbiting them. It’s not much fun being near a pulsar, but if you’re a star far enough away, you can survive, even in a binary system.

But some stars are much closer and face the fury of hell.

An approaching star is more likely to be hit by these rotating beams of radiation, and the effects of the beams on them are also more powerful. They can zap the binary companion star so energetically that it heats up and loses material – the pulsar almost literally boils it. This material then leaves the star and flies off into space… where some will fall on the pulsar, feeding the energy and the beams.

Again, black widow pulsar is surprisingly appropriate.

So that’s what astronomers were looking for in the data, and not only did they find one, they found a strange one. Called ZTF J1406+1222, it lies about 3,700 light-years away from us and has the shortest orbital period of any known Black Widow, at just 62 minutes. This means that the two objects are very close to each other; for a standard neutron star mass of 1.4 times the mass of the Sun, the companion has a mass just one-twentieth that of the Sun – thus about 50 times that of Jupiter – and their orbit is only about 800 000 km in diameter. This is less than three times the distance from the Moon to Earth.

This secondary object becomes fried.

It’s already weird, but it’s getting weirder. The mass of the second object is too small to be a normal star. It may be a brown dwarf, an object more than about 13 but less than about 77 times the mass of Jupiter. It could also be a star that once looked like the Sun but lost a lot of mass to the pulsar, exposing its core. Anyway, its density is 10 grams per cubic centimeter, which is high. Twice as dense as Earth and 10 times that of a normal star. On the low end for a brown dwarf, but again maybe also an old, mostly eaten stellar core.

The amount of energy emitted by pulsars is staggering, about 10 times the Sun’s total energy output. Astronomers calculated the temperature of the secondary object and found that on the side away from the pulsar’s beams it was around 6,300°C, but on the side facing the beams it was over 10,000°C . Ouch. These beams warm him by about 4000°C, and it is likely that heat from the day side is transmitted to the night side via circulation in the atmosphere. Interestingly, it’s hotter than many stars, so if this secondary is a brown dwarf, it’s very different from the others, which are usually quite cold temperature-wise. And if it’s a stripped core, that would be amazing to know as well.

There is also a third star; astronomers measured the movement of the pulsar through space and found another star traveling with it. If it’s actually a second companion in what is then a trinary system, that’s quite a long way off, with the third star taking about 10,000 years or so to orbit once. It’s far, but still, what a view!

If you could survive it. I think I prefer to see it from Earth. Yet the third star, if physically associated, will help astronomers understand this system, because somehow it has remained bound to binary even at such a distance. A supernova is an extremely violent event; being able to retain a third star makes the physics tricky.

Everything about this object is delicate. I imagine astronomers will be looking for more data because it’s full of surprises, and it’ll be a fun puzzle to solve. And this is one of the main reasons why we observe the sky.

About Johnnie Gross

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