Astronomers Catch a Pulsar Eating an Asteroid

Eating an asteroid causes a sudden change in the rotation rate of a pulsar, say astrophysicists who have caught one in the act of digestion

Pulsars are among the more extraordinary objects in the universe. They are hugely dense nuggets of matter, called neutron stars, left over after a supernova. When neutron stars with powerful magnetic fields rotate at high speed, they emit radiation in a narrow beam that sweeps the heavens like cosmic light houses.

When these beams sweep across Earth, astronomers see them as regular pulses. Jocelyn Bell Burnell discovered the first one in 1967 and astronomers have been carefully measuring them ever since.

One thing they’ve discovered is that pulsars gradually slow down as they emit energy. But as well as this gradual decrease, the pulse rates sometimes change abruptly for reasons that aren’t well understood.

One idea is that the huge gravitational forces in the star can cause the surface to settle, generating starquakes. When they strike, starquakes disrupt the entire star’s rotation suddenly.

Today, Paul Brook at the University of Oxford in the UK and few pals say that an asteroid crashing into a pulsar ought to change its rotation rate in a measurable way too. And they say they’ve seen exactly this signature in a pulsar called PSR J0738−4042 that astronomers have been studying for 24 years.

The behaviour of an asteroid as it falls into a pulsar is complex. Brook and co say the intense radiation emitted by the neutron star evaporates the asteroid and ionises the matter involved.

These resulting electrically-charged particles then enter the star’s magnetosphere where they interact with other charged particles, generating radiation, for example,via gamma ray pair production. It is the emission of this radiation that dissipates energy changing the rotation rate of the pulsar.

That gives rise to specific signature. First, ingesting an asteroid should trigger a sudden change in the pulsar’s rotation rate that continues as the pulsar digests this material.

But once all the new matter has been converted to radiation and emitted, the rotation rate should revert to its original state. The time this takes is a measure of the mass of the asteroid.

That’s exactly what they’ve seen in PSR J0738−4042. Back in September 2005, they noticed a sudden change in the pulsar’s rotation rate which has continued until today. That’s equivalent to a mass of about 10^15 grams, about the size of asteroids in the Solar System.

This line of reasoning leads to an interesting prediction. “At the point in time when the injected mass is exhausted, we would expect the pulsar to return to its previous spin-down state,” say Brook and co.

So if these guys are right, astronomers should observe another change in the rotation rate of PSR J0738−4042 when it has finished its meal, probably in the next few years. That’ll be an important test of these ideas.

An interesting corollary to this is that Brook and co ask why this would be the first time that PSR J0738−4042 has tucked into a nearby asteroid. After a careful study of the pulsar’s rotation rate PSR J0738−4042 over 24 years, they say that the 2005 glitch isn’t the only one.

It turns out that PSR J0738−4042 also appears to have gobbled up small neighbours in 1992 and in 2010. This is clearly one hungry pulsar.

Of course, it stands to reason that pulsars ought to interact with the debris around them—after all, these are the remains of supernovas. And that means PSR J0738−4042 is unlikely to be the only pulsar behaving in this way. So don’t be surprised if astronomers suddenly find evidence that other pulsars are hungry too.

Ref: http://arxiv.org/abs/1311.3541: Evidence Of An Asteroid Encountering A Pulsar