Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse
Object: Spinning star called a pulsar
Rotation rate: 7 times a second
A glitch is not normally something to boast about. But when one is found in an already rare type of pulsar – curious dense, rotating stars that flash regularly like lighthouses – it's like striking gold.
Such a find makes recently discovered pulsar J1838-0537 among the most precious of this kind of star. Its glitch – an unexpected change in the tempo of its rotation that occurred in 2009 – is the biggest ever discovered in this much rarer type of pulsar, which is detected via its highly energetic gamma-rays rather than by its lower-energy radio waves.
"Very few pulsars exhibit glitches," says Holger Pletsch of the Max Planck Institutes for Gravitational Physics and for Radio Astronomy, who led the team that discovered J1838-0537.
Studying glitch pulsars such as J1838-0537 could give us an insight into some of the strange physics thought to go on inside pulsars, from the bizarre, frictionless superfluids thought to lurk at their cores to the starquakes that may occur in their crusts. In turn, this could open up a new way to study extremely dense matter, which is impossible on Earth.
Clock stopped
Finding J1838-0537 in the first place wasn't easy. Using a data analysis method originally designed to detect gravitational waves, Pletsch's team dug through data from the Fermi Gamma-ray Space Telescope in the hope of uncovering previously missed pulsars.
Most of the 2000 or so known pulsars emit radio waves. Since the Fermi satellite launched, astronomers have discovered that 60 of them also emit gamma-rays. Another 30 appear to emit only gamma-rays. This is the type that Pletsch's team were looking for.
They found nine of them last year. And now they have a new one to add to the list. But before they could file it away they had to find out where it had disappeared to.
Pulsars tend to be reliable cosmic timekeepers thanks to their regular rotation. This is visible because their intense magnetic field accelerates particles at their surface to produce a beam of radiation along the magnetic field axis. As the pulsar spins, this beam sweeps into view once per rotation, rather like a lighthouse.
Starry weirdness
Pulsar J1838-0537 had been spinning like clockwork, seven times a second, when it seemed to disappear in 2009. Eventually, Pletsch and his team managed to find it again in their data – and when they did, it was clear why it had disappeared.
The rare pulsar had in fact stepped up the speed of its rotation to about 38 millionths of a Hertz faster than before. As astronomers depend on pulsars' regularity to track them, this made it appear as if J1838-0537 had disappeared.
This might not sound like a big jump, but it is the biggest increase ever seen in this type of pulsar. It hid the pulsar from sight for about a year – and means the pulsar could be even weirder than we know. "What the pulsar did in between is still unclear," says Pletsch: it could have started spinning even faster than we think it did, or it could even have been through several glitches.
The cause of the glitch is unclear. One explanation could be a starquake. These mysterious disturbances are thought to be triggered by stress in the pulsar's crust caused by the strong magnetic fields inside it.
Extreme heart
Alternatively, the glitch could be caused by a pulsar's frictionless superfluid interior spinning faster than its solid crust. The jury is still out, because so few glitches have ever been seen.
Studying these glitches could give us an insight into the poorly understood workings of pulsars, enabling us to "look at the real heart of these enormously powerful machines", says Pletsch.
That's interesting in terms of astrophysics – but also to the fundamental behaviour of materials under extreme conditions. "These conditions are irreproducible on Earth," says Pletsch.
Reference: arxiv.org/abs/1207.5333
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