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After keeping a dead star under close observation for over two decades, astronomers have finally caught it acting out.
A pulsar called PSR J0908-4913 (J0908 for short) has, for the first time, been seen glitching – a peculiar and poorly-understood “hiccup” in rotation speed that some pulsars undergo.
This not only adds to the pool of glitching pulsars astronomers can study to try to understand the guts of neutron stars – it shows that there may be many more glitching pulsars out there we are yet to find.
The findings have been posted as a non-peer-reviewed research note in Research Notes of the AAS.
“It’s quite remarkable the Parkes radio telescope had observed this pulsar for over 20 years without seeing a single glitch,” astrophysicist Marcus Lower of Swinburne University of Technology in Australia told ScienceAlert.
“It implies large scale pulsar monitoring of hundreds or even thousands of pulsars by the future Square Kilometer Array (many of which are too faint to be observed regularly by Parkes and Molonglo) will uncover a significantly larger sample of glitchy pulsars than current telescopes can provide.”
Pulsars are a type of rapidly rotating type of dead star called a neutron star – the core left behind after a star of a certain mass has gone supernova. These stars are oriented in such a way that, as they rotate, they flash a beam of radiation towards us from the polar region, like a cosmic lighthouse.
Because the rotation rate is regular, this flashing is often incredibly precise, and is a useful tool for a plethora of science applications.
But, of the roughly 2,700 known pulsars in the Milky Way, a small percentage – somewhere between 130 and 190, or 5 to 7 percent – glitch. This is when the rotation speed hitches and speeds up, and they’re thought to be caused by processes inside the neutron star.
J0908, a pulsar with a 107-millisecond spin period, was discovered in 1988 using the Molonglo Observatory Synthesis Telescope (MOST). Follow-up observations were conducted for over two decades using the Parkes Observatory; MOST started tracking it again in 2015 as part of the UTMOST project, a collaboration to study transient objects in real-time.
It was through MOST that, on 9 October 2019, data came through showing a hiccup in the pulsar’s rotation speed – what the researchers describe in the research note as “permanent change in spin-frequency.”
And, well, it looks pretty normal for its type.
“J0908 is what radio astronomers refer to as a ‘young pulsar’. These are the pulsars that undergo the most glitches and have unstable spin-periods that can vary by tens to hundreds of milliseconds over a few years. It’s not clear whether the stability of their spin-periods is related to how glitchy they are,” Lower explained.
“The glitch itself is comparable to small glitches seen in pulsars with similar spin-down rates.”
These similar pulsars glitch around every 6 to 25 years, so it’s likely that J0908 has glitched many times before in the past – and possible that long-term observation kicked in just after a glitch.
Not all glitching pulsars glitch alike, though. Vela, for instance, glitches once every three years; recently, astronomers discovered that its rotation rate gradually slows down before glitching, and that it recovers to its regular speed after.
J0908’s glitch actually occurred between observations, so the team was unable to determine what happened immediately before and after the glitch; but they have observed no evidence of a change in spin-down rate, or spin recovery.
Spin recovery isn’t guaranteed, and can take some time, so this is also pretty normal. What’s unusual is the long observational history of the star. This, Lower said, provides an excellent record of the star’s behaviour before the glitch that they can use to determine if it’s acting weird now.
And, of course, the more glitching pulsars we observe, the better we can understand the phenomenon.
“Each glitch we detect provides additional insight into the processes behind them. For instance, we don’t know if all pulsars glitch. So finding new glitchy pulsars helps us understand how widespread the glitch phenomena is among neutron stars,” Lower told ScienceAlert.
“There is some debate as to the exact role glitches play in the long-term rotational evolution of pulsars, particularly if they’re responsible for some pulsars that appear to be either slowing down too quickly, or not fast enough. In order to find answers to these problems, we need a large, diverse set of pulsars to look at.”
It may be some time before J0908 glitches again, but there’s still plenty to study about the star – such as any post-glitch weirdness, how long it takes to return to normal, and whether the glitch produced any long-term changes.
The findings are available in the Research Notes of the AAS.