The fastest-spinning neutron star ever found has been discovered in a crowded star cluster near the centre of the Milky Way, a new study reveals. The star rotates 716 times per second – faster than some theories predict is possible – and therefore may force researchers to revise their models.

Neutron stars form when a massive star explodes at the end of its life and leaves behind a super-dense, spinning ball of neutrons. These stellar corpses emit intense beams of radio waves from their poles and are called pulsars.

Most pulsars rotate just a few times per second, but some spin hundreds of times faster. These so-called millisecond pulsars whip around so quickly because they are thought to have stripped mass – and angular momentum – from companion stars at some point in their histories.

Astronomers led by Jason Hessels of McGill University in Montreal, Canada, used the 100-metre Green Bank Telescope in West Virginia, US, to clock the newly discovered pulsar at 716 rotations per second, or 716 hertz. The previous record holder, which spins at 642 Hz, was discovered in 1982.


Gravity waves

The new discovery, called PSR J1748-2446ad, is interesting because it spins faster than 700 Hz, which astronomers had considered a stellar speed limit. Although most pulsars should have enough self-gravity to spin as fast as 3000 times per second before they split apart, all of the previously discovered millisecond pulsars, of which there are 150 or so, spin slower than 700 Hz.

That led researchers to believe that they were emitting gravitational waves – theoretical ripples in space-time – that kept their spins in check. Massive, fast-spinning objects that are not perfectly symmetrical are predicted to radiate away energy in these waves, with faster objects unleashing much more energy than slower ones.

Bryan Jacoby, a pulsar expert at the National Research Council in Washington DC, US, and not part of the discovery team, says this “brick wall” effect is thought to be very effective at limiting spin rates.

“If you make it spin just a little bit faster, you’re emitting a lot more very strong gravitational waves,” he told New Scientist.”So, the existence of this pulsar suggests the numbers [for this speed limit] aren’t quite right, or that something is wrong with this idea.”

The authors agree, saying that slowing from gravitational wave emission must become significant at spin rates higher than 700 Hz or that theorists should revise their models of neutron star crusts, which were used to arrive at the 700 Hz limit.

Bloated companion

The researchers say the pulsar’s speed means it would fly apart if it was larger than 16 kilometres across. They estimate its mass at about twice that of the Sun – in line with previous pulsar observations. But they can not measure it directly, since it is orbited by a bloated star that blocks the pulsar’s light about 40% of the time.

Because four out of five of the fastest known pulsars are eclipsed by companions, the researchers say the new find suggests that “even faster spinning neutron stars exist” but may be difficult to observe.

Finding those stars will be key since “determining the maximum achievable rotation rate is important for a variety of astrophysical problems”, say the researchers.

Jacoby agrees, saying a pulsar’s speed can ultimately help reveal its density and so shed light on how matter behaves at such crushing densities. “This is fundamental physics that’s important in a much larger sense than just studying neutron stars,” he told New Scientist.

The pulsar was found in a cluster of 10-billion-year-old stars called Terzan 5, which lies 28,000 light years away near the centre of the galaxy. Terzan 5 contains 33 known millisecond pulsars – the highest number in any star cluster.

The research was presented at a meeting of the American Astronomical Society in Washington DC, US.

Journal reference: Science (DOI: 10.1126/science.1123430)