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2nd June 2013

Astronomers observe a spinning neutron star suddenly slowing down

Researchers using NASA's Swift X-ray Telescope have observed a spinning neutron star suddenly slowing down, yielding clues they can use to understand these extremely dense objects.

 

neutron star manhattan size comparison
Size comparison of a neutron star (weighing as much as 500,000 Earths) and New York City.

 

Neutron stars are among the most remarkable objects in the known universe. Composed almost entirely of neutrons (subatomic particles without net electrical charge), they form when a star's core is compressed during a supernova and collapses under its own gravity. They are the second densest objects after black holes, with a single teaspoon of their material having a mass of nearly 6 billion tons. The gravitational force of a typical neutron star is such that if an object were to fall from a height of one metre, it would travel at 4.5 million mph (7.2 million km/h) taking only a microsecond to reach the surface. They can spin at over 700 times per second, emitting a beam of radiation with a very precise period. From that blinking effect, astronomers can determine how the star is rotating.

The neutron star 1E 2259+586 is located about 10,000 light-years away, toward the constellation Cassiopeia. It is one of about two dozen neutron stars known as "magnetars", which have extremely powerful magnetic fields and occasionally produce high-energy explosions or pulses. Observations of X-ray pulses on 28th April 2012 indicate that 1E 2259+586 abruptly decreased its spin rate – by 2.2 millionths of a second – a phenomenon that has never been witnessed before. Accompanying this sudden slowdown was a large increase in the X-ray output of the magnetar, tell-tale evidence of a major event inside or near its surface.

 

1e 2259+586 neutron star image
1E 2259+586. Credit: ESA/XMM-Newton/M. Sasaki et al.

 

"Astronomers have witnessed hundreds of events – known as glitches – associated with sudden increases in the spin of neutron stars, but this sudden spin-down caught us off guard," said Victoria Kaspi, a professor of physics at McGill University in Montreal, who leads a team that monitor magnetars routinely. "This is telling us something brand new about the insides of these amazing objects."

Astronomers dubbed the event an "anti-glitch," said Neil Gehrels, principal investigator of the Swift mission at NASA's Goddard Space Flight Center in Greenbelt, Md. "It affected the magnetar in exactly the opposite manner of every other clearly identified glitch seen in neutron stars."

"I looked at the data and was shocked," said Rob Archibald, co-author and MSc student at McGill University. "These stars are not supposed to behave this way."

The discovery has important implications for understanding the extreme physical conditions present within neutron stars, where matter becomes squeezed to densities several times greater than an atomic nucleus. No laboratory on Earth can replicate these conditions.

It is possible that 1E 2259+586 gained some mass – perhaps caused by a planet or other large object colliding and being sucked in. However, the exact reasons for its slowdown remain unknown. A report on the findings appeared this week in the journal Nature.

 

neutron star
Artist's concept of a magnetar. These stellar remnants are the most magnetised objects in the universe. Credit: NASA

 

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