Closest neutron star to Earth discovered

Astronomers from the McGill University and Penn State University have identified what they say is the closest neutron star to Earth.

Washington, Aug 21 : Astronomers from the McGill University and Penn State University have identified what they say is the closest neutron star to Earth.

Given that the seven previously known isolated neutron stars are collectively known as 'The Magnificent Seven' after the 1960 Western directed by John Struges, astronomers have named the new star Calvera after the villain in the film by the same name.

"The seven previously known isolated neutron stars are known collectively as 'The Magnificent Seven' within the community and so the name Calvera is a bit of an inside joke on our part," said co-discoverer Derek Fox of Penn State University.

Fox said, if confirmed, this would be only the eighth known "isolated neutron star" - meaning a neutron star that does not have an associated supernova remnant, binary companion, or radio pulsations.

The object is located in the constellation Ursa Minor.

First author Robert Rutledge of McGill University in Montreal, Quebec, called attention to the source after comparing a catalogue of 18,000 X-ray sources from the German-American ROSAT satellite, which operated from 1990 to 1999.

He compared this with catalogues of objects that appear in visible light, infrared light, and radio waves and found that the ROSAT source known as 1RXS J141256.0+792204 did not appear to have a counterpart at any other wavelength.

Then the astronomers aimed NASA's Swift telescope at the object in August 2006.

Swift's X-ray Telescope showed that the source was still there, and emitting about the same amount of X-ray energy as it had during the ROSAT era.

However, the Swift observations enabled the group to pinpoint the object's position more accurately, and showed that it was not associated with any known object.

"The Swift observation of this source is what got the show going. As soon as I saw the data, I knew Calvera was a great neutron-star candidate," said coauthor and Penn State undergraduate Andrew Shevchuk.

Observations with the 8.1 metre Gemini North Telescope in Hawaii, further revealed that the object is not associated with any optical counterpart down to a very faint magnitude.

Rutledge said there are no widely accepted alternate theories for objects like Calvera that are bright in X-rays and faint in visible light.

"Exactly which type of neutron star it is remains a mystery? Either Calvera is an unusual example of a known type of neutron star, or it is some new type of neutron star, the first of its kind," he said.

Rutledge said Calvera's location high above the plane of our Milky Way Galaxy is also a mystery.

"In all likelihood, the neutron star is the remnant of a star that lived in our galaxy's starry disk before exploding as a supernova. In order to reach its current position, it had to wander some distance out of the disk. But exactly how far?" he said.

"The best guess is that it is still close to its birthplace, and therefore close to Earth. If this interpretation is correct, the object is 250 to 1,000 light-years away. This would make Calvera one of the closest known neutron stars -- possibly the closest," he added.

A paper describing the research will be published in the Astrophysical Journal.

ANI