Our neighbour, the neutron star

Possibly the closest neutron star to Earth discovered by McGill and Penn State astronomers

McGill University and Penn State University astronomers using NASA's Swift satellite have discovered what they believe to be the closest neutron star to Earth. If confirmed, it would also be only the eighth so-called isolated neutron star ever discovered. Unlike the majority of known neutron stars, the isolated variety do not have associated supernova remnants or binary companions, and do not emit radio pulsations.

In a paper to be published in the Astrophysical Journal, Robert Rutledge of McGill University in Montreal, Quebec, co-author Derek Fox of Penn State University and their colleagues describe a stellar object nicknamed "Calvera," located in the constellation Ursa Minor. First author Rutledge originally called attention to Calvera based on X-ray data derived from the German-American ROSAT satellite, which operated from 1990 to 1999.

An object like Calvera that is bright in X-rays and faint in visible light is almost certainly a neutron star, said Rutledge, assistant professor in astrophysics at McGill's Department of Physics, although exactly which type remains a mystery. "Either Calvera is an unusual example of a known type of neutron star," he explained, "or it is some new type of neutron star, the first of its kind."

The nickname Calvera derived from the villain in the movie The Magnificent Seven, explained Fox, assistant professor in astrophysics at Penn State's Department of Astronomy and Astrophysics. "The seven previously known isolated neutron stars are known collectively as 'The Magnificent Seven.' And so the name Calvera is a bit of an inside joke on our part."

The group first aimed the Swift satellite's X-ray telescopes at the object in August 2006, and confirmed that the source was still there, and was still emitting about the same level of X-ray energy. Further observations with the Gemini North Telescope in Hawaii and NASA's orbiting Chandra X-ray Observatory reinforced the interpretation that it was a neutron star.

Calvera's location high above the plane of our Milky Way galaxy is part of its mystery. In all likelihood, it is the remnant of a star that exploded in a supernova and subsequently wandered out of the galactic disk. "The best guess is that it is still close to its birthplace, and therefore close to Earth," said Rutledge. If this interpretation is correct, the object is 250 to 1,000 light-years away, which would make Calvera one of the closest known neutron stars, if not the closest.

"Because it is so bright, and probably close to Earth, it is a promising target for many types of observations," said Fox. Indeed, to clear up the mysteries surrounding Calvera the team plans additional observations with Chandra and a radio telescope to determine if Calvera pulsates in the X-rays and radio wave spectra.

Calvera could represent the tip of the iceberg for isolated neutron stars, said the researchers, with Swift's sensitive X-ray telescope the key to finding them. "There could be dozens," said Fox.

Web:
Robert Rutledge's website
Derek Fox's website

Images:
A high-resolution illustration