Astronomers using NASA's Swift X-ray telescope have detected a neutron star within 250 to 1,000 light-years of Earth, making it the closest neutron star ever known.
The object, located in the constellation Ursa Minor, is nicknamed Calvera, after the villain in the movie "The Magnificent Seven." If confirmed, it would be only the eighth known "isolated neutron star," or one that does not have supernova remnants, binary companions or radio pulsations.
"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 Derek Fox, an astronomer at Penn State University who co-discovered the mysterious object.
Neutron stars are supernova leftovers that are too small to form black holes. Instead, the leftover gas and dust crams into a glowing, incredibly dense body only a few miles wide--a teaspoon, for example, would weigh millions of tons. A study of the newly discovered neutron star will be detailed in an upcoming edition of the Astrophysical Journal.
Robert Rutledge of McGill University in Montreal, Quebec, originally called attention to the curious X-ray source. He compared a catalog of 18,000 X-ray sources from the German-American ROSAT satellite, which operated from 1990 to 1999, with catalogs of objects that appear in visible light, infrared light and radio waves. He realized that the ROSAT source known as 1RXS J141256.0+792204 did not appear to have a counterpart at any other wavelength.
The group aimed Swift at the object in August 2006, showing that it was still there and emitting about the same amount of X-ray energy as it had during the 1990s. The Swift observations, however, enabled the group to pinpoint the object's position more accurately and show that it was not associated with any known object.
"The Swift observation of this source is what got the show going," said Andrew Shevchuk, an astronomy student at Penn State and co-author of the study. "As soon as I saw the data, I knew Calvera was a great neutron-star candidate."
To show that the object is not associated with any other energy source, the team targeted the 8.1-meter Gemini North Telescope in Hawaii on Calvera. The team also used the Chandra X-ray Observatory, which showed the object as point-like-an observation consistent with neutron-star appearance.
Rutledge said there are no widely accepted alternate theories for objects bright in X-rays and faint in visible light like Calvera. Exactly which type of neutron star it is, however, 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," Rutledge said.
Calvera's location high above the plane of our Milky Way Galaxy is part of its mystery, but researchers think the neutron star is the remnant of a star before exploding as a supernova. In order to reach its current position, it had to wander some distance out of the disk.
"The best guess is that it is still close to its birthplace, and therefore close to Earth," Rutledge said. If true, the object is 250 to 1,000 light-years away — making it one of the closest known neutron stars.
"Because it is so bright, and probably close to Earth, it is a promising target for many types of observations," Fox said.
The team added that Calvera could represent the tip of the iceberg for isolated neutron stars.
"There could easily be dozens," Fox said. "The key point is that until our Swift survey, no one was able to refine the X-ray positions of large numbers of ROSAT sources to the point where it became clear which ROSAT sources were 'missing' their optical counterparts."
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