Neutron stars form when massive stars exhaust their fuel

The object, known as a neutron star, was studied using space telescopes and ground-based observatories.

But this one, located in the constellation Ursa Minor, seems to lack some key characteristics found in other neutron stars.

Details of the study, by a team of US and Canadian researchers, will appear in the Astrophysical Journal.

If confirmed, it 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.

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

Robert Rutledge, McGill University

"The seven previously known isolated neutron stars are known collectively as The Magnificent Seven within the community," said co-author Derek Fox, of Pennsylvania State University, US.

"So the name Calvera is a bit of an inside joke on our part."

The authors estimate that the object is 250 to 1,000 light-years away. This would make Calvera one of the closest neutron stars to Earth - and possibly the closest.

Neutron stars are one of the possible end points for a star. They are created when stars with masses greater than four to eight times those of our Sun exhaust their nuclear fuel, and undergo a supernova explosion.

This explosion blows off the outer layers of the star, forming a supernova remnant. The central region of the star collapses under gravity, causing protons and electrons to combine to form neutrons - hence the name "neutron star".

Data search

Robert Rutledge of McGill University in Montreal, Canada, originally noticed the object.

He compared a catalogue of 18,000 X-ray sources from the German-American Rosat satellite, which operated from 1990 to 1999, with catalogues of objects that appeared in visible light, infrared light, and radio waves.

Swift was launched to observe gamma-ray bursts

The group aimed Nasa's Swift satellite at the object in August 2006. Swift's X-ray telescope showed that the source was still there, and was emitting about the same amount of X-ray energy as it had during the Rosat era.

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

The researchers followed up with the 8.1m Gemini North Telescope in Hawaii and a short observation by Nasa's Chandra X-ray Observatory.

Unusual properties

Exactly what type of neutron star Calvera is remains a mystery. According to Dr Rutledge, there are no widely accepted alternative theories to explain objects such as this that are bright in X-rays and faint in visible light.

"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," said Dr Rutledge.

Calvera's location high above the plane of our Milky Way galaxy is also a mystery. The researchers believe the object is the remnant of a star that lived in our galaxy's starry disc before exploding as a supernova.

In order to reach its current position, it had to wander some distance out of the disc.