Unusual duo detected

The pulsar is not visible and the red star indicated may not be its companion

By BBC News Online science editor Dr David Whitehouse

Astronomers have found what they think is a pulsar that has a black hole as a companion.

Such a binary would be a first for science and a superb natural laboratory for testing theories about space and time.

The companion's mass is at least 11 times the mass of the Sun, and probably more like 16 times

Prof Andrew Lyne, Jodrell Bank

A pulsar is a small, dense, rapidly rotating object left over from a stellar explosion. It sweeps out radiation across the cosmos much like a lighthouse flashes at sea.

When stellar remnants are very massive, they are thought to collapse beyond the pulsar-like stage to become black holes. And to now find what appears to be both types of object occupying the same region of space is very exciting.

The suspected pulsar-black hole binary system is located near the centre of our galaxy.

Crowded space

The discovery was presented to the American Astronomical Society meeting in San Diego, California, US. The research team behind the find is led by Dr Ingrid Stairs of the Jodrell Bank Observatory, UK.

Timing observations of the pulsar, called PSR J1740-3052, were made with Jodrell's Lovell Telescope and with the equipment at the Parkes Radio Observatory in Australia.

Minute variations in the arrival times of the pulses indicate the presence of an unseen companion in orbit around it every 231 days. "The companion's mass is at least 11 times the mass of the Sun, and probably more like 16 times," says Professor Andrew Lyne of Jodrell Bank Observatory.

With such a mass the companion is either a very heavy ordinary star or possibly a black hole.

Astronomers have looked for the companion but their images are crowded with stars because the pulsar lies in the general direction of the centre of our galaxy and its large stellar population.

Closest approach

The team doubts that a red star they detected in correct position is actually the pulsar's companion.

Dr Ingrid Stairs said: "A typical star of this mass would extend beyond the pulsar's orbit. You would expect the pulsar to be eclipsed every orbit and it isn't."

Precise timing of the pulsar shows a small change of the orbital position where the pulsar and its companion have their closest approach.

This is the result of an effect attributable to a combination of general relativistic and classical effects.

"The value you'd predict for this number if the companion were a supergiant star just doesn't square with what we observe," said Dr Richard Manchester of the Australia Telescope National Facility.

The astronomers have applied for time on the European Southern Observatory's Very Large Telescope in Chile to look in more detail at the system.