Irish scientist helps to fill us in on black holes

An Irish astrophysicist has played a part in two new discoveries about one of the great mysteries of space, the black hole. Scientists this week will present new findings about these objects which have such a gravitational pull that not even light can escape them.

The research is being presented at one of the largest gatherings of astronomers to take place this year at the 197th annual meeting of the American Astronomical Society, under way in San Diego, California. Ireland is playing its part in this international research effort as a member of the research team.

"As an Irish astronomer, I have access to this data," explained Dr Paul Callanan, of the Department of Physics at University College Cork.

The discoveries focus on black holes, collapsed stars which have become so dense and compact that their original bulk has been reduced to something little more than a dozen kilometres across.

One finding involves new evidence for something thought to surround every black hole, its "event horizon". The event horizon is the place where the gravitational pull becomes so powerful that no light can escape. While Einstein and others have theorised on the existence of the event horizon, there had been little proof until now.

The research team, led by scientists from Harvard and involving Dr Callanan, used the orbiting Chandra observatory, a satellite designed specifically to identify and analyse X-rays.

Many black holes are part of a binary system which includes both an observable star and an accompanying but invisible black hole. The star can be like our sun or can be a neutron star, a much denser, older and near dark star.

The two binary objects wheel around one another like dance partners and the hole's presence can be detected because of the way it disturbs the orbit of the star which can be seen. "It is not what we saw but what we didn't see," Dr Callanan said. "We measured with Chandra the X-ray emissions from the binary star systems."

They discovered a hundred-fold difference in the amount of X-rays coming from a binary pair involving a conventional star compared to the X-rays from a neutron star binary. "The simplest interpretation for this is the event horizon," Dr Callanan said. "The neutron star can still emit X-rays but at a much lower rate than a conventional star binary. But a hole will emit no X-rays."

While some X-rays will manage to escape just before crossing the event horizon, no X-rays will escape once this point is crossed. The researchers believe that the X-ray data makes the theoretical case for a surrounding event horizon much stronger.

In a separate research project in which Dr Callanan had a role, a black hole binary has provided evidence of a massive explosion, a gamma-ray burst within our own galaxy. Gamma-ray bursts are the most powerful stellar events and emit inconceivable amounts of energy.

The team, which involved US and Dutch astronomers, believes that the gamma-ray burst came from a collapsing star which later formed a black hole. It discovered an unusual mix of elements in the hole's partner star and concluded that the mix could only have arisen because of material blown into it from the burst.