Black hole brightens at galactic core

Simulation of matter and light swirling around a super-massive black hole

By BBC News Online science editor Dr David Whitehouse

A sudden increase in brightness from a cloud of very hot gas near the heart of the Milky Way seems to confirm the idea that a super-massive black hole exists at the galactic core.

Most astronomers believe that such an object does indeed reside there, but the evidence so far, based on the motions of nearby stars, has not been conclusive.

Now, new observations of the core have detected a flare of X-ray energy that dimmed and brightened in just 10 minutes.

The duration, combined with the earlier evidence, allows astronomers to calculate a mass and a size for their suspected black hole. They come out with a mass more than two million times that of our Sun, contained in a region of space just 150 million kilometres (93 million miles) across.

A black hole is the only explanation for such figures, the scientists say.

X-ray flare

Other theories to explain the very fast motion of the galaxy's central stars include the presence of some other form of invisible matter, like a cluster of dark stars.

The flare from the centre of the Galaxy

Whatever is there, it has to be truly massive to pull the closest visible stars along at an incredible five million kilometres per hour.

But confirmation that what we are looking at is almost certainly a black hole comes from something going on at its fringes.

In the journal Nature, Frederick Baganoff at the Massachusetts Institute of Technology, US, and colleagues report observations made by the Chandra X-ray telescope. It has detected a flare of X-ray energy produced from where the edge of where the black hole should be.

The flare dimmed and brightened over a period of 10 minutes, the time taken for light to travel about 150 million km (93 million miles). This means that it originated from an object no further across than the distance from the Earth to the Sun.

Follow-up studies

This implies that the mass at the centre of our Galaxy - about 2.6 million times the mass of our Sun - is crammed into a very small space. And according to the laws of physics, only a black hole can be that dense.

Commenting on the research, Fulvio Melia of the University of Arizona, Tucson, US, said: "Baganoff and colleagues appear to have excluded all other possible alternatives."

But although this study makes the case for a black hole at the galactic centre a lot stronger, it is not the final proof. Melia says that Chandra could have seen a flaring object that just happened to be in the same direction as the centre of our Galaxy.

He adds that follow-up work, being done by several observatories, is underway to exclude this possibility.