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M82's hidden supernova

...A supernova has recently exploded in the nearby galaxy M82, but you won't be able to see it with any ordinary telescope. Shrouded in obscuring gas and dust, only the radio emission of the stellar explosion was seen...

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Victoria gives Opportunity a view into Mars' history

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Did life need asteroid bombardment?

...A period 3.9 billion years ago when Earth was peppered with impacts by large asteroids may have created an environment in which primitive life could take hold, rather than destroying that life...

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On the edge of a hungry black hole



Posted: 28 May, 2009

Gas and dust equal to the mass of two Earths are being gobbled up every hour by a hungry black hole in a distant galaxy, according to a space telescope probing the Universe in X-rays that has peered closer to a black hole than ever before.

An artist's concept of a spiralling disc of hot gas and dust around a black hole.

Image: ESA.

The European Space Agency’s orbiting XMM-Newton observatory studied the black hole in the core of an active galaxy known only as 1H0707-495. This particular galaxy is described as Seyfert 1 galaxy, which typically have bright centres emitting a broad range of hydrogen emission but narrow emission lines of heavier elements such as iron. Four out of five Seyfert galaxies are barred spirals. The bars funnel large amounts of gas into the core, which then feeds into a spiralling disc around the supermassive black hole located in the centre of the galaxy.

As the gas spins around the black hole, it heats up, producing X-rays that then illuminate clumps of matter in the disc. To get a better grasp of the size and distribution of material in this disc, astronomers used XMM-Newton to search for X-rays emitted by iron atoms. They chose iron because it has a distinctive signature that is affected by the spin of the black hole, the velocity at which the iron atoms are orbiting around the black hole, and the energy required to escape the black hole’s gravity. As such, they can tell us a lot about the characteristics of the black hole.

In addition, there is a 30-second time lag between changes in X-rays observed directly, and the reflected X-rays from clumps in the disc. This delay is caused by the time it takes for the

X-rays to travel to the clumps of gas, and has allowed a measurement of the size of the disc to be made. It turns out that it is twice the radius of the black hole itself – we are seeing structures around the black hole that are incredibly close to its event horizon (the fabled ‘point of no return’). Knowing the size of the disc and the speed at which the iron atoms are orbiting the black hole allows an estimate of the black hole’s mass to be made, which is between three and five million times the mass of our Sun. For comparison, the black hole at the centre of our Galaxy is 4.3 million solar masses and has a radius of 22 million kilometres (and to put that in context, compare it with the average distance between Earth and the Sun, which is 149.6 million kilometres).

“We can now start to map out the region immediately around the black hole,” says Professor Andrew Fabian of the University of Cambridge who led the observations, and who is also the current President of the Royal Astronomical Society. The research is published in the 28 May edition of Nature, and should allow us to far better understand the mechanisms by which material is swallowed up by black holes (in 1H0707-495 it is practically being thrown down the black hole’s maw). The next big step will be to try and image the event horizon itself; the closest we can ever get to directly observing a black hole.


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