Music of the black holes: they all play the same tune

Astronomers at the University of Southampton have uncovered a remarkable connection between the monstrous black holes residing at the hearts of distant galaxies and their comparatively tiny cousins which inhabit star systems in our own Milky Way: they are playing the same tunes. Dr Phil Uttley presents these findings in a talk called `The music of black holes` at the National Astronomy Meeting in Bristol on Tuesday 9 April.

Massive black holes, a million to a billion times heavier than the Sun but smaller than our own solar system, are thought to lurk in the centres of most galaxies, including our own Milky Way. From time to time, gas in the central regions of the galaxy will fall in towards the black hole. As the gas gets closer to the black hole, more and more energy is released in the form of light and other electromagnetic radiation, including X-rays originating close to the black hole where the gas is hottest.

Galaxies where this process is taking place are called `active` galaxies. But the X-ray output from them is not constant, probably due to turbulence in the gas supply. The X-rays vary slowly on different time-scales ranging from hours to years, with some active galaxies showing much slower variations than others.

For the past six years, Dr Phil Uttley and Prof. Ian McHardy at the University of Southampton, together with other colleagues, have used NASA`s Rossi X-ray Timing Explorer (RXTE) satellite to monitor the X-ray variations of several active galaxies. Their aim is to compare the slow variations in the X-ray output of active galaxies with the much more rapid variations (on time-scales of milliseconds to seconds) of black hole X-ray binary systems (BHXRBs): black holes that are a million times smaller than the monsters in galaxy centres and feed off gas from normal, `companion` stars.

Dr Uttley explains: `The X-ray variations of active galaxies and BHXRBs can be likened to music, showing small variations – single notes – on short time-scales, and larger variations – whole key changes – on longer time-scales. What we are finding with our RXTE monitoring is that the time-scales for these note and key changes to take place are about a million or more times longer in active galaxies than in BHXRBs. In other words, take the tune played out in X-rays by a black hole X-ray binary and slow down the tape by a factor of a million or so and you get the kind of variations we are seeing in active galaxies.`

It has long been thought that, despite the huge difference in size, the innermost regions of active galaxies and BHXRBs are essentially the same. From a theoretical point of view this makes sense, because those regions are dominated by the black holes` enormous gravity, and do not care about the external environment which feeds them – whether there is a normal companion star, like in an X-ray binary, or the gas-rich environment in the centre of a galaxy.

Dr Uttley said `The tape speed setting is the only major difference, and it`s governed by the black hole`s mass. Bigger black holes show slower variations, so we can use the X-ray variability to measure the mass of the black holes in active galaxies. But more importantly, we can now be confident about pushing the analogy between active galaxies and black hole X-ray binary systems even further, to learn more about these incredible objects.`