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

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UK National Astronomy Meeting Web site:

Issued by: RAS Press Officers

Dr Jacqueline Mitton
Phone: +44 (0)1223-564914    Fax:    +44 (0)1223-572892
E-mail:  Mobile phone: 07770-386133

Peter Bond
Phone: +44 (0)1483-268672      Fax:    +44 (0)1483-274047
E-mail:    Mobile phone: 07711-213486

National Astronomy Meeting Press Room phones:
+44 (0)117 928-4337, (0)117 928-4338, (0)117 954-5913, (0)117 928-7901

Dr Phil Uttley, Department of Physics and Astronomy,
University of Southampton, S017 1BJ    Tel:+44(0)23 8059 2089    Fax:+44(0)23 8059