Astronomers at the University of Southampton have made a discovery that
promises to explain why X-ray binary stars are so variable — a phenomenon
that has been a long-standing mystery in X-ray astronomy.

Since the dawn of X-ray astronomy in the 1970s, astronomers have puzzled
over why the powerful X-ray emission from double star systems known as X-ray
binaries is so variable. An X-ray binary system contains either a neutron
star or a black holes. These bizarre objects are only the size of a large
city yet are more massive than our Sun and in X-rays alone emit ten thousand
times more power than the total output of the Sun. This immense amount of
energy is generated because material from a normal companion star flows
onto the neutron star or black hole, attracted by its very strong gravity.

But the X-ray output is not steady. It flickers very rapidly, varying by
small amounts on very short time-scales (fractions of a second). At the
same time, the X-ray output varies by larger amounts on longer time-scales.
No satisfactory explanation for this variability has ever been proved but
a favourite idea amongst astronomers is called the ‘shot-noise model’
(borrowing a term from electronics). According to this theory, the
variability is caused by random flares or ‘shots’ occurring in the X-ray
emitting region within the star system. Like building blocks piling up,
the shots combine together to produce variations on all time-scales.

Because the individual shots in the shot-noise model occur randomly and are
independent of all the other contributions to the total X-ray output, the
amount of variability on short time-scales should always be independent of
what happens to the X-ray output on longer time-scales. But using data from
observations of X-ray binaries made with NASA’s Rossi X-ray Timing Explorer
satellite, researchers Dr. Phil Uttley and Prof. Ian McHardy have discovered
that just the opposite is true. By comparing the amount of variability in
many small segments of data with the overall X-ray output (or ‘flux’)
measured in each segment, they found that the amplitude of short-time-scale
variations increases in direct proportion as the X-ray flux increases on
longer time-scales. In fact, they found a perfect linear relationship
between X-ray variability and X-ray flux.

This result implies that the short-time-scale variations in flux must
somehow ‘know’ what is happening to the flux on longer time-scales. This
finding is totally inconsistent with standard shot-noise models. However,
there is an alternative theory that fits with the observations. A number of
theorists have noted that variations in the X-ray output of X-ray binaries
might be caused by fluctuations in the flow of material that fuels the X-ray
emission, taking place far outside the region of space where the X-rays are
actually produced. In this scenario, slow, long-time-scale variations in
the flow of gas from the normal star happen far from the black hole and
move inwards like ripples on a pool of water. As they move inwards, the
slow variations in the fuel supply pick up more rapid variations, which are
larger when the fuel supply is larger, so that when the fuelling rate —
and hence X-ray output — is high, so is the degree of variability.

Dr Uttley said, “With this discovery we have dealt a fatal blow to the
most popular theory for why X-ray binaries are so variable. We are close
to pinning down the real answer and resolving a mystery that has intrigued
X-ray astronomers for 30 years.”

One twist in the story is that this important property of X-ray binary
systems could have been discovered in the early days of X-ray astronomy,
had astronomers known what to look for. In X-ray binaries, the pattern of
increasing variability with flux cannot easily be seen by just looking at
the data with the eye; computer analysis is needed to reveal it. Dr. Uttley
and Prof. McHardy were pointed to the existence of this pattern in X-ray
binaries by their observations of the much slower variations in distant
‘active galaxies’, which contain black holes a million times larger than
those in X-ray binaries and vary much more slowly. In a few of these
objects, the pattern of increasing variability with flux is easily
recognisable in the data without the need for computer analysis, although
a consequence of the slower variations is that the perfect linear nature
of the relationship found for X-ray binaries cannot be discerned.

This research is published in the 11 May issue of the Monthly Notices of
the Royal Astronomical Society, and on the web at:

http://xxx.soton.ac.uk/abs/astro-ph/0103367