5 April 2002   Ref. PN 02/15 (NAM9)
EMBARGOED until 7 a.m. BST Friday 12 April 2002

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



Astronomers probing the intimate details of apparently quiescent stellar
black holes have discovered that in reality they are dynamic, lively places,
subject to flares that briefly illuminate the whole of the gas disc around
the black hole. Their observations are helping to build up a picture of
precisely where X-rays are generated in the gas as it heats up to extreme
temperatures and swirls around under incredible gravitational forces before
cascading into the black hole itself. On Friday 12 April, Dr Robert Hynes of
Southampton University will tell the National Astronomy Meeting in Bristol
about detailed observations of flares lasting a few hours, made with the
William Herschel Telescope on the island of La Palma, and even more recent
observations made with the brand new Gemini South telescope in Chile of the
shortest such flares ever spotted from a quiescent black hole, lasting only
a matter of minutes, or less.

The best evidence astronomers have for the existence of black holes within
our own galaxy comes from X-ray binary stars where a black hole or neutron
star is fed gas by an ordinary star in orbit around it. The gas becomes so
hot that it glows with X-rays. Some of these binaries have a 'quiescent'
state in which the X-rays they emit are more than a million times less
powerful than normal. It is believed that less gas is falling onto the black
hole or neutron star at these times, but quiescent systems with black holes
appear even fainter than the ones with neutron stars. This might be because
energy is disappearing past the black hole's event horizon - the point of no
return beyond which energy is irretrievably lost. But to be sure,
astronomers need to know more about how the dribble of gas flows onto the
black hole during the quiescent period.

To investigate this, Robert Hynes, collaborating with Professor Phil Charles
also at Southampton, Dr Carole Haswell of the Open University and Cristina
Zurita and Dr Jorge Casares of the Instituto de Astrofisica de Canarias on
Tenerife, has used the William Herschel Telescope to look at the visible
light from the gas disc of a quiescent black-hole X-ray binary star (V404
Cygni). The glow from the disc varied by a large amount - during flares
lasting for a few hours, gas all around the black hole was lit up, most
likely by X-rays shining on it. 'We have yet to observe visible and X-ray
flares simultaneously,' says Dr Hynes, 'but if this explanation for the
visible flares is correct, we can use them to pinpoint more accurately where
the X-rays are coming from.'

In the most recent observations with the Gemini South Telescope, the team
have found even more rapid variations. They saw the visible brightness of
one system increase by 25% in about one minute. 'These are the most rapid
variations of these faint, quiescent black holes that anyone has found so
far,' says Dr Hynes. 'They are far from being the dormant objects we
imagined, and there must still be dramatic activity going on where gas falls
onto the black hole.'


Artist's impressions of X-ray binary systems are available from


More information about the William Herschel Telescope is available at

More information about the Gemini telescopes is available at

RAS Web site:

UK National Astronomy Meeting Web site:

Dr Robert I. Hynes, Dept. of Physics and Astronomy,
University of Southampton SO17 1BJ
Tel:+44(0) 2380 592112    Fax:+44(0)2380 593910

Prof. Phil Charles, Dept. of Physics and Astronomy,
University of Southampton SO17 1BJ
Tel:+44(0) 2380 592076    Fax:+44(0)2380 593910