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ROYAL ASTRONOMICAL SOCIETY PRESS NOTICE

Issued by: 

Jacqueline Mitton
RAS Press Officer
Phone: +44 (0)1223-564914                
E-mail: jmitton{at}dial.pipex.com         

RAS Web site: http://www.ras.org.uk 

CONTACTS

Dr Christine Done,
Dept. of Physics, University of Durham
Durham DH1 3LE, UK
Chris.Done{at}durham.ac.uk
Phone: (+44) (0) 191 334 3614
Fax: (+44) (0) 191 334 3521

Dr Marek Gierlinski
Dept. of Physics, University of Durham
Durham DH1 3LE, UK
Marek.Gierlinski{at}durham.ac.uk
Phone: (+44) (0) 191 334 3516
Fax: (+44) (0) 191 334 3521

31 March 2003

BLACK HOLES REALLY ARE HOLES, SAY ASTRONOMERS

Black holes really are holes -- objects without a
surface -- say Drs Christine Done and Marek Gierlinski in
a paper accepted for publication in the Monthly Notices
of the Royal Astronomical Society. Having an "event
horizon" rather than a surface is the property that
makes something a black hole but, by definition, it's
impossible ever to see one directly. However, these
new results give direct evidence of the existence of
such holes in spacetime.

Drs Done and Gierlinski set out to track down the
signatures of event horizons by looking for differences
between objects thought to be black holes and collapsed
bodies of a different kind that are only slightly less
extreme -- neutron stars. Any material captured by the
strong gravity of either type of object will spiral
inwards in much the same way, reaching speeds of up to
half the speed of light and transforming some of the
immense gravitational energy into X-ray emission. The
crucial difference is that, in the case of a black
hole, material should simply pour into the hole, taking
its remaining energy with it and disappearing forever,
whereas with a neutron star material smashes onto the
surface, releasing whatever energy is left. As a
consequence, the X-ray emissions from neutron stars
and black holes should look different.

"The idea is simple in theory, and has been known for
a long time, but until now it has been hard to put
into practice because the X-ray emission even from a
single type of object can show a bewildering variety
of properties that are not well understood," says
Chris Done.

Only recently, with over 6 years of operation of
NASA's Rossi X-ray Timing Explorer, an orbiting X-ray
observatory, have there been enough data to cover the
whole range of behaviour of both neutron stars and
black holes. Drs Done and Gierlinski took advantage
of this enormous dataset, reducing all the information
contained in the complex X-ray spectra down to just
two numbers, which describe the slope of a spectrum
at low and high energies. Plotting these two numbers
against each other as an X-ray source changes in
brightness gives a good overview of how the shape of
the object's X-ray spectrum is changing. "What we see
is that neutron stars and black holes behave in
distinctly different ways as more material falls onto
them," says Chris Done, "and the only big difference
we know of that can account for the observations is
that neutron stars have a surface, while black holes
don't."

NOTE 

More information about the Rossi X-ray Timing Explorer
(RXTE)
     http://heasarc.gsfc.nasa.gov/docs/xte/xte_1st.html

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