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.”
CONTACTS
Dr Christine Done,
Dept. of Physics, University of Durham, Durham DH1 3LE, UK
Chris.Done@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@durham.ac.uk
Phone: (+44) (0) 191 334 3516
Fax: (+44) (0) 191 334 3521
NOTE
More information about the Rossi X-ray Timing Explorer (RXTE)
