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The Question
(Submitted April 11, 1998)
Are there two types of blackhole? One is like the Cygnus X-1, and the
other is a super massive blackhole in the active galactic nuclei?
The Answer
Thank you for contacting our Ask a High-Energy Astronomer service. The
only real difference between these two kinds of black hole is size. A
black hole in an X-ray binary will only be a few times as massive as the
sun. A black hole at the center of an active galaxy can be millions of
times as massive as the sun. It can get so big because the density of
matter in a galactic center is high so there is lots of matter for it to
accrete.
Both kinds of black hole can have an accretion disk. Usually most of
the radiation we detect comes from the accretion disk. The accretion disk of
a super-massive black hole will be much larger than that of one in an X-ray
binary. This means that the radiation that we detect varies on a much longer
timescale (days instead of milliseconds).
Damian Audley
for Ask a High-Energy Astronomer
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