Black Hole Q:

"Bryan Heit" wrote in message
...
I was reading through "A Brief History of Time" - a little late, but
better late then never. One thing that caught my attention was the idea
of "hot" blackholes, as in all black holes should emit gamma radiation
(hawking radiation) and thus be detectable by looking for gamma
radiation sources. I did a quick google search and didn't find much on
detecting hawking radiation. Has this ever been observed, or is it
still a theory?

It has not been observed. There are two major problems
facing the prospective observer. First, all black hole
candidates to date are very far away and so very hard
to see with the necessary resolution. Second, the
nature of the radiation given off by Hawking radiation
depends upon the temperature of the black hole, which
in turn depends upon the size (mass) of the black hole.

Large black holes are cold. Small black holes are warmer,
and tiny black holes are hot. That is to say, if a black
hole could be kept from eating, it would start to
evaporate. Slowly at first, and then more rapidly as its
size diminished and it became hotter. Eventually it would
explode in a furious final evaporation.

Gamma rays are pretty hard (energetic) radiation. It would
be a very small black hole near the end of its evaporative
life that would be required to give them off in any
substantial quantity. The black holes that we think we
"see" are not in any danger of starving, so they won't be
good candidates for detecting gamma radiation due to
hawking radiation.

On the other hand, black holes that are actively feeding
can give off hard radiation from their accretion rings,
which get very, very hot indeed.