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.

Bryan Heit wrote:
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?

Bryan


It is an as-of-yet unverified hypothesis. Hawking posited that miniblack holes
the mass of mounts might reveal themselves in this era. These would be created
in the early moments after the big bang. Though bursts of gamma rays have been
detected, the model for their production has not included miniblack holes.

As to star-produced black holes, we will need to wait some time (I recall a
number like 10^60 or so years) for the evaporation process to bring them to the
point where they will spontaneously evaporate into a host of gamma rays.

"BH" == Bryan Heit writes:

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

As others have explained already, Hawking radiation is still a
hypothesis and has not yet been detected. So-called primordial black
holes, those that were formed soon after the Big Bang, are small
enough that, *if they existed today*, they would be evaporating.
There's been a recent revival of interest in such objects, e.g.,
URL:http://xxx.lanl.gov/abs/astro-ph/0304528 (although none has been
detected).

--
Lt. Lazio, HTML police | e-mail:
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html