Re: Distant Gamma Ray Burster Nailed

Carl Feynman (carlf@atg.com)
Fri, 23 May 1997 15:06:45 -0400

At 10:17 PM 5/19/97 +0200, Eugene Leitl wrote:
>On Mon, 19 May 1997, Carl Feynman wrote:
>[...]
>> like the antimatter fountain. Since they vary over time scales as short as
>> a millisecond, they can't be bigger than a light-millisecond across, i.e.
>> 3000 km. I just did a back of the envelope calculation (well, in my head,
>> actually) and determined that to get all the gamma rays from a burst into a
>> volume that small, they would have to be compressed to the density of lead.
>> Cool, huh? ^^^^^^^^^^^^^^^^^^^^
>
>!!!!!????? Gamma at the density of lead? Now this is so wildly ridiculous,
>it fails even to send a shiver down my spine.

It *should* send a shiver down your spine. Okay, that's not creepy enough,
get this: gamma rays at the density of lead are as opaque to gamma rays as
lead is. In other words, if you could pile up such a ball of gammas, they
wouldn't be able to get out! Only the ones at the surface would be able to
escape, then the ones beneath them, etc. The time it would take for all the
radiation to dribble out would be longer than the duration of a gamma ray
burst. So it impossible to get gamma rays out of an object as small as a
burster in times as fast as a burster.

How to solve this problem? Well, if whathever makes the burst is moving
towards us at close to the speed of light, it will be running just behind
the light that we see it with, so we will see things on it happening faster
than they actually occur. It turns out that it is possible to get the
energy release of a burster if the source is moving toward us at 0.9995 c or
faster. Moreover, it turns out that pretty much anything moving at that
speed through the interstellar medium will make a gamma ray burst when it
plows into interstellar hydrogen atoms. The plowing process takes many
hours before the projectile slows down, but we see it compressed into a few
seconds because the projectile is traveling only a smidgen behind the gamma
rays it produces. So *something* is throwing around chunks of stuff at
unheard-of speeds, by mechanisms unknown.

If the chunks are not moving directly toward us, we wouldn't see them nearly
as easily because almost all the radiation is emitted in the forward
direction.

>Which processes can be _that_ energetic? Even if we assume colliding
>neutron stars, travelling at their usual kinetical + gravitational
>potential energy, and assume a high conversion efficiency (several
>10%) to gamma, intuitively this doesn't suffice.

Actually, a conversion efficiency of 0.1% of the mass would be entirely
sufficient. Remember that a 3000 km ball of lead is only the mass of a
largish planet. Convert Neptune into pure energy and you'd have a pretty
good gamma ray burst. Since stars are thousands of times more massive than
planets, a conversion efficiency of 10^-3 is plenty.

>I don't have
>Carl's envelopes at the house, could anyone give some ballpark figures
>on energetics of such events?

10^50 ergs at 3000 megaparsecs. It's a big envelope.

>If a hole eats a pulsar in one fell gulp, how much energy is released
>then?

10^53 ergs. Most of the energy in that case goes into neutrinos and gravity
waves, but there might be enough left over to make a burst (by mechanisms
unknown, ha ha!)

>And why does my every darn sentence seem to end with a question
>mark, today?

Well, nobody understands these things. At least now that we know they come
from cosmic distances, we can throw out *half* of the theories that people
have come up with. A few years, there were 100 theories and less than 100
known bursts. Now there are 1000 bursts and only about two dozen theories.

>
>Since we are at the holes, and evaporating primeval singularties should
>give a fair gamma flash (how fast? did somebody record the GBR spectral
>changes)? before winking out, can one look out of them? (But these should
>be very rapid, and extremely faint, being observable just in low lightyear
>range).

Doesn't work. The intensity-vs-time curves of observed bursts are very
variable, and often start fast and end slow, while evaporating primordial
black holes all look the same, and start slow and end fast.