Re: Distant Gamma Ray Burster Nailed

Carl Feynman (
Fri, 23 May 1997 15:06:46 -0400

At 10:39 PM 5/22/97 -0700, Hal Finney wrote:

>I'm not sure how Carl Feynmann got those incredible numbers about the
>energy density within the GRB. Presumably you look at its (huge) distance
>from us, look at the energy we saw, assume that it was radiating that
>much energy in all directions, and get a whole lot of energy.

Yup. A typical burst is 10^50 ergs if the source is 3000 megaparsecs away
and radiating equally in all directions. The intensity here is 10^-7
ergs/cm^2. If it was light instead of gamma rays, it would be as bright as
a 10 watt bulb two miles away, i.e. clearly visible to a dark-adapted eye.

>One thing I was thinking was that the radiation could perhaps be going
>out in a tight beam. GRB's would actually be much more common than we
>see, but most of them miss us. However you would still have extremely
>high energy densities unless the beam is super tight. Also the fact
>that something was visible for hours or days afterwards doesn't sound
>right for a tight-beam phenomenon.

Tight-beam vs omnidirectional is hard to decide obsrvationally. As far as I
know, we can't tell yet.

If they do originate with stuff plowing into the interstellar medium at
>.9995 c, since we only see the radiation from the stuff that's aimed right
at us, there could perfectly well be a shell of stuff expanding in all
directions, and sending a burst in all directions, and it would look to us
like a tight-beam phenomenon. In this case, the visible effect hours or
days afterward would be the fireball produced by the stuff after it had
decellerated to a moderate .99 c (say).