Comments on: Followup: naked eye gamma-ray burst

By: EH Ooi

EH Ooi — Thu, 11 Sep 2008 05:41:19 +0000

Hi Phil.
I visit your blog once in a while, probably I missed the first announcement on GRB (if any). Maybe I got to start visiting your blog more often. Did you (and other astronomers out there) know about this GRB before hand or was it a coincident that they detected it. What I am trying to say is, were you anticipating this event? If yes and if in future there are more stuffs like that, could you post them early so I may get a chance to see it (although I dont think Singapore is a good place to observe, light pollution). By the way, when is your second book coming out?

By: Tom Marking

Tom Marking — Thu, 27 Mar 2008 23:45:28 +0000

“Again, I ask. If the amount of energy put out by these things is what makes them difficult to understand, then how does “beaming” solve the problem since it would take more energy to concentrate a beam than is in the beam itself?”

For an isotropic (i.e., in all directions equally) radiator we have:

F = P / (4*pi*r^2)

where F is power flux (watts per square meter), P is power (watts), and r is distance (meters)

For a tightly focused beam:

F = P / (Sigma*r^2)

where Sigma is the solid angle (steradians) of the beam. Or:

P = Sigma*F*r^2

Sigma = 2*pi*(1 – cos(0.5*theta))

where theta is the full beam width (radians)

For small theta we use a Taylor series approximation:

P = 0.25*pi*theta^2*F*r^2

Thus, if you cut the beam width in half the power needed to produce the given flux (i.e., brightness) goes down by a factor of four. So small beams are good to project power a long way. Hope that helps.

By: R.K.

R.K. — Wed, 26 Mar 2008 18:01:53 +0000

By the way, the GRB came from roughly the direction of USNOA2 1200-07324239, near the Bootes Void.

http://server6.sky-map.org/starview?object_type=1&object_id=1148174927

By: Alec

Alec — Tue, 25 Mar 2008 18:57:25 +0000

I wonder how many alien philosophers were explaining how the universe was finely and intelligently tuned to be a hospitable place for conscious life, when they were hit by this?

By: Barton Paul Levenson

Barton Paul Levenson — Tue, 25 Mar 2008 18:25:36 +0000

I’m familiar with the “no molecules of justice” mode of thought; it’s something we hear constantly from the likes of Richard Dawkins. It’s wrong, though. Justice can be real even if it’s not made of molecules. Ditto love. And God. If you want to live without those things, feel free, but don’t claim that your nihilism is somehow backed by science.

By: dr_lha

dr_lha — Mon, 24 Mar 2008 19:08:27 +0000

Mark: My explanation was not meant to contain all the arguments for why there are jets are present in GRBs. It is merely one of the simplest arguments for them, i.e. that an object at such a distance could not physically output the amount of energy required if the GRB output its energy uniformly in all directions.

Although its true that GRBs are still poorly understood, as Phil states there are many phenomenological characteristics in GRB afterglows that point towards there being jets in the system.

I would point out that I am not a GRB theorist, but know many that are and the concept to beaming and jets in GRBs is non-controversial these days.

By: The Bad Astronomer

The Bad Astronomer — Mon, 24 Mar 2008 17:13:47 +0000

There is quite a bit of evidence of beaming, actually. There is a phenomenon called the jet break, where the material in the beam suddenly slows and the angle of beam widens. This has been seen in many many GRBs, and is excellent evidence that the energy is focused into beams and not emitted in all directions.