Re: The Extinction Challenge
Thu, 29 Jul 1999 10:48:15 -0700

Jeff Davis, <>, writes:
> The latest issue of Scientific American has an article on GRB's (the cover
> article in fact),

Actually I believe you mean the July 10 issue of Science News.

> according to which (the latest theory) the bursts are
> part of a hypernova explosion of stars larger than 35 solar masses, and are
> tightly confined "jets" directed axially outward, north and south, along
> the axis of rotation. According to this model, the grb's are detectable by
> us only when the distant hypernova is oriented so that the axis of rotation
> points more or less directly at the earth. (Presumably in a galaxy so
> oriented but safely distant.) Also the total energy released is
> substantially smaller when "the signal" we see is the result of axial jets,
> than it would be if it were at that same level in all radial directions.

Yes, this was a very interesting article (unfortunately not available on line), which suggests that they are close to getting a good model for GRBs as super-intense supernovas which form black holes. What some consider the "smoking gun" was the discovery that a GRB left behind what seems to have been a supernova.

Essentially what happens is that when a sufficiently massive star goes supernova, the force of the explosion is unable to drive the mass of the star to escape velocity. The stellar material falls back and leads to the formation of a black hole. An accretion disk forms in the process blocking radiation from escaping anywhere but the poles. The actual gamma rays are formed when high energy particles (neutrinos, I think) run into slower matter, either the interstellar medium or gas ejected earlier by the star.

I think, although the article did not say this, that this is essentially the classical model for how stellar black holes would be formed. So it seems that GRBs are a byproduct of black hole formation.

> If it should turn out that the axes of rotation of stars in a galaxy are
> mostly aligned with the axis of rotation of the galaxy itself, then the
> grb jets will mostly be directed out of the plane of the ecliptic into
> relatively "empty" space, and only a very few stars radially nearby or
> those immediately above or below in the galactic disc will be subject to
> obliteration.

I don't know about this alignment theory. The Milky Way is in the plane of the galactic disk, but I don't believe it is aligned with the ecliptic (the plane of the planets' orbits).

> Thus, according to this model of grb's, the destruction wrought on the
> local galaxy should be substantially less than previously predicted.

Yes, it should only affect those stars along the direction of the beam. I saw something in another article suggesting that the beam was extremely narrow, a small fraction of a degree, but this article indicated that more work was needed to determine the width.