GRB's A Primer-Part two: Some theories.

[Astroman 6]

Now, theory on what causes these things is firming up in some areas, and not so much in others.

There appear to be two distinct types. When the energies of BATSE bursts are plotted according to energy and length, two groups appear. Short/hard bursts and long/soft bursts.

Short/hard bursts refer to the quick burst, under 10 seconds or so, and at energies in the mid to upper gamma ray spectrum-MeV.

Long/soft bursts last seconds to minutes and radiate mostly in the upper x-ray to mid gamma range-50Kev to 1MeV.

Both are “flavored” with variations within the light curve. They may have peaks of extremely high energies, while the rest is relatively low ultraviolet level.

The short/hard bursts seem to have a connection with supernova explosions. Two have been detected and have shown the connection. The GRB arrives first and is so bright it “drowns out” the light from the supernova. Spectra from the SN is detected later, sometimes weeks later and indicate a Type Ic which indicates a companion star of some kind. These GRB’s are extremely far away and are red shifted toward the x-ray or even UV band. It could be that these are the beginnings of black holes, neutron stars or more exotic objects. (Though how you get more exotic than a neutron star is beyond me…)

The actual mechanism for the creation of the burst is not well understood, beyond the apparent tie to supernova. There are SNe detected regularly, (in fact one was detected in M51 just a couple weeks ago), but only two have been definitely tied to a GRB, as I said. It could be that the SN connection is beamed straight at us, and although the burst is high energy, the actual event is of low total output, while the long/soft burst has more total output at lower energies. (That’s tough to get your head around I know, but take it slow and it’ll sink in.)

Other theories for producing GRB’s are: Convergence of two black holes, convergence of two neutron stars or combinations thereof, the collapse of a very rapidly spinning neutron star into a black hole, a “hypernova”-read SN on steroids.

Convergences of BH’s may satisfy the demands for energy release, but just barely. It depends on the total mass of both participants. Equally large BH’s, say 10 solar masses each may do the trick. Convergences of neutron stars and anything but a BH would not have enough mass to explode, much less create a GRB, unless the NS swallowed a BH quickly and with enough total mass to make trouble. Velocity and trajectory enter into this scenario, and the math is hellish, so it seems unlikely anyway.

According to Einstein’s general relativity, such collisions would also generate gravity waves of high order. There are some smaller gravitational wave detectors up today, but I don’t believe any are sensitive enough to detect these collisions, given their necessary distance. Maybe once LIGO and the orbital one, (name, anyone?), get up and running, they can, but none have been detected to date. So far, it looks like collisions are out.