Deep Physics stuff i don't know :P

[Nishroch_Order]

one more new question

4. with referance to this page : BBC News | SCI/TECH | Cosmic catastrophe 'a certainty'

gamma-ray bursts from collapsing supermassive stars are shot across the cosmos in narrow beams

Why in narrow beams? Why dosen’t it explode and shoot radiation out in all directions like in a conventional explosion?

[JS_Princeton]

Jets and beams are somewhat understood features of many cataclysmic high-energy systems. The specific details are gory, long, and for the most part boring. Other astrophysical objects that exhibit such behavior are quasars (a bit of a mystery there for the way we see such jets), pulsars, and active galactic nuclei.

[Nishroch_Order]

details are gory, long, and for the most part boring

Try me.

[JS_Princeton]

Well, okay.

For Gamma-ray bursts, you can start out with the Monthly Notice of the Royal Astronomical Society paper by Stephan Rosswog and Enrico Ramirez-Ruiz entitled: Jets, winds and bursts from coalescing neutron stars (Volume 336, Issue 1, pp. L7-L11). With the following abstract:

Recent high-resolution calculations of neutron star coalescences are used to investigate whether annihilation can provide sufficient energy to power gamma-ray bursts, especially those belonging to the short duration category. Late time slices of the simulations, where the neutrino emission has reached a maximum, stationary level are analysed to address this question. We find that annihilation can provide the necessary driving stresses that lead to relativistic jet expansion. Maximum Lorentz factors of the order of 15 are found along the binary rotation axis, but larger values are expected to arise from higher numerical resolution. Yet the accompanying neutrino-driven wind must be absent from the axis when the burst occurs or prohibitive baryon loading may occur.

We argue that even under the most favourable conditions, annihilation is unlikely to power a burst in excess of ~1048 erg. Unless the emission is beamed into less than one per cent of the solid angle, which we argue is improbable if it is collimated by gas pressure, this may fail to satisfy the apparent isotropic energies inferred at cosmological distances. This mechanism may none the less drive a precursor fireball, thereby evacuating a cavity into which a later magnetically driven jet could expand. A large range of time delays between the merger and the black hole formation are to be expected. If the magnetically driven jet occurs after the black hole has formed, a time span as long as weeks could pass between the neutrino powered precursor and the magnetically driven GRB.

You’re basically looking at a number of factors including angular momentum of the parent objects, the energy scales, gas/radiation pressures and gradients, and magnetic fields. All of these are possible origins of collimated jets. In pulsars, for example, the large magnetic field is considered the origin or the attenuation while in most black hole accretion disks with jets modelling, the physical angular momentum is the driving condition. In GRBs (gamma ray bursters) we are looking at any number of possible theoretical models for collimation.