instead of the bipolar explosion
We don't know yet. This article outlines some of the current models though, so have a look.
It's an effect of the magnetic field of a rapidly rotating black hole forming. Look it up on Google for details
Son of a gun. I thought it WAS known to be an effect of a magnetic filed. Well, live and learn.
Also the reason that we think that gamma ray bursts are beamed is that if they weren't beamed then we are talking about some totally horrendous explosion that is more 1000 times powerful than anything we've seen. If it's beamed, then it's still a massive boom, but the level of energy is comparable to explosions that we know about (i.e. supernova).
One of the biggest mysteries about GRB's is the energy release rivals that of an anti-matter/matter explosion at their putative distances, which appears unlikely. Hence, the beamed idea ... IOW, what twofish said.
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