GRBs are sorted into 2 groups by their duration. Bursts with durations
(T90) longer than 2 seconds are called long GRBs; bursts with
durations shorter than 2 seconds are called short GRBs. These two
groups show very different features and they are believed to have
different origins.
Short bursts are believe to be mergers of compact objects; either a
Neutron star - Neutron star merger or a Neutron star - Blackhole
merger. It's hard to determine their distance, but they can happen in
our galaxy.
Long GRBs are believed to have a cosmological origin. People have 3
ways to measure the distance of long bursts: 1. Localize the burst by
its optical afterglow, find the host galaxy and thus the redshift of
the host galaxy. 2. By spectral feature (rare) 3. By the correlation
between the total energy released and the peak in the spectrum. The
measured redshifts (by first 2 ways) range from 0.8 to 6. They are all
located in cosmological distance.
Hypernova is one of many explanations for the long GRBs. Woosley first
suggested this idea in 1993. You may know that massive stars (with 3
to 6 solar masses typically) will experience core collapse and result
in a Type 1b supernova explosion (SNE). However, a star heavier than
10 solar masses has a bigger iron core, which makes it more difficult
to explode. Therefore, it will experience a "failed supernova" and
directly collapse into a blackhole. The new forming blackhole
continues to accrete surrounding materials left from the progenitor
(outer layers) with a high accretion rate, which will generate a so
called "relativistic fireball" that required to produce long GRBs. But
since GRBs are 100 times more energetic than supernova, in 1998
Paczynski think it is not proper to call it a "failed supernova".
Instead he renamed it as a "hypernova". So the difference between a
Type 1b supernova and a hypernova is that Type 1b supernove produces a
neutron star, while a hypernova produces a black hole and causes a
GRB.
Now you may understand why all observed long bursts are not in our own
galaxy: Long GRBs require massive progenitors, which have shorter life
time. Our galaxy is too old, all these massive stars have already been
evolved. But long bursts did happen long time ago in our galaxy. Only
when we look back in time (look at deeper universe, far galaxies), we
can see long GRBs happening there. |
