How do you tell the difference between a gamma-ray burst and a
star just from a picture of a nebula, in which it cannot flash
on and off here and there? GRBs have a spectra far more Red-
shifted than that of a star.. In fact, GRBs have such an unique
spectra that nothing matches with it. Its profile has one or two
high peaks of 1/2 seconds during which most of the energies will
be emitted.. An ordinary star would be bright in optical
wavelength... But, doesn't have any crazy peaks & hence won't be
extremely red-shifted (Otherwise, we won't be able to see them)
A GRB's redshift can be measured only with the afterglow when
the initial gamma ray flare would be over. Also, Crab nebula
doesn't host GRB phenomenon... It only flares up because of some
abnormal neutron star spin-down or magnetic disturbances. And
we're able to catch the ultra high energy gamma ray photons from
Crab only because it's so nearer to us than any GRB you can name
of... GRBs are always highly red-shifted extra-galactic
explosions.