Strange supernova is ‘missing link’ in gamma-ray burst connection

Astronomers find that ‘central engines’ in supernova explosions can come in different strengths, and include those that produce powerful blasts of gamma rays, and weaker versions that produce no such bursts.

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I had trouble understanding that news report, wasn’t sure about orientation and how it tied in with the different types of core collapse supernovae, so turned to the original article:
http://arxiv.org/pdf/1408.1606.pdf

The spectral features of core-collapse supernovae (SN)
provide a basis of classification that reflects properties
of their progenitor stars and explosion dynamics. By standard definition, Type Ib
supernovae lack conspicuous absorptions attributable
to hydrogen, and Type Ic supernovae lack conspicuous
absorptions attributable to hydrogen and helium. These two subgroups, however,
may have many deviant cases, and a possible
continuum between them is sometimes acknowledged
by using the designation Type Ibc (hereafter SN Ibc).
SN Ibc are thought to originate from stars that
have been largely stripped of their outer envelopes.

Broad-lined Type Ic (SN Ic-bl) supernovae are a subset
of SN Ibc that show exceptionally high expansion
velocities in their bulk ejecta reaching ∼ 0.1 c.
Generally, SN Ic-bl are associated with large kinetic
energies (several 10^52 erg) approximately 10 times
those of normal SN Ibc. The handful of these known
are diverse and can vary considerably in these explosion
properties.

It is an open question as to why some SN Ic-bl are associated
with Gamma Ray Bursters (GRBs) and others are not.
The gamma ray bursts are highly directional and gamma rays
can only be seen along the axis. But radio observations
seem to rule out the possibility that all SN Ic-bl without
a gamma ray detection are off-axis GRBs.
Thus, certain properties of the progenitor systems
and explosion dynamics of these SN must dictate
why some explosions are not sufficient to generate
a GRB.

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OK, so now that’s clearer.