  Supernovae connected with long-duration gamma-ray bursts (GRBs) are
hyper-energetic explosion resulting from the collapse of very massive stars
(about 40Mo, where Mo is the mass of the Sun) stripped of their outher hydrogen
and helium envelopes. A very massive progenitor, collapsing to a black hole,
was thought to be a requirement for the launch of a GRB. Here we report the
results of modelling the spectra and light curve of SN 2006aj, which
demonstrate that the supernova had a much smaller explosion energy and ejected
much less mass than the other GRB-sueprnovae, suggesting that it was produced
by a star whose initial mass was only about 20Mo. A star of this mass is
expected to form a neutron star rather than a black hole when its core
collapses. The smaller explosion energy of SN 2006aj is matched by the weakness
and softness of GRB 060218 (an X-ray flash), and the weakness of the radio flux
of the supernova. Our results indicate that the supernova-GRB connection
extends to a much broader range of stellar masses than previously thought,
possibly involving different physical mechanisms: a `collapsar' for the more
massive stars collapsing to a black hole, and magnetic activity of the nascent
neutron star for the less massive stars.

Astrophysics