A neutron-star-driven X-ray flash associated with supernova SN 2006aj

Supernovae connected with long-duration gamma-ray bursts(1-3) (GRBs) are hyper-energetic explosions resulting from the collapse of very massive stars (similar to 40M(.), where M-. is the mass of the Sun) stripped of their outer hydrogen and helium envelopes(4-7). A very massive progenitor, collapsing to a black hole, was thought to be a requirement for the launch of a GRB(8). Here we report the results of modelling the spectra and light curve of SN 2006aj ( ref. 9), which demonstrate that the supernova had a much smaller explosion energy and ejected much less mass than the other GRB - supernovae, suggesting that it was produced by a star whose initial mass was only similar to 20 M-.. 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(10) of GRB 060218 ( an X-ray flash), and the weakness of the radio flux of the supernova(11). 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' ( ref. 8) for the more massive stars collapsing to a black hole, and magnetic activity of the nascent neutron star(12) for the less massive stars.