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

## Abstract

Supernovae connected with long-duration γ-ray bursts1,2,3 (GRBs) are hyper-energetic explosions resulting from the collapse of very massive stars (40 M, where M is the mass of the Sun) stripped of their outer hydrogen and helium envelopes4,5,6,7. A very massive progenitor, collapsing to a black hole, was thought to be a requirement for the launch of a GRB8. 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 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 softness10 of GRB 060218 (an X-ray flash), and the weakness of the radio flux of the supernova11. 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 star12 for the less massive stars.

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## Acknowledgements

We thank S. Kulkarni, C. Fryer, T. Janka, W. Hillebrandt and C. Kouveliotou for many discussions. This work was supported in part by the European Union, by the JSPS and MEXT in Japan, and by the USA NSF.

## Author information

Correspondence to Paolo A. Mazzali or Jinsong Deng.

## Ethics declarations

### Competing interests

Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.

## Supplementary information

### Supplementary Figure 1

The spectra of 3 broad-lined SNe Ic a few days before maximum brightness. This figure shows a comparison of the spectrum of SN2006aj and those of two other Type Ic supernovae: SN1998bw, which produced a GRB (980425) and SN2002ap, which was not accompanied by a GRB. (PDF 38 kb)

### Supplementary Figure Legend

This file contains text to accompany the above Supplementary Figures. (PDF 40 kb)

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https://doi.org/10.1038/nature05081

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