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Relativistic ejecta from X-ray flash XRF 060218 and the rate of cosmic explosions

Abstract

Over the past decade, long-duration γ-ray bursts (GRBs)—including the subclass of X-ray flashes (XRFs)—have been revealed1,2,3 to be a rare variety of type Ibc supernova. Although all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary type Ibc supernovae by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just a supernova, is still unknown. Here we report radio and X-ray observations of XRF 060218 (associated4 with supernova SN 2006aj), the second-nearest5,6 GRB identified until now. We show that this event is a hundred times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary type Ibc supernovae by the presence of 1048 erg coupled to mildly relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) that produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs or XRFs and ordinary supernovae, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218.

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Acknowledgements

GRB research at Caltech is supported in part by funds from NSF and NASA. We are, as always, indebted to S. Barthelmy and the GCN. The VLA is operated by the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. A.M.S. and S.B.C. are supported by NASA Graduate Research Fellowships. E.B. and A.G.-Y. acknowledge support by NASA through a Hubble Fellowship grant. D.N.B. and J.A.N. acknowledge support by NASA.

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Correspondence to A. M. Soderberg.

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

Supplementary information

Supplementary Notes

This file contains two Supplementary Discussion sections: I. Estimates for the Rate of Sub-energetic GRBs; and II. Estimates for the rate of Type Ibc supernovae like GRB980425 and XRF060218. (PDF 40 kb)

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Further reading

Figure 1: Radio and X-ray light-curves of XRF 060218.
Figure 2: Radio observations for a large sample of local type Ibc supernovae.
Figure 3: Energy as a function of velocity for GRBs, XRFs, and type Ibc supernovae.

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