1. Caltech Optical Observatories 105-24,
2. Theoretical Astrophysics 130-33,
3. Space Radiation Laboratory 220-47, California Institute of Technology, Pasadena, California 91125, USA
4. Carnegie Observatories, 813 Santa Barbara Street, Pasadena, California 91101, USA
5. Department of Astronomy, Pennsylvania State University, University Park, Pennsylvania 16802, USA
6. National Radio Astronomy Observatory, PO Box 0, Socorro, New Mexico 87801, USA
7. Department of Astronomy, University of Virginia, PO Box 3818, Charlottesville, Virginia 22903, USA
8. Racah Institute of Physics, Hebrew University, Jerusalem 91904, Israel
9. Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822, USA
10. RSAA, ANU, Mt Stromlo Observatory, via Cotter Road, Weston Creek, Australian Capital Territory 2611, Australia
11. Mullard Radio Astronomy Observatory, Cavendish Laboratory, Cambridge CB3 0HE, UK
12. Pomona College Dept. of Physics & Astronomy, 610 North College Ave, Claremont, California 91711, USA
13. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

Correspondence to: A. M. Soderberg¹ Correspondence and requests for materials should be addressed to A.M.S. (Email: ams@astro.caltech.edu).

Abstract

Over the past decade, long-duration -ray bursts (GRBs)—including the subclass of X-ray flashes (XRFs)—have been revealed^{1, 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-nearest^{5, 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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