Long-duration γ-ray bursts (GRBs) are associated with type Ic supernovae1 that are more luminous than average2,3,4,5 and that eject material at very high velocities. Less-luminous supernovae were not hitherto known to be associated with GRBs, and therefore GRB–supernovae were thought to be rare events6. Whether X-ray flashes—analogues of GRBs, but with lower luminosities and fewer γ-rays—can also be associated with supernovae, and whether they are intrinsically ‘weak’ events or typical GRBs viewed off the axis of the burst7, is unclear. Here we report the optical discovery and follow-up observations of the type Ic supernova SN 2006aj associated with X-ray flash XRF 060218. Supernova 2006aj is intrinsically less luminous than the GRB–supernovae, but more luminous than many supernovae not accompanied by a GRB. The ejecta velocities derived from our spectra are intermediate between these two groups, which is consistent with the weakness of both the GRB output8 and the supernova radio flux9. Our data, combined with radio and X-ray observations8,9,10, suggest that XRF 060218 is an intrinsically weak and soft event, rather than a classical GRB observed off-axis. This extends the GRB–supernova connection to X-ray flashes and fainter supernovae, implying a common origin. Events such as XRF 060218 are probably more numerous than GRB–supernovae.
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This work is based on data collected by the GRACE consortium with ESO Paranal telescopes. The ESO staff astronomers at Paranal are acknowledged for their professional assistance. We are grateful to S. R. Kulkarni, M. Modjaz, A. Rau, and S. Savaglio for helpful interactions and to R. Wilman for allowing us to implement our Target-of-Opportunity programme with the VLT during his scheduled observing time. We thank S. Barthelmy for providing information about the Swift/BAT performance. This work has benefited from collaboration within the EU FP5 Research Training Network ‘Gamma-Ray Bursts: an Enigma and a Tool’. IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc, under contract to the National Science Foundation (NSF). A.V.F.’s group at the University of California, Berkeley, is supported by the NSF and by the TABASGO Foundation. Author Contributions E. Pian, N.M., P.F., S.K., E. Palazzi, A.V.F., R.J.F., W.L., F.P., P.M.V., E.W.G., C.D., O.H., D.S.W., D.B., L.W., S.E. and C.L. organized the observations and were responsible for data acquisition, reduction and analysis. P.A.M., E.R.-R., S.E.W., J.D., K.N., D.N.S. and K.M. contributed to the interpretation and discussion of the data. J.P.U.F., D.A.K., J.H., J.S., A.L., P.O'B., L.A., E.C., A.J.C.-T., F.F., A.S.F., J.G., K.K., P.M., L.N. and E.R. provided expertise on specific aspects of the data presentation and discussion. E. Pian, P.A.M., E.R.-R., S.E.W., C.K., K.N., N.R.T., R.A.M.J.W., E.C. and R.S. wrote the manuscript.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Pian, E., Mazzali, P., Masetti, N. et al. An optical supernova associated with the X-ray flash XRF 060218. Nature 442, 1011–1013 (2006). https://doi.org/10.1038/nature05082
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