An optical supernova associated with the X-ray flash XRF 060218.

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.