1. INAF—Osservatorio Astronomico di Brera, via E. Bianchi 46, I-23807 Merate (LC), Italy
2. INAF—Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, via U. La Malfa 153, I-90146 Palermo, Italy
3. UCL Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
4. Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, Pennsylvania 16802, USA
5. Università degli studi di Milano Bicocca, piazza delle Scienze 3, I-20126 Milano, Italy
6. NASA—Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
7. National Research Council, 2101 Constitution Avenue NW, Washington DC 20418, USA
8. INAF—Osservatorio Astrofisico di Arcetri, largo E. Fermi 5, I-50125 Firenze, Italy
9. Kavli Institute for Theoretical Physics, UC Santa Barbara, California 93106, USA
10. International School for Advanced Studies (SISSA-ISAS), via Beirut 2-4, I-34014 Trieste, Italy
11. Department of Physics, Pennsylvania State University, University Park, Pennsylvania 16802, USA
12. Physics Faculty, Weizmann Institute, Rehovot 76100, Israel
13. Department of Physics, University of Nevada, Box 454002, Las Vegas, Nevada 89154-4002, USA
14. Department of Astronomy, Nanjing University, Nanjing, 210093, China
15. PPARC, Polaris House, North Star Avenue, Swindon SN2 1SZ, UK
16. Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
17. Department of Physics and Astronomy, Sonoma State University, Rohnert Park, California 94928-3609, USA
18. ASI Science Data Center, via G. Galilei, I-00044 Frascati (Roma), Italy
19. Universities Space Research Association, 10211 Wincopin Circle Suite 500, Columbia, Maryland 21044-3431, USA
20. Los Alamos National Laboratory, PO Box 1663, Los Alamos, New Mexico 87545, USA

Correspondence to: S. Campana¹ Correspondence and requests for materials should be addressed to S.C. (Email: sergio.campana@brera.inaf.it).

Abstract

Although the link between long -ray bursts (GRBs) and supernovae has been established^{1, 2, 3, 4}, hitherto there have been no observations of the beginning of a supernova explosion and its intimate link to a GRB. In particular, we do not know how the jet that defines a -ray burst emerges from the star's surface, nor how a GRB progenitor explodes. Here we report observations of the relatively nearby GRB 060218 (ref. 5) and its connection to supernova SN 2006aj (ref. 6). In addition to the classical non-thermal emission, GRB 060218 shows a thermal component in its X-ray spectrum, which cools and shifts into the optical/ultraviolet band as time passes. We interpret these features as arising from the break-out of a shock wave driven by a mildly relativistic shell into the dense wind surrounding the progenitor⁷. We have caught a supernova in the act of exploding, directly observing the shock break-out, which indicates that the GRB progenitor was a Wolf–Rayet star.

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