A&A 419, L21-L25 (2004)
DOI: 10.1051/0004-6361:20040133

Letter

The supernova 2003lw associated with X-ray flash 031203

B. Thomsen¹, J. Hjorth², D. Watson², J. Gorosabel³, J. P. U. Fynbo^{1, 2}, B. L. Jensen², M. I. Andersen⁴, T. H. Dall⁵, J. R. Rasmussen¹, H. Bruntt¹, E. Laurikainen⁶, T. Augusteijn⁷, T. Pursimo⁷, L. Germany⁵, P. Jakobsson² and K. Pedersen<sup>2

1</sup>  Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, 8000 Århus C, Denmark
²  Niels Bohr Institute, Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
³  Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado de Correos, 3004, 18080 Granada, Spain
⁴  Astrophysikalisches Institut Potsdam, 14482 Potsdam, Germany
⁵  European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
⁶  Department of Physical Sciences, University of Oulu, Box 3000, 90014 Oulu, Finland
⁷  Nordic Optical Telescope, Apartado 474, 38700 St. Cruz de La Palma, Canary Islands, Spain

(Received 19 March 2004 / Accepted 4 April 2004 )

Abstract
The X-Ray Flash (XRF), 031203 with a host galaxy at z=0.1055, is, apart from GRB 980425, the closest -Ray Burst (GRB) or XRF known to date. We have monitored its host galaxy from 1-100 days after the burst. In spite of the high extinction to the source and the bright host, a significant increase and subsequent decrease has been detected in the apparent brightness of the host, peaking between 10 and 33 days after the GRB. The only convincing explanation is a supernova (SN) associated with the XRF, SN2003lw. This is the earliest time at which a SN signal is clearly discernible in a GRB/XRF (apart from SN1998bw). SN2003lw is extremely luminous with a broad peak and can be approximately represented by the lightcurve of SN1998bw brightened by ~ 0.55 mag, implying a hypernova, as observed in most GRB-SNe. The XRF-SN association firmly links XRFs with the deaths of massive stars and further strengthens their connection with GRBs. The fact that SNe are also associated with XRFs implies that Swift may detect a significant population of intermediate redshift SNe very soon after the SN explosions, a sample ideally suited for detailed studies of early SN physics.

Key words: gamma rays: bursts -- supernovae: general

Offprint request: B. Thomsen, bt@phys.au.dk

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