  The X-Ray Flash (XRF), 031203 with a host galaxy at z=0.1055, is, apart from
GRB980425, the closest Gamma-Ray Burst (GRB) or XRF known to date. We 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.
