1 Introduction

Supernova (SN) 1998bw was discovered in April 1998 in a spiral arm of the face-on SB galaxy ESO 184-G82 at a redshift of z=0.0085 (Galama et al. 1998a). This exceptional type Ic supernova was born famous because of its positional and temporal coincidence with the gamma-ray burst GRB 980425 (Galama et al. 1998b). This association was further supported by the very early onset of the bright radio luminosity (Kulkarni et al. 1998). Also in the optical, the supernova was unusually luminous and showed very high expansion velocities (Patat et al. 2001).

The earliest photometric evolution of SN 1998bw was presented in Galama et al. (1998b). McKenzie & Schaefer (1999) monitored the early exponential decay, and the late phases were added by Sollerman et al. (2000). Patat et al. (2001) combined these data to make a rough bolometric light curve, which has been the basis of several model investigations (Nomoto et al. 2001; Nakamura et al. 2001a). The aim of this paper is to improve and extend this light curve to the final phases of SN 1998bw.

Modelling of the early light curve and spectra suggested an extremely energetic explosion ((2- $5)\times~10^{52}$ erg) of a massive star, composed mainly of carbon and oxygen (Iwamoto et al. 1998; Woosley et al. 1999). As this kinetic energy is more than ten times that of a canonical core-collapse supernova, the term hypernova was suggested for this event. The mass of ⁵⁶Ni needed to power the early light curve in these early models, 0.5-0.7 ${M}_\odot$, is much larger than the $\la$0.1 ${M}_\odot$typical for "normal'' core-collapse SNe (e.g., Sollerman 2002).

This large mass of ⁵⁶Ni was disputed by Höflich et al. (1999), who suggested that the early light curve could be reproduced by a normal SN Ic, given the right viewing angle and degree of asymmetry. The modelling of Sollerman et al. (2000) indicated, however, that at least 0.3 ${M}_\odot$ of ⁵⁶Ni was required to power the late emission of the supernova. This conclusion was later supported by Nakamura et al. (2001a), see also Mazzali et al. (2001).

In this paper we focus on the very late light curve of SN 1998bw. In Sect. 2 we present new VLT observations and the technique used to achieve the revised optical light curves. Here we also present some late near-IR photometry. In Sect. 3 we present the very late Hubble Space Telescope (HST) observations of SN 1998bw. The results are presented in Sect. 4. After a discussion in Sect. 5, we briefly summarize our conclusions in Sect. 6.