Supernova 1998bw – the final phases^*

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
² Stockholm Observatory, Department of Astronomy, SCFAB, 106 91 Stockholm, Sweden
³ Physics Department, University of Notre Dame, Notre Dame, IN, USA
⁴ Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, USA
⁵ Department of Astronomy, University of California, Berkeley, CA, USA
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, USA
⁷ Department of Astronomy, University of Texas, Austin, TX, USA

Corresponding author: J. Sollerman, jesper@astro.su.se

Received: 3 January 2002
Accepted: 28 February 2002

Abstract

The probable association with GRB 980425 immediately put SN 1998bw at the forefront of supernova research. Here, we present revised late-time BVRI light curves of the supernova, based on template images taken at the VLT. To follow the supernova to the very last observable phases we have used HST/STIS. Deep images taken in June and November 2000 are compared to images taken in August 2001. The identification of the supernova is firmly established. This allows us to measure the light curve to ~1000 days past explosion. The main features are a rapid decline up to more than 500 days after explosion, with no sign of complete positron trapping from the decay. Thereafter, the light curve flattens out significantly. One possible explanation is powering by more long lived radioactive isotopes, if they are abundantly formed in this energetic supernova.