Letter to the Editor

Late-time emission of type Ia supernovae: optical and near-infrared observations of SN 2001el 
 ^*,**

¹ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark e-mail: [max,jesper]@dark-cosmology.dk
² Stockholm Observatory, AlbaNova, Department of Astronomy, 106 91 Stockholm, Sweden

Received: 21 December 2006
Accepted: 6 May 2007

Abstract

Aims.To elucidate the nature of the late-phase emission of the normal type Ia supernova SN 2001el.

Methods.We present optical and near-infrared light curves of SN 2001el from 310 to 445 days past maximum light, obtained with the Very Large Telescope.

Results.The late-time optical () light curves decay in a nearly linear fashion with decay time scales of , , , , and magnitudes (per hundred days) in the U, B, V, R, and I bands, respectively. In contrast, in the near-infrared () bands the time evolution of the flux appears to be nearly constant at these epochs. We measure decline rates (per hundred days) of and magnitudes in the J and H bands, respectively. We construct a UVOIR light curve, and find that the late-time luminosity has a decay time scale nearly consistent with complete depostion of positron kinetic energy.

Conclusions.The late-time light curves of the normal type Ia SN 2001el demonstrate the increased importance of the near-infrared contribution. This was previously observed in the peculiar SN 2000cx, and the results for SN 2001el thus ensure that the conclusions previously based on a single peculiar event are applicable to normal type Ia supernovae. The measured late-time UVOIR decline rate suggests that a majority of the positrons are trapped within the ejecta. This result does not favor the prediction of a weak and/or radially combed magnetic field configuration.