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

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 (UBVRI) light curves decay in a nearly linear fashion with decay time scales of 1.43 +/- 0.14, 1.43 +/- 0.06, 1.48 +/- 0.06, 1.45 +/- 0.07, and 1.03 +/- 0.07 magnitudes ( per hundred days) in the U, B, V, R, and I bands, respectively. In contrast, in the near-infrared (JHKs) bands the time evolution of the flux appears to be nearly constant at these epochs. We measure decline rates ( per hundred days) of 0.19 +/- 0.10 and 0.17 +/- 0.11 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.