The late-time light curve of the type Ia supernova 2000cx

\n We have conducted a systematic and comprehensive monitoring programme\nof the type Ia supernova 2000cx at late phases using the VLT and\nHST. The VLT observations cover phases 360 to 480 days past maximum\nbrightness and include photometry in the BVRIJH bands, together with a \nsingle epoch in each of U and Ks. While the optical\nbands decay by about 1.4 mag per 100 days, we find that the\nnear-IR magnitudes stay virtually constant during the observed\nperiod. This means that the importance of the near-IR to the\nbolometric light curve increases with time. The finding is also in\nagreement with our detailed modeling of a type Ia supernova in the\nnebular phase. In these models, the increased importance of the\nnear-IR is a temperature effect. We note that this complicates\nlate-time studies where often only the V band is well monitored. In\nparticular, it is not correct to assume that any optical band follows\nthe bolometric light curve at these phases, and any conclusions based\non such assumptions, e.g., regarding positron-escape, \nmust be regarded as premature.\nA very simple model where all positrons are trapped\ncan reasonably well account for the \nobservations. The nickel\nmass deduced from the positron tail of this light curve is\nlower than found from the peak brightness, providing an estimate of the \nfraction of late-time emission that is \noutside of the observed wavelength range.\nOur detailed models show the signature of an infrared catastrophe at these \nepochs, which is not supported by the observations. \n\n\n