The Unique Type Ia Supernova 2000cx in NGC 524

We present extensive photometric and spectroscopic observations of the TypeIa supernova (SN Ia) 2000cx in the S0 galaxy NGC 524, which reveal it to bepeculiar. Photometrically, SN 2000cx is different from all known SNe Ia, andits light curves cannot be fit well by the fitting techniques currentlyavailable. There is an apparent asymmetry in the $B$-band peak, in which thepremaximum brightening is relatively fast (similar to that of the normal SN1994D), but the postmaximum decline is relatively slow (similar to that of theoverluminous SN 1991T). The color evolution of SN 2000cx is also peculiar: the$(B - V)_0$ color has a unique plateau phase and the $(V - R)_0$ and $(V -I)_0$ colors are very blue. Although the premaximum spectra of SN 2000cx are similar to those of SN1991T-like objects (with weak Si II lines), its overall spectral evolution isquite different. The Si II lines that emerged near maximum $B$-band brightnessstay strong in SN 2000cx until about three weeks past maximum. The change inthe excitation stages of iron-peak elements is slow. Both the iron-peak and theintermediate-mass elements are found to be moving at very high expansionvelocities in the ejecta of SN 2000cx. We discuss theoretical models for SN 2000cx. SN 2000cx may be an overluminousobject like SN 1991T, but with a larger yield of $^{56}$Ni and a higher kineticenergy in the ejecta. We also briefly discuss the implications of ourobservations for the luminosity vs. light-curve width relation.Comment: Accepted to be published in the Oct issue of PAS