Optical and Infrared Photometry of the Unusual Type Ia Supernova 2000cx

We present optical and infrared photometry of the unusual Type Ia supernova 2000cx. With the data of Li et al. and Jha, this constitutes the largest data set ever assembled for a Type Ia SN, more than 600 points in UBVRIJHK. We confirm the finding of Li et al. regarding the unusually blue B−V colors as SN 2000cx entered the nebular phase. Its I‐band secondary hump was extremely weak given its B‐band decline rate. The V minus near‐infrared colors likewise do not match loci based on other slowly declining Type Ia SNe, although V−K is the least “abnormal.” In several ways, SN 2000cx resembles other slow decliners, given its B‐band decline rate [Δm15(B) = 0.93], the appearance of Fe iii lines and weakness of Si ii in its premaximum spectrum, the V−K colors, and postmaximum V−H colors. If the distance modulus derived from surface brightness fluctuations of the host galaxy is correct, we find that the rate of light increase prior to maximum, the characteristics of the bolometric light curve, and the implied absolute magnitude at maximum are all consistent with a subluminous object with Δm15(B) ≈ 1.6–1.7 having a higher than normal kinetic energy.