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We present extensive optical (UBVRI), near-infrared (JK) light curves andoptical spectroscopy of the Type Ia supernova (SN) 2006X in the nearby galaxyNGC 4321 (M100). Our observations suggest that either SN 2006X has anintrinsically peculiar color evolution, or it is highly reddened [E(B -V)_{host} = 1.42+/-0.04 mag] with R_V = 1.48+/-0.06, much lower than thecanonical value of 3.1 for the average Galactic dust. SN 2006X also has one ofthe highest expansion velocities ever published for a SN Ia. Compared with theother SNe Ia we analyzed, SN 2006X has a broader light curve in the U band, amore prominent bump/shoulder feature in the V and R bands, a more pronouncedsecondary maximum in the I and near-infrared bands, and a remarkably smallerlate-time decline rate in the B band. The B - V color evolution shows anobvious deviation from the Lira-Phillips relation at 1 to 3 months aftermaximum brightness. At early times, optical spectra of SN 2006X displayedstrong, high-velocity features of both intermediate-mass elements (Si, Ca, andS) and iron-peak elements, while at late times they showed a relatively bluecontinuum, consistent with the blue U-B and B-V colors at similar epochs. Alight echo and/or the interaction of the SN ejecta and its circumstellarmaterial may provide a plausible explanation for its late-time photometric andspectroscopic behavior. Using the Cepheid distance of M100, we derive a Hubbleconstant of 72.7+/-8.2 km s^{-1} Mpc^{-1}(statistical) from the normalizeddereddened luminosity of SN 2006X. We briefly discuss whether abnormal dust isa universal signature for all SNe Ia, and whether the most rapidly expandingobjects form a subclass with distinct photometric and spectroscopic properties.

Optical and Near-Infrared Observations of the Highly Reddened, Rapidly Expanding Type Ia Supernova 2006X in M100