Early Spectral Evolution of the Rapidly Expanding Type Ia Supernova 2006X

We present optical spectroscopic and photometric observations of Type Ia supernova (SN Ia) 2006X from 10d before the B-band maximum to 91d after. This object exhibits one of the highest expansion velocities ever published for SNe Ia. At its premaximum phases, the spectra show strong and broad features of intermediate-mass elements, such as Si, S, Ca, and Mg, while the OI λ7773 line is weak. The extremely high velocities of the SiII and SII lines and the weak feature of the OI line suggest that an intense nucleosynthesis might take place in the outer layers, favoring a delayed detonation model. Interestingly, the SiII λ5972 feature is quite shallow, resulting in an unusually low depth ratio of SiII λ5972 to SiII λ6355, \cal{R} (SiII). The low \cal{R} (SiII) is usually interpreted as being a high photospheric temperature. However, the weak SiIII λ4560 line suggests a low temperature, in contradiction with an interpretation on the low \cal{R} (SiII). This could imply that the SiII λ5972 line might be contaminated by underlying emission. We propose that \cal{R} (SiII) may not be a good temperature indicator for a rapidly expanding SN Ia in the premaximum phases.