We present optical spectroscopic and photometric observations of Type Ia supernova (SN) 2006X from --10 to +91 days after the $B$-band maximum. This SN exhibits one of the highest expansion velocity ever published for SNe Ia. At premaximum phases, the spectra show strong and broad features of intermediate-mass elements such as Si, S, Ca, and Mg, while the O{ \sc i}$\lambda$7773 line is weak. The extremely high velocities of Si{ \sc ii} and S{ \sc ii} lines and the weak O{ \sc i} line suggest that an intense nucleosynthesis might take place in the outer layers, favoring a delayed detonation model. Interestingly, Si{ \sc ii}$\lambda$5972 feature is quite shallow, resulting in an unusually low depth ratio of Si{ \sc ii}$\lambda$5972 to $\lambda$6355, $\cal R$(Si{ \sc ii}). The low $\cal R$(Si{ \sc ii}) is usually interpreted as a high photospheric temperature. However, the weak Si{ \sc iii}$\lambda$4560 line suggests a low temperature, in contradiction to the low $\cal R$(Si{ \sc ii}). This could imply that the Si{ \sc ii}$\lambda$5972 line might be contaminated by underlying emission. We propose that $\cal R$(Si{ \sc ii}) may not be a good temperature indicator for rapidly expanding SNe Ia at premaximum phases.
Categories: astro-ph.HE astro-ph.SR