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We present optical photometry and spectra for the Type Ia supernova (SN Ia)2007gi in the nearby galaxy NGC 4036. SN 2007gi is characterized by extremelyhigh-velocity (HV) features of the intermediate-mass elements (Si, Ca, and S),with expansion velocities ($v_{\rm exp}$) approaching $\sim$15,500 km s$^{-1}$near maximum brightness (compared to $\sim$10,600 km s$^{-1}$ for SNe Ia withnormal $v_{\rm exp}$). SN 2007gi reached a $B$-band peak magnitude of13.25$\pm$0.04 mag with a decline rate of $\Delta m_{15}(B)$(true) =1.33$\pm$0.09 mag. The $B$-band light curve of SN 2007gi demonstrated aninteresting two-stage evolution during the nebular phase, with a decay rate of1.16$\pm$0.05 mag (100 days)$^{-1}$ during $t = 60$--90 days and 1.61$\pm0.04$mag (100 days)$^{-1}$ thereafter. Such a behavior was also observed in the HVSN Ia 2006X, and might be caused by the interaction between supernova ejectaand circumstellar material (CSM) around HV SNe Ia. Based on a sample of a dozenwell-observed $R$-band (or unfiltered) light curves of SNe Ia, we confirm thatthe HV events may have a faster rise time to maximum than the ones with normal$v_{\rm exp}$.

Optical Observations of the Rapidly Expanding Type Ia Supernova 2007gi