Optical Observations of the Rapidly Expanding Type Ia Supernova 2007gi
  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 extremely
high-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 with
normal $v_{\rm exp}$). SN 2007gi reached a $B$-band peak magnitude of
13.25$\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 an
interesting two-stage evolution during the nebular phase, with a decay rate of
1.16$\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 HV
SN Ia 2006X, and might be caused by the interaction between supernova ejecta
and circumstellar material (CSM) around HV SNe Ia. Based on a sample of a dozen
well-observed $R$-band (or unfiltered) light curves of SNe Ia, we confirm that
the HV events may have a faster rise time to maximum than the ones with normal
$v_{\rm exp}$.
