Optical Observations of the Rapidly Expanding Type Ia Supernova 2007gi

We present optical photometry and spectra for the Type Ia supernova (SN Ia) 2007 gi in the nearby galaxy NGC 4036. SN 2007 gi is characterized by extremely high-velocity (HV) features of the intermediate-mass elements (Si, Ca, and S), with expansion velocities (v _exp) approaching ∼15,500 kms ^-1 near-maximum brightness (compared to ∼10,600 kms ^-1 for SNe Ia with normal v _exp). SN 2007gi reached a B-band peak magnitude of 13.25 ±0.04 mag with a decline rate of Δm15 (B) (true) = 1.33 ±0.09 mag. The B-band light curve of SN 2007 gi demonstrated an interesting two-stage evolution during the nebular phase, with a decay rate of 1.16 ± 0.05 mag (100 days) ^-1 during t = 60-90 days and 1.61 ± 0.04 mag (100 days) ^-1 thereafter. Such 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 those with normal v _exp.