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 (vexp) approaching ∼15;500 kms � 1 near-maximum brightness (compared to ∼10;600 kms � 1 for SNe Ia with normal vexp). SN 2007gi reached a B-band peak magnitude of 13:25 � 0:04 mag with a decline rate of Δm15ðBÞðtrue Þ¼ 1:33 � 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 � 0:05 mag ð100 daysÞ � 1 during t ¼ 60-90 days and 1:61 � 0:04 mag ð100 daysÞ � 1 thereafter. Such behavior was also ob- served 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 vexp.