The Tsinghua University-National Astronomical Observatories of China (NAOC) Transient Survey (TNTS) is an automatic survey for a systematic exploration of optical transients (OTs), conducted with a 60/90 cm Schmidt telescope at Xinglong station of NA
OC. This survey repeatedly covers ~ 1000 square degrees of the north sky with a cadence of 3-4 days. With an exposure of 60 s, the survey reaches a limited unfiltered magnitude of about 19.5 mag. This enables us to discover supernovae at their relatively young stages. In this paper, we describe the overall performance of our survey during the first year and present some preliminary results.
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 ve
locities ($v_{rm exp}$) 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 $v_{rm exp}$.
