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Photometric and spectroscopic observations of type Ia supernova (SN) 2017fgcwhich cover the period from $-$12 to +137 days since the $B$-band maximum arepresented. SN 2017fgc is a photometrically normal SN Ia with the luminositydecline rate, $ \Delta m_{15} (B)_{true} $= 1.10 $ \pm $ 0.10 mag.Spectroscopically, it belongs to the High Velocity (HV) SNe Ia group, with theSi II $\lambda$6355 velocity near the $B$-band maximum estimated to be 15,200 $\pm $ 480 km $s^{-1}$. At the epochs around the near-infrared secondary peak,the $R$ and $I$ bands show an excess of $\sim$0.2 mag level compared to thelight curves of the normal velocity (NV) SNe Ia. Further inspection of thesamples of HV and NV SNe Ia indicates that the excess is a generic featureamong HV SNe Ia, different from NV SNe Ia. There is also a hint that the excessis seen in the V band, both in SN 2017fgc and other HV SNe Ia, which behaveslike a less prominent shoulder in the light curve. The excess is not obvious inthe B band (and unknown in the U band), and the color is consistent with thefiducial SN color. This might indicate the excess is attributed to thebolometric luminosity, not in the color. This excess is less likely caused byexternal effects, like an echo or change in reddening but could be due to anionization effect, which reflects an intrinsic, either distinct or continuous,difference in the ejecta properties between HV and NV SNe Ia.

Light curve properties of SN 2017fgc and HV SNe Ia