Light curve properties of SN 2017fgc and HV SNe Ia

TL;DR

This study presents detailed photometric and spectroscopic analysis of SN 2017fgc, a high velocity Type Ia supernova, revealing a consistent near-infrared excess compared to normal velocity SNe Ia, likely due to intrinsic ejecta differences.

Contribution

It is the first comprehensive analysis of SN 2017fgc showing the near-infrared excess as a common feature among HV SNe Ia, suggesting intrinsic ejecta property differences.

Findings

SN 2017fgc is a photometrically normal SN Ia with high velocity features.

HV SNe Ia show a near-infrared excess around the secondary peak.

The excess is likely due to intrinsic ejecta properties, not external effects.

Abstract

Photometric and spectroscopic observations of type Ia supernova (SN) 2017fgc which cover the period from $-$12 to +137 days since the $B$-band maximum are presented. SN 2017fgc is a photometrically normal SN Ia with the luminosity decline rate, $ \Delta m_{15} (B)_{true} $= 1.10 $ \pm $ 0.10 mag. Spectroscopically, it belongs to the High Velocity (HV) SNe Ia group, with the Si 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 the light curves of the normal velocity (NV) SNe Ia. Further inspection of the samples of HV and NV SNe Ia indicates that the excess is a generic feature among HV SNe Ia, different from NV SNe Ia. There is also a hint that the excess is seen in the V band, both in SN 2017fgc and other HV SNe Ia, which behaves like a less prominent shoulder in the light curve. The excess is not obvious in the B band (and unknown in the U band), and the color is consistent with the fiducial SN color. This might indicate the excess is attributed to the bolometric luminosity, not in the color. This excess is less likely caused by external effects, like an echo or change in reddening but could be due to an ionization effect, which reflects an intrinsic, either distinct or continuous, difference in the ejecta properties between HV and NV SNe Ia.