Optical Observations of the Rapidly Expanding Type Ia Supernova 2007gi

TL;DR

This paper reports optical observations of the rapidly expanding Type Ia supernova 2007gi, highlighting its high-velocity features, unique light curve evolution, and possible interaction with circumstellar material, contributing to understanding supernova diversity.

Contribution

It provides detailed optical photometry and spectra of SN 2007gi, revealing its high expansion velocities and two-stage nebular phase decay, suggesting ejecta-CSM interaction as a key factor.

Findings

SN 2007gi has expansion velocities of ~15,500 km/s near maximum.

The light curve shows a two-stage decay in the nebular phase.

High-velocity SNe Ia may have faster rise times to maximum.

Abstract

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 ($v_{\rm exp}$) 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 $v_{\rm exp}$.