H$\alpha$ Luminosity of ATLAS18qtd Does Not Plateau in the Nebular Phase

TL;DR

This study reports non-detections of H-alpha emission in a Type Ia supernova during the nebular phase, setting upper limits on luminosity and suggesting the emission does not plateau or increase at late times.

Contribution

The paper provides new observational constraints on H-alpha luminosity evolution in a supernova, challenging previous claims of emission plateauing or increasing during the nebular phase.

Findings

H-alpha luminosity decreased by at least a factor of 4 since last detection.

Upper limit on H-alpha luminosity is approximately 1.1×10^{36} erg/s.

H-alpha emission does not show evidence of plateauing or increasing in late nebular phase.

Abstract

We present new spectroscopic and photometric observations of ATLAS18qtd/SN 2018cqj, a fast-declining Type Ia supernova with variable H$\alpha$ emission in previously-published nebular phase spectra. ATLAS18qtd is undetected in both spectroscopic and photometric observations which occurred at $\sim 540~\rm d$ after maximum light and $\sim 230~\rm d$ after the last H$\alpha$ detection. With these new non-detections, we place an upper limit on the H$\alpha$ luminosity of $\lesssim 1.1\times 10^{36}~\rm{erg}~\rm s^{-1}$ indicating the H$\alpha$ flux decreased by a factor of $\gtrsim 4$ since the previous detection. This upper limit excludes H$\alpha$ emission that plateaus or increases since the previous detection but cannot confirm that the H$\alpha$ emission decay rate is equivalent to the supernova decay rate.