HOLISMOKES -- V. Microlensing of type II supernovae and time-delay inference through spectroscopic phase retrieval

TL;DR

This paper explores how microlensing affects spectroscopic observations of strongly lensed type II supernovae, proposing a phase inference method based on spectral lines to accurately measure time delays for cosmology.

Contribution

It introduces a novel spectral phase retrieval technique for type II supernovae under microlensing, enabling precise time-delay measurements with uncertainties below 2 days.

Findings

Color curves are weakly affected by microlensing during the plateau phase.

Spectral absorption lines can accurately determine supernova phase within 2 days.

The phase inference method is promising for time-delay cosmography.

Abstract

We investigate strongly gravitationally lensed type II supernovae (LSNe II) for time-delay cosmography incorporating microlensing effects, which expands on previous microlensing studies of type Ia supernovae (SNe Ia). We use the radiative-transfer code ${\rm \small TARDIS}$ to recreate five spectra of the prototypical SN 1999em at different times within the plateau phase of the light curve. The microlensing-induced deformations of the spectra and light curves are calculated by placing the SN into magnification maps generated with the code ${\rm \small GERLUMPH}$. We study the impact of microlensing on the color curves and find that there is no strong influence on them during the investigated time interval of the plateau phase. The color curves are only weakly affected by microlensing due to the almost achromatic behavior of the intensity profiles. However, the lack of non-linear structure in the color curves makes time-delay measurements difficult given the possible presence of differential dust extinction. Therefore, we further investigate SN phase inference through spectral absorption lines under the influence of microlensing and Gaussian noise. As the spectral features shift to longer wavelengths with progressing time after explosion, the measured wavelength of a specific absorption line provides information on the epoch of the SN. The comparison between retrieved epochs of two observed lensing images then gives the time delay of the images. We find that the phase retrieval method using spectral features yields accurate delays with uncertainties $\small {\lesssim}$2 days, making it a promising approach.