HOLISMOKES -- III. Achromatic Phase of Strongly Lensed Type Ia Supernovae

TL;DR

This study investigates the achromatic phase in strongly lensed Type Ia supernovae, finding it lasts around three weeks on average and can be used for more accurate time-delay measurements in cosmology.

Contribution

The paper provides a detailed analysis of the achromatic phase duration across different models and microlensing configurations, highlighting its potential for improved time-delay measurements.

Findings

Achromatic phase lasts around three weeks on average.

Longer achromatic phases occur with smoother microlensing maps and lower macro-magnifications.

Color curves with features like extreme points are promising for time-delay measurements.

Abstract

To use strongly lensed Type Ia supernovae (LSNe Ia) for cosmology, a time-delay measurement between the multiple supernova (SN) images is necessary. The sharp rise and decline of SN Ia light curves make them promising for measuring time delays, but microlensing can distort these light curves and therefore add large uncertainties to the measurements. An alternative approach is to use color curves where uncertainties due to microlensing are significantly reduced for a certain period of time known as the achromatic phase. In this work, we investigate in detail the achromatic phase, testing four different SN Ia models with various microlensing configurations. We find on average an achromatic phase of around three rest-frame weeks or longer for most color curves but the spread in the duration of the achromatic phase (due to different microlensing maps and filter combinations) is quite large and an achromatic phase of just a few days is also possible. Furthermore, the achromatic phase is longer for smoother microlensing maps, lower macro-magnifications and larger mean Einstein radii of microlenses. From our investigations, we do not find a strong dependency on the model or on asymmetries in the SN ejecta. Further, we find that three independent LSST color curves exhibit features such as extreme points or turning points within the achromatic phase, which make them promising for time-delay measurements. These curves contain combinations of rest-frame bands $u$, $g$, $r$, and $i$ and to observe them for typical LSN Ia redshifts, it would be ideal to cover (observer-frame) filters $r$, $i$, $z$, $y$, $J$, and $H$.