Image Flux Ratios of Gravitationally Lensed HS 0810+2554 with High Resolution Infrared Imaging

TL;DR

This study uses high-resolution infrared imaging to analyze flux ratios in a gravitationally lensed quasar, revealing microlensing effects and substructure influences on observed flux variations across wavelengths.

Contribution

It provides the first high-resolution infrared flux ratio measurements of HS 0810+2554, constrains microlensing and millilensing effects, and introduces a simple model to interpret flux ratio variations.

Findings

Flux ratio of images A/B varies with wavelength, indicating microlensing.

Longer wavelengths show excess flux from larger regions unaffected by microlensing.

The product of microlensing and millilensing factors is tightly constrained to 1.79.

Abstract

We report near simultaneous imaging using LMIRCam on the LBTI of the quadruply imaged lensed quasar HS 0810+2554 at wavelengths of 2.16, 3.7 and $4.78~\mu$m with a Full Width Half Max (FWHM) spatial resolution of $0^{\prime\prime}\!\!.13$, $0^{\prime\prime}\!\!.12$ and $0^{\prime\prime}\!\!.15$ respectively, comparable to HST optical imaging. In the $\rm{z} = 1.5$ rest frame of the quasar, the observed wavelengths correspond to 0.86, 1.48, and $1.91~\mu$m respectively. The two brightest images in the quad, A and B, are clearly resolved from each other with a separation of $0.187^{\prime\prime}$. The flux ratio of these two images (A/B) trends from 1.79 to 1.23 from 2.16 to $4.78~\mu$m. The trend in flux ratio is consistent with the $2.16~\mu$m flux originating from a small sized accretion disk in the quasar that experiences only microlensing. The excess flux above the contribution from the accretion disk at the two longer wavelengths originates from a larger sized region that experiences no microlensing. A simple model employing multiplicative factors for image B due to stellar microlensing $(m)$ and sub-structure millilensing $(M)$ is presented. The result is tightly constrained to the product $m\times M=1.79$. Given the observational errors, the 60\% probability contour for this product stretches from $m= 2.6$, $M = 0.69$ to $m= 1.79$, $M = 1.0$, where the later is consistent with microlensing only.