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

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 μm with a full width at half maximum spatial resolution of 0.″13, 0.″12, and 0.″15 respectively, comparable to Hubble Space Telescope optical imaging. In the z = 1.5 rest frame of the quasar, the observed wavelengths correspond to 0.86, 1.48, and 1.91 μm respectively. The two brightest images in the quad, A and B, are clearly resolved from each other with a separation of 0.″187. The flux ratio of these two images (A/B) trends from 1.79 to 1.23 at wavelengths from 2.16 to 4.78 μm. The trend in flux ratio is consistent with the 2.16 μm 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 substructure millilensing (M) is presented. The result is tightly constrained to the product m 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.