A Study of Gravitational Lens Chromaticity using Ground-based Narrow Band Photometry

We present observations of wavelength-dependent flux ratios for four gravitational lens systems (SDSS~J1650+4251, HE~0435$-$1223, FBQ 0951+2635, and Q~0142$-$100) obtained with the Nordic Optical Telescope. The use of narrowband photometry, as well as the excellent seeing conditions during the observations, allows us to study their chromatic behavior. For SDSS~J1650+4251, we determine the extinction curve of the dust in the $z_L=0.58$ lens galaxy and find that the 2175 Å\ feature is absent. In the case of HE~0435$-$1223, we clearly detect chromatic microlensing. This allows us to estimate the wavelength-dependent size of the accretion disk. We find an R-band disk size of $r^{R}_s=13\pm5$ light days for a linear prior on $r^{R}_s$ and $r^{R}_s=7\pm6$ light days for a logarithmic prior. For a power-law size-wavelength scaling of $r_s\proptoλ^{p}$, we were able to constrain the value of the exponent to $p=1.3\pm0.3$ for both $r^{R}_s$ priors, which is in agreement with the temperature profiles of simple thin disk models ($p=4/3$).