2015ApJ...806..251J

We use X-ray and optical microlensing measurements to study the shape of the dark matter density profile in the lens galaxies and the size of the (soft) X-ray emission region. We show that single epoch X-ray microlensing is sensitive to the source size. Our results, in good agreement with previous estimates, show that the size of the X-ray emission region scales roughly linearly with the black hole mass, with a half-light radius of R_1/2 ~= (24 ± 14)r_g where r_g = GM_BH/c². This corresponds to a size of log(R_1/2/cm) = 15.6_–0.3^+0.3 or ∼1 lt-day for a black hole mass of M_BH = 10⁹M_☉. We simultaneously estimated the fraction of the local surface mass density in stars, finding that the stellar mass fraction is α = 0.20±0.05 at an average radius of e 1.9_e, where R_e is the effective radius of the lens. This stellar mass fraction is insensitive to the X-ray source size and in excellent agreement with our earlier results based on optical data. By combining X-ray and optical microlensing data, we can divide this larger sample into two radial bins. We find that the surface mass density in the form of stars is α = 0.31±0.15 and α = 0.13±0.05 at (1.3 ± 0.3)R_e and xxi(2.3 ± 0.3)R_e, respectively, in good agreement with expectations and some previous results.