Finding Gravitational Lenses With X-rays

There are $\sim 1$, 0.1 and 0.01 gravitationally lensed X-ray sources per square degree with soft X-ray fluxes exceeding ${10}^{-15},{10}^{-14}$ and ${10}^{-13}ergs\mathrm{/}sc{m}^{-2}$ respectively. These sources will be detected serendipitously with the Chandra X-ray Observatory at a rate of 1–3 lenses per year of high resolution imaging. The low detection rate is due to the small area over which the HRC and ACIS cameras have the $<1^{\mathrm{\prime }\mathrm{\prime }}5$ FWHM resolution necessary to find gravitational lenses produced by galaxies. Deep images of rich clusters at intermediate redshifts should yield one wide separation ($\mathrm{\Delta }\theta \text{gtorder}{5}^{\mathrm{\prime }\mathrm{\prime }}0$) multiply-imaged background X-ray source for every $\sim 10$, 30 and 300 clusters imaged to the same flux limits.