1999MNRAS.305..412C

Foreground massive galaxy clusters are expected to gravitationally lens background sources. Here, we calculate the expected number of gravitationally lensed X-ray sources on the sky owing to foreground galaxy clusters for different cosmological models. We describe the background sources using flux and redshift distribution observed in the ROSAT Deep Survey towards the `Lockman' hole, and model the foreground galaxy clusters as isothermal spheres. We account for the redshift evolution of lensing clusters using a Press–Schechter analysis with normalizations based on the observed cluster temperature function. For a flat cosmology with Ο_m=0.3 and Ο_Λ = 0.7, we predict ∼ 10³ lensed sources with fluxes larger than ∼ 5x10^–15 erg.cm^–2.s^–1 and ∼ 10⁴ sources with fluxes larger than ∼ 10^–16 erg.cm^–2.s^–1 in the 0.5–2.5 keV band. We discuss the possibility of detecting lensed X-ray sources towards clusters in deep AXAF observations. The planned AXAF guaranteed time observation of A370 with an exposure of 90 ks is likely to recover at least two lensed active galactic nuclei (AGNs) above the 3 σ level. Aided by amplification owing to gravitational lensing, the search for lensed X-ray sources towards clusters is likely to recover AGNs at redshifts of 1–4.