Testing the Cosmic Opacity at Higher Redshifts: Implication from Quasars with Available UV and X-Ray Observations

In this paper, we present a cosmological model-independent test for the cosmic opacity at high redshifts (z similar to 5). We achieve this with the opacity-dependent luminosity distances derived from the nonlinear relation between X-ray and UV emissions of quasars, combined with two types of opacity-independent luminosity distances derived from the Hubble parameter measurements and simulated gravitational wave (GW) events achievable with the Einstein Telescope (ET). In the framework of two phenomenological parameterizations adopted to describe cosmic opacity at high redshifts, our main results show that a transparent universe is supported by the current observational data at a 2 sigma confidence level. However, the derived value of the cosmic opacity is slightly sensitive to the parameterization of tau(z), which highlights the importance of choosing a reliable parameterization to describe the optical depth tau(z) in the early universe. Compared with previous works, the combination of the quasar data and theH(z)/GW observations in similar redshift ranges provides a novel way to confirm a transparent universe (epsilon = 0 at higher redshiftsz similar to 5), with an accuracy of Delta epsilon similar to 10(-2). More importantly, our findings indicate that a strong degeneracy between the cosmic-opacity parameter and the parameters characterizing theL(UV) - L(X)relation of quasars, which reinforces the necessity of proper calibration for this new type of high-redshift standard candle (in a cosmological model-independent way).