Using quasar X-ray and UV flux measurements to test the cosmic opacity with cosmography

Abstract The X-ray and UV flux measurements of 1598 quasars (QSOs) compiled by Risaliti and Lusso cover the redshift range of 0 . 036 ≤ z ≤ 5 . 1003 , and they are one of the few data sets that can probe the redshift range of z ∼ 2 . 4 − 5 . 1 . In this paper, we use these quasar measurements combining with the opacity-independent observational Hubble parameter data (OHD) to test the cosmic opacity. To be model-independent, we use the cosmography to describe the cosmic expansion history. In addition, four parameterizations of the optical depth, i.e. τ ( z ) = 2 ɛ z , τ ( z ) = ( 1 + z ) 2 ɛ − 1 , τ ( z ) = τ 1 z + τ 2 z 2 , τ ( z ) = τ 1 z + τ 2 z 2 + τ 3 z 3 , are used. Our results show that in a global fitting on the quasar data, the observations are consistent well with a transparent universe within a 1 σ confidence level (CL), whichever parametrization is used. And the cosmic opacity parameters have negligible influences on the cosmographic parameters and the quasar parameters characterizing the nonlinear relation between X-ray and UV flux measurements of quasars.