Can Reverberation-measured Quasars Be Used for Cosmology?

Abstract

Quasars have been proposed as a new class of standard candles analogous to supernovae, since their large redshift range and high luminosities make them excellent candidates. The reverberation mapping (RM) method enables one to estimate the distance to the source from the time delay measurement of the emission lines with respect to the continuum, since the time delay depends on the absolute luminosity of the source. The radius–luminosity relation exhibits a low scatter and offers a potential use in cosmology. However, in recent years, the inclusion of new sources, particularly the super-Eddington accreting QSO, has increased the dispersion in the radius–luminosity relation, with many objects showing time delays shorter than the expected. Using 117 H β reverberation-mapped active galactic nuclei with 0.002 <  z  < 0.9 and 41.5 < log  L 5100  < 45.9, we find a correction for the time delay based on the dimensionless accretion rate ( ) considering a virial factor anticorrelated with the FWHM of H β . This correction decreases the scattering of the accretion parameters compared with the typical values used, which is directly reflected by suppressing the radius–luminosity relation dispersion. We also confirm the anticorrelation between the excess of variability and the accretion parameters. With this correction, we are able to build the Hubble diagram and estimate the cosmological constants Ω m and Ω Λ , which are consistent with the Λ Cold Dark Matter model at 2 σ confidence level. Therefore, reverberation mapping results can be used to constrain cosmological models in the future.