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. Reverberation mapping (RM) method enables 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 the 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 Hβ RM AGN 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 (Ṁ ) considering a virial factor anti-correlated with the FWHM of Hβ. This correction decreases the scattering of the accretion parameters compared with typical values used, which is directly reflected by suppressing the radius-luminosity relation dispersion. We also confirm the anti-correlation 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 standard cosmological model at 2σ confidence level. Therefore, RM results can be used to constrain cosmological models in the future.

Current and Future Applications of Reverberation-Mapped Quasars in Cosmology

“…Hence, the Eddington ratio that we estimate will be higher. This trend of decrease of the time delay with rising Eddington ratio seems systematic Martínez-Aldama et al, 2019a). Being able to constrain their physical parameter space (in terms of the ionisation parameter, cloud density and accretion rate) and link this to the observables, such as emission line ratios and shape of the emission line profiles, would provide answers to this anomaly (Panda et al in prep).…”

Reverberation mapping of distant quasars: Time lag determination using different methods

“…In addition, the rest‐frame time lag of days is consistent within uncertainties in the radius–luminosity relationship (Czerny et al ), which was previously studied and determined for a lower‐redshift sample of quasars (Bentz et al ). In principle, this opens up the possibility of using quasars for constraining cosmological parameters due to the validity of the radius–luminosity relationship toward higher redshifts (Martínez‐Aldama et al ). In parallel, quasars have been used in cosmological studies using the X‐ray and UV–luminosity relationship and its application to the Hubble diagram of quasars (Risaliti & Lusso ).…”

“…Another important aspect of the radius–luminosity relation is that it can be utilized to determine the luminosity of AGN, and based on the measured flux, the luminosity distance can be obtained. Thus, the radius–luminosity relation provides a way to turn AGN into another population of standard candles that could be used for testing cosmological models over a larger range of redshifts than supernovae Ia (Bentz et al ; Martínez‐Aldama et al ).…”

Main trends of the quasar main sequence - effect of viewing angle

“…The prediction would be specially valuable for high accretors that show high R FeII ( 1) shorter time delays with respect to the ones derived from the scaling law of Bentz et al (2013). By the same token, we may become able to constrain the r BLR -L 5100 relation that has been shown to offer prospects of application for cosmology by building the Hubble diagram for quasars (Risaliti & Lusso, 2015;Martínez-Aldama et al, 2019). M .…”