Towards equation of state of dark energy from quasar monitoring: Reverberation strategy

TL;DR

This paper proposes a reverberation mapping method using high-redshift quasars to independently measure luminosity distances, aiming to constrain dark energy properties through a five-year monitoring strategy.

Contribution

It models the expected measurement accuracy of quasar-based distance estimates and assesses their potential for constraining cosmological models.

Findings

Monitoring with Mg II lines can achieve 0.06-0.32 mag accuracy in distance modulus.

CIV line monitoring is less reliable due to higher intrinsic variability.

Five-year quasar monitoring can provide valuable constraints on dark energy.

Abstract

High redshift quasars can be used to deduce the distribution of dark energy in the Universe, as a complementary tool to SN Ia. The method is based on determination of the size of the Broad Line Region from the emission line delay, determination of the absolute monochromatic luminosity either from the observed statistical relation or from a model of the formation of the Broad Line Region, and determination of the observed monochromatic flux from photometry. This allows to obtain the luminosity distance to a quasar independently from its redshift. The accuracy of the measurements is however, a key issue. We model the expected accuracy of the measurements by creating artificial quasar monochromatic lightcurves and responses from the Broad Line Region under various assumptions about the variability of a quasar, Broad Line Region extension, distribution of the measurements in time, accuracy of the measurements and the intrinsic line variability. We show that the five year monitoring based on Mg II line should give the accuracy of 0.06 - 0.32 magnitude in the distance modulus which allows to put interesting constraints on the cosmological models. Monitoring of higher redshift quasars based on CIV lines is problematic due to much higher level of the intrinsic variability of CIV in comparison with Mg II.