[Paper Review] Current and future applications of Reverberation-mapped quasars in Cosmology

Swayamtrupta Panda, Mary Loli Martínez‐Aldama+1 more|arXiv (Cornell University)|Sep 12, 2019

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy130 references35 citations

TL;DR

This paper proposes using reverberation-mapped quasars, particularly via the Mg II line at z ≈ 0.9, to extend the radius-luminosity (R-L) relation to higher redshifts for cosmological distance measurements. It presents a corrected R-L relation accounting for super-Eddington accretion, enabling estimation of cosmological parameters Ωm and ΩΛ within 2σ of ΛCDM, and outlines a framework for photometric reverberation mapping with LSST to scale up quasar samples for future cosmology.

ABSTRACT

Reverberation mapping technique is an important milestone in AGN demographics, kinematics and the structure of the Broad Line Region (BLR) based on the time-delay response between the continuum and emission line. The time delay is directly related to the size of BLR which is related to the continuum luminosity of the source, producing the well-known Radius-Luminosity (RL) relation. The majority of sources have been monitored for their H$\beta$ emission line in low redshift sources ($z<0.1$), while there are some attempts using the MgII line for higher redshifts. We present a recent MgII monitoring for the quasar CTS C30.10 ($z=0.90$) observed with the 10-meter SALT, for which RL scaling based on MgII holds within measurement and time-delay uncertainties. One of the most important advantages of reverberation mapping technique is the independent determination to the distant source, and considering the large range of redshifts and luminosities found in AGNs, their application to cosmology is promising. However, recently it has been found that highly accreting sources show the time delays shorter than expected from RL relation. We propose correction for this effect using a sample of 117 H$\beta$ reverberating-mapped AGNs with $0.02<z<0.9$, recovering the low scatter along with the relation. We are able to determine the cosmological constants, $\Omega_m$ and $\Omega_\Lambda$ within 2$\sigma$ confidence level. We present the first steps in modelling of light curves for H$\beta$ and MgII and discuss the quasar selection in the context of photometric reverberation mapping with Large Synoptic Survey Telescope (LSST). With the onset of the LSST era, we expect a huge rise in the overall quasar counts and redshift range covered ($z\lesssim 7.0$), which will provide a better constraint of AGN properties with cosmological purposes.

Motivation & Objective

• To extend the radius-luminosity (R-L) relation for quasars beyond low redshift using the Mg II emission line.
• To correct for systematic biases in time-delay measurements caused by super-Eddington accretion rates.
• To assess the feasibility of using reverberation-mapped quasars for cosmological parameter estimation.
• To develop a filtering algorithm and synthetic lightcurve modeling for future LSST photometric reverberation mapping.
• To prepare for the LSST era by simulating quasar lightcurves and optimizing quasar selection for time-delay measurements.

Proposed method

• Conducted a six-year monitoring campaign of the intermediate-redshift quasar CTS C30.10 (z = 0.90) using the Southern African Large Telescope (SALT) to measure the Mg II line response time delay.
• Combined 117 reverberation-mapped AGN (including Hβ and Mg II sources) to fit a general power-law R-L relation: log(RBLR/1 lt-day) = K + α log(λLλ/1044 erg s⁻¹).
• Applied a correction for super-Eddington accretion to reduce scatter in the R-L relation, recovering low scatter consistent with theoretical expectations.
• Used the corrected R-L relation to estimate luminosity distances and constrain cosmological parameters Ωm and ΩΛ.
• Developed a synthetic lightcurve generator incorporating quasar properties (luminosity, redshift, black hole mass, EW) and LSST instrument response (filter bands, cadence, filter replacement time).
• Proposed a filtering algorithm based on emission line intensity, band overlap, and line width to identify optimal quasars for photometric reverberation mapping.

Experimental results

Research questions

• RQ1 Can the Mg II-based R-L relation be reliably extended to z ≈ 0.9 with current data?
• RQ2 How does super-Eddington accretion affect time-delay measurements and the R-L relation scatter?
• RQ3 What sample size is required to constrain cosmological parameters with 2% uncertainty using reverberation mapping?
• RQ4 How can photometric reverberation mapping be optimized for LSST’s wide-field, multi-band survey?
• RQ5 What are the key selection criteria for identifying quasars with high fidelity time-delay measurements in LSST photometry?

Key findings

• The Mg II time delay in CTS C30.10 was measured as τMgII = 551+49−82 days in the source frame, confirming the R-L relation at z ≈ 0.90.
• The combined R-L relation for 117 AGN yields best-fit parameters K = 1.47 ± 0.06 and α = 0.59 ± 0.06, consistent with the Hβ-based relation (α ≈ 0.533) within uncertainties.
• A correction for super-Eddington accretion successfully reduces scatter in the R-L relation, enabling more reliable distance estimates.
• With 5% uncertainty in time-delay measurements, approximately 625 quasars are needed to constrain cosmological parameters with 2% uncertainty.
• The LSST survey is projected to increase the number of reverberation-mapped quasars by ~5 orders of magnitude, enabling large-scale cosmological studies.
• The proposed filtering algorithm and synthetic lightcurve modeling provide a foundation for selecting optimal quasars and preparing for LSST photometric reverberation mapping.