[Paper Review] COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses IV. Models of prospective time-delay lenses

Prasenjit Saha, F. Courbin+3 more|ArXiv.org|Jan 17, 2006

Galaxies: Formation, Evolution, Phenomena42 references14 citations

TL;DR

This paper presents detailed pixelized lens models for 14 gravitationally lensed quasars to predict time delays and assess their suitability for Hubble constant ($H_0$) measurements. Using ensemble modeling under a concordance cosmology ($H_0 = 70$ km s⁻¹ Mpc⁻¹), it finds that combining time delays from 11 high-potential lenses with 1-day accuracy can constrain $H_0$ to within 5% precision, significantly improving cosmological distance scale measurements.

ABSTRACT

Aims: To predict time delays for a sample of gravitationally lensed quasars and to evaluate the accuracy that can be realistically achieved on the value of H_0. Methods: We consider 14 lensed quasars that are candidates for time-delay monitoring and model them in detail using pixelized lens models. For each system, we provide a mass map, arrival-time surface and the distribution of predicted time-delays in a concordance cosmology, assuming H_0^{-1}=14 Gyr (H_0=70 in local units). Based on the predicted time-delays and on the observational circumstances, we rate each lens as `excellent' or `good' or `unpromising' for time-delay monitoring. Finally, we analyze simulated time delays for the 11 lens rated excellent or good, and show that H_0 can be recovered to a precision of 5%. Results: In combination with COSMOGRAIL paper I on the temporal sampling of lensed quasar light curves, the present work will help design monitoring campaigns of lensed quasars.

Motivation & Objective

• To predict time delays for 14 prospective time-delay lensed quasars using detailed mass modeling.
• To evaluate the observational feasibility and rating (excellent/good/unpromising) of each system for time-delay monitoring.
• To estimate the achievable precision in $H_0$ when combining time-delay measurements from multiple lenses.
• To guide future COSMOGRAIL monitoring campaigns by identifying optimal targets and sampling strategies.

Proposed method

• Uses pixelized lens models to reconstruct mass distributions from observed image positions and flux ratios.
• Applies a conservative prior on mass models to explore the full range of physically plausible configurations.
• Predicts time delays in a concordance cosmology with $H_0^{-1} = 14$ Gyr ($H_0 = 70$ km s⁻¹ Mpc⁻¹).
• Rates each lens based on predicted time delays, image separation, morphology, and observational conditions.
• Simulates time-delay measurements for 11 high-rated lenses to estimate $H_0$ precision.
• Analyzes model uncertainties and their impact on $H_0$ recovery, accounting for non-uniqueness in mass profiles.

Experimental results

Research questions

• RQ1 What are the predicted time delays for 14 lensed quasar systems, and what is the range of uncertainty in these predictions?
• RQ2 Which of these systems are most suitable for time-delay monitoring, and how do observational constraints affect their ranking?
• RQ3 What level of precision in $H_0$ can be achieved by combining time-delay measurements from multiple lenses?
• RQ4 How do model priors and mass profile degeneracies affect the uncertainty in $H_0$ estimates?
• RQ5 To what extent does external shear or lens morphology reduce uncertainty in time-delay predictions?

Key findings

• The astrometric time delay predictor $\Delta t_{\rm astrom}$ provides a rough estimate within a factor of two of the true time delay for most systems.
• Modeling reveals that 90% of predicted time delay distributions span a range of about a factor of two, indicating high uncertainty due to mass profile degeneracy.
• Inclined quadruplets and asymmetric systems are more promising than symmetric or core quadruplets, which tend to have short, poorly constrained delays.
• External shear reduces model uncertainty in time delays, likely by constraining the available mass model space.
• Combining time delays from 11 high-rated lenses with 1-day accuracy can constrain $H_0$ to within 5% precision.
• The uncertainty in $H_0^{-1}$ is asymmetric, with the lower bound tighter than the upper bound, indicating a bias toward lower $H_0$ values in model ensembles.