FBQ 0951+2635: time delay and structure of the main lensing galaxy

TL;DR

This study accurately measures the time delay in the gravitational lens system FBQ 0951+2635, analyzes the lens galaxy's mass structure, and discusses implications for galaxy modeling using extensive optical data.

Contribution

It provides a robust measurement of the time delay and detailed analysis of the lens galaxy's mass profile without relying on prior assumptions.

Findings

Measured time delay of 13.5 +/- 1.6 days.

The lens galaxy's mass profile is close to isothermal.

A constant mass-to-light ratio model is also plausible.

Abstract

As there is a long-standing controversy over the time delay between the two images of the gravitationally lensed quasar FBQ 0951+2635, we combined early and new optical light curves to robustly measure a delay of 13.5 +/- 1.6 d (1sigma interval). The new optical records covering the last 17 yr were also used to trace the long-timescale evolution of the microlensing variability. Additionally, the new time delay interval and a relatively rich set of further observational constraints allowed us to discuss the mass structure of the main lensing galaxy at redshift 0.26. This lens system is of particular interest because the external shear from secondary gravitational deflectors is relatively low, but the external convergence is one of the highest known. When modelling the galaxy as a singular power-law ellipsoid without hypotheses/priors on the power-law index, ellipticity and position angle, we demonstrated that its mass profile is close to isothermal, and there is good alignment between mass and near-IR light. We also recovered the true mass scale of the galaxy. Finally, it is worth mentioning that a constant mass-to-light ratio model also worked acceptably well.