# Doubly imaged quasar SDSS J1515+1511: time delay and lensing galaxies

## TL;DR

This study measures the time delay of 211 days between two images of a gravitationally lensed quasar, analyzes lensing effects, and confirms the lens galaxy's redshift using optical observations and spectroscopy.

## Contribution

First robust measurement of the time delay in SDSS J1515+1511, and confirmation of the lens galaxy's redshift at 0.742 through optical spectroscopy.

## Key findings

- Time delay between images A and B is 211 ± 5 days.
- Lens galaxy G1 is confirmed at redshift 0.742.
- Mass scale and external shear estimates remain consistent with previous studies.

## Abstract

We analyse new optical observations of the gravitational lens system SDSS J1515+1511. These include a 2.6-year photometric monitoring with the Liverpool Telescope (LT) in the $r$ band, as well as a spectroscopic follow-up with the LT and the Gran Telescopio Canarias (GTC). Our $r$-band LT light curves cover a quiescent microlensing period of the doubly imaged quasar at $z_{\rm s}$ = 2.049, which permits us to robustly estimate the time delay between the two images A and B: 211 $\pm$ 5 days (1$\sigma$ confidence interval; A is leading). Unfortunately, the main lensing galaxy (G1) is so faint and close to the bright quasar that it is not feasible to accurately extract its spectrum through the GTC data. However, assuming the putative redshift $z_{\rm G1}$ = 0.742, the GTC and LT spectra of the distant quasar are used to discuss the macrolens magnification, and the extinction and microlensing effects in G1. The new constraints on the time delay and macrolens magnification ratio essentially do not change previous findings on the mass scale of G1 and external shear, while the redshift of the lensing mass is found to be consistent with the assumed value of $z_{\rm G1}$. This is a clear evidence that G1 is indeed located at $z_{\rm G1}$ = 0.742. From the GTC data we also obtain the redshift of two additional objects (the secondary galaxy G2 and a new absorption system) and discuss their possible role in the lens scenario.