Spectroscopic characterization of a remarkable temporally varying, triple-lensed quasar at z=2.67

TL;DR

This work identifies and characterizes a remarkable triple-lensed quasar, DR3Gaia2107-1611, at $z=2.67$, discovered via Gaia astrometry and Pan-STARRS. A detailed lens model yields a halo mass of $M_h = (2.78 \pm 0.05) \times 10^{10}\,M_\odot$ and inter-sightline time delays $\Delta\tau$ on the order of weeks to years, enabling construction of an intrinsic quasar timeseries over $\sim 18$ months. Multi-epoch spectroscopy reveals sightline-dependent and temporally evolving emission-line properties, with increases in ionization along two images and a potential microlensing signature in the third, illustrating the power of combining astrometry with spectroscopy for lensing studies. The paper also demonstrates the effectiveness of Gaia-based astrometric selection for uncovering rare lens configurations and provides optimized criteria to guide future searches.

Abstract

Gravitationally lensed quasars are viable cosmic tools for constraining a diversity of fundamental astrophysical phenomena; They enable identification of faint, low-mass supermassive black holes, provide unique constraints on the intervening intergalactic or interstellar medium in their sightlines, and can be used to determine key cosmological quantities such as the Hubble constant, $H_0$. However, they are rare phenomena, and it has proven difficult to define efficient, unbiased selection methods.} In this study, we report the independent spectroscopic identification of a remarkable triple-lensed quasar at $z=2.67$, identified based on astrometric measurements from the {\em Gaia} mission, previously identified in Pan-STARRS. Furthermore, a larger spectroscopic follow-up survey of {\em Gaia}-detected candidate lensed quasars. We characterize in detail the three mirror images of the quasar and their spatial and temporal spectroscopic coverage, with focus on the emission-line properties which shows variation across sigthlines and temporal evolution over the $\sim 11$months spectroscopic campaign. We construct a lens model of the foreground source from a combination of the multiple spectra and deep optical imaging, providing a robust halo mass of $M_{\rm h} = (2.78 \pm 0.05)\times 10^{10}M_\odot$. Based on the lens model, the time delay between each sightline is translated into an intrinsic quasar time, allowing us to construct a quasar timeseries over $\sim18$months with monthly cadence. Over months timescales the broad emission lines vary in both velocity offset and equivalent width (EW) as well as an overall increase in ionization. This exemplary triple-lensed quasars demonstrates the viability of identifying such rare lens configurations based purely on the astrometric measurements from the {\em Gaia} mission, which we here provide optimized selection criteria for, for future studies.

Figures (9)

• Figure 1: Example spectrum of the quasar DR3Gaia0205-3233 at $z=0.843\pm0.003$ (blue), fit with the dust extinction parameter, $A_V$, red (see Eq. \ref{['eq:dust_model']}). The redshift is determined from the identified emission lines in the spectrum.
• Figure 2: Histograms of redshift, $z$, and visual extinction parameters, $A_V$, for the parent quasar sample observed in this study.
• Figure 3: $S/N$ for proper motion against G - RP, the difference in the Gaia bands G- and RP-bands. The plotted points are sources from the study discarded as lens candidates (red) and remaining lens candidates, each identified with the source ID and an index separating individual sight lines in the source. To enhance the statistics, known lenses are added, either as lines to connect lenses where photometry of both objects exist in Gaia, or "+" if not. A line is added, showing that the lens and lens candidate population can be separated by an additional photometric criterion. Note, that we deselect only sources where all sight lines complies to the condition in Eq. \ref{['eq:SelectionCondition']} -- meaning they will be to the right of the line.
• Figure 4: (Left): Spectra of each sight line in the lensed quasar DR3Gaia2107-1611, as seen in Fig. \ref{['fig:DR3Gaia']}. The variability between sightlines are especially clear in the shape of C IV, which is different for sight line B, while Ly$\alpha$ vary substantially across all sightlines.
• Figure 5: Imaging of DR3Gaia2107-1611 with each slit used for the observing the three sightlines, A, B and C, as seen in Fig. \ref{['fig:epoch1']}. The lens galaxy is vaguely visible in the center of the lens system. We mark the tentative detection of Ly$\alpha$ emission on the opposite side of sight line A, detected from the 2D-spectrum of the system before extraction.