TDCOSMO II. Six new time delays in lensed quasars from high-cadence monitoring at the MPIA 2.2m telescope

We present six new time-delay measurements obtained from Rc-band monitoring data acquired at the Max Planck Institute for Astrophysics (MPIA) 2.2m telescope at La Silla observatory between October 2016 and February 2020. The lensed quasars HE0047 1756, WG0214 2105, DES 0407 5006, 2M1134 2103, PSJ 1606 2333, and DES 2325 5229 were observed almost daily at high signal-to-noise ratio to obtain highquality light curves where we can record fast and small-amplitude variations of the quasars. We measured time delays between all pairs of multiple images with only one or two seasons of monitoring with the exception of the time delays relative to image D of PSJ 1606 2333. The most precise estimate was obtained for the delay between image A and image B of DES 0407 5006, where tau AB = 128:4+(3:5) (3:8) d (2.8% precision) including systematics due to extrinsic variability in the light curves. For HE0047 1756, we combined our high-cadence data with measurements from decade-long light curves from previous COSMOGRAIL campaigns, and reach a precision of 0.9 d on the final measurement. The present work demonstrates the feasibility of measuring time delays in lensed quasars in only one or two seasons, provided high signal-to-noise ratio data are obtained at a cadence close to daily.

COSMOGRAIL XVII. Time delays for the quadruply imaged quasar PG 1115+080

Vivien François Bonvin, Hung-Hsu Chan, Frédéric Courbin, Sun Hee Kim, Georges Meylan, Martin Raoul Robert Millon, Eric Gérard Guy Paic, Karina Alexandra Rojas Olate, Malte Tewes

We present time-delay estimates for the quadruply imaged quasar PG 1115+080. Our results are based on almost daily observations for seven months at the ESO MPIA 2.2m telescope at La Silla Observatory, reaching a signal-to-noise ratio of about 1000 per quasar image. In addition, we re-analyze existing light curves from the literature that we complete with an additional three seasons of monitoring with the Mercator telescope at La Palma Observatory. When exploring the possible source of bias we considered the so-called microlensing time delay, a potential source of systematic error so far never directly accounted for in previous time-delay publications. In 15 yr of data on PG 1115+080, we find no strong evidence of microlensing time delay. Therefore not accounting for this effect, our time-delay estimates on the individual data sets are in good agreement with each other and with the literature. Combining the data sets, we obtain the most precise time-delay estimates to date on PG 1115+080, with Delta t(AB) = 8.3(-1.6)(+1.5) days (18.7% precision), Delta t(AC) = 9.9(-1.1)(+1.1) days (11.1%) and Delta t(BC) = 18.8(-1.6)(+1.6) days (8.5%). Turning these time delays into cosmological constraints is done in a companion paper that makes use of ground-based Adaptive Optics (AO) with the Keck telescope.

EDP SCIENCES S A2018

Time-delay Cosmography with Strongly Lensed Quasars

Martin Raoul Robert Millon

The LCDM model has emerged as the concordance model of cosmology for its ability to explain a variety of observations, ranging from the anisotropies of the Cosmic Microwave Background (CMB) to the accelerated expansion of the Universe. However, with the ever-increasing precision of the measurements, tensions have recently emerged between the latest CMB observations and other cosmological probes. The most prominent one concerns the expansion rate of the Universe, that is the Hubble constant, H0, with a statistically significant discrepancy between local and early measurements. If not arising from unaccounted systematic errors, this discrepancy is an exciting opportunity to explore missing physics beyond the standard LCDM model.Time-delay cosmography has emerged as a competitive and single-step method to measure locally the Hubble constant, providing an alternative to the cosmic distance ladder. The two techniques do not share any known source of errors, allowing us to obtain truly independent estimates. This technique is based on the strong lensing phenomenon, occurring when the light coming from a background source is split into multiple images by a massive foreground galaxy. In asymmetric configurations, the travel time is then slightly different for each of the lensed images, leading to measurable time delays in the signals received from the background object. The Hubble constant can then be directly inferred from these time delays, provided that an accurate reconstruction of the mass distribution of the lens galaxy is available.The first part of my PhD work focuses on the precise determination of the time delays in strongly lensed quasars, an essential ingredient of the method. The COSMOGRAIL collaboration has been monitoring lensed quasars for two decades to record enough quasar variations that can be unambiguously matched in all light curves. I compiled these data obtained from the Euler 1.2m Swiss telescope in La Silla, Chile, and measured the time delays in 18 systems, more than doubling the current sample of lensed quasars with known delays. This work required the development and automation of curve-shifting algorithms presented in a dedicated chapter. As the sample of known lensed quasars is growing rapidly, I also present a novel monitoring strategy to obtain the time delays efficiently, in only one or two monitoring seasons. Thanks to this long-term monitoring effort, the TDCOSMO collaboration published the most precise determination of the Hubble constant obtained with time-delay cosmography from a sample of seven lensed quasars. We found H0 = 73.7+/- 1.5 km/s/Mpc, at 2% precision. This result confirmed the existing tension, now reaching a high significance when combined with the distance ladder results. "As extraordinary claims require extraordinary evidence", the second part of my work focused on the verification of the assumptions made to obtain this result. In the last chapter of this thesis, I present my work on the search for unaccounted systematic errors in every step of the analysis. I also show the results of our participation to a blind data challenge designed to test lens modelling codes and the assumptions used to reconstruct the mass distribution of the galaxies. I conclude this thesis by presenting the new approach proposed by the TDCOSMO collaboration to relax all assumptions about the density profile of massive elliptical galaxies and adopt a parametrisation entirely constrained by stellar kinematic.

EPFL2021

Gravitational lenses

Christel Vuissoz

The Hubble constant H0 is one of the most important parameters in cosmology, as it encodes the age of the Universe and is necessary for any distance determination at a cosmological scale. It is, however, only poorly constrained by traditional methods. The current favored value, H0 = 72±8 km s-1 Mpc-1, is provided by the HST Hubble constant Key Project (Freedman et al. 2001), which combines several Cepheid-calibrated distance indicators. This roughly 10% error nevertheless denotes only the statistical uncertainty in the determination of H0, while the possible systematical errors in the first step of the distance ladder (the distance to the Large Magellanic Cloud) may be of the same order of magnitude. Time delays between gravitationally lensed images of distant quasars can yield a more precise measurement of the Hubble constant, on a truly cosmic scale, and independently of any local distance calibrator. At the beginning of this thesis, time delays had been measured in only ten lensed systems, nine of which gave H0 estimates. However before 2004, no concerted and long term action has succeeded to apply the time delay method at a level of precision really competitive with other techniques. The major difficulties arise from the modeling of the lens mass distribution, and from the uncertainty on the time delay measurement itself, which was typically of about 10% in past monitoring programs. COSMOGRAIL (COSmological MOnitoring of GRAvItational Lenses) is an international collaboration initiated in April 2004 at the Laboratory of Astrophysics of EPFL, and which aims at measuring precise time delays for most known lensed quasars, in order to determine the Hubble constant down to an uncertainty of a few percent. This thesis took place at the beginning of COSMOGRAIL and consisted in setting up this large photometric monitoring. It addressed both issues of carrying out accurate photometry of faint blended sources and of obtaining well sampled light curves, in order to measure precise time delays. As part of the COSMOGRAIL project, I have been managing the monitoring of over twenty gravitationally lensed quasars with the 1-2m telescopes involved both in the Northern and Southern hemispheres, and organizing the data. The first crucial work of this thesis was then to develop an automated reduction pipeline able to produce an homogeneous data set from images acquired with very different telescopes. This pipeline was also needed to perform aperture photometry of all lensed quasars, in order to study their variability and define the monitoring priorities. The powerful MCS deconvolution algorithm (Magain, Courbin, & Sohy 1998) was greatly used in this work and allowed to highly improve the image resolution, with the aim of obtaining accurate photometric measurements of the individual quasar lensed images. I have finally tested and improved three different numerical techniques to determine time delays between the quasar components from their light curves. In this thesis, time delays have been determined in four systems. The first one was measured in the doubly imaged quasar SDSS J1650+4251, after two years of monitoring with the 1.5m telescope of Maidanak Observatory, in Uzbekistan. The quadruply lensed system RXS J1131–1231 was then studied and three time delays determined from 3-year observations with the Swiss Euler 1.2m telescope located at La Silla, in Chile. The photometric monitoring of the quadruple WFI J2033–4723 was also carried out with the Euler telescope, and data were then merged with those obtained by a second monitoring group, with the SMARTS 1.3m telescope at the Cerro Tololo Interamerican Observatory (CTIO), also located in Chile. Two time delays were measured in this system, after three years of observations, the close pair A1 – A2 remaining unresolved. Three time delays were determined in the quadruply imaged quasar HE0435–1223, after four years of optical monitoring with Euler, Mercator and Maidanak telescopes, to which photometric measurements by SMARTS 1.3m telescope were added. Euler and SMARTS merged data for the doubly imaged quasar QJ0158–4325 were also analysed, the size of the source accretion disk was measured, but we failed to determine a time delay due to the high amplitude of the microlensing variability in this system. The accuracies on time delay measurements reached in this thesis are of the order of 3-4% and show a clear improvement from the typical 10% uncertainties of past monitoring programs. These results were finally converted into estimates of the Hubble constant following different models of the lensing mass potential. The H0 mean value obtained when considering the individual determinations from twelve gravitationally lensed quasars with known time delays is H0 = 60 ± 7 km s-1 Mpc-1. This result is consistent with the current favored value, and above all promising, as including additional systems to this ensemble will surely provide tighter bounds on H0. In conclusion, the increasing number of time delay measurements and improvements in lens modeling should reduce the errors on the Hubble constant estimate provided by gravitational lensing. Conversely, the determination of more time delays should put further constraints on lens galaxy density profiles when using a prior on H0 from other studies.