COSMOGRAIL: Measuring Time Delays of Gravitationally Lensed Quasars to Constrain Cosmology

Astronomical Science COSMOGRAIL: Measuring Time Delays of Gravitationally Lensed Quasars to Constrain Cosmology Malte Tewes1 We live in a golden age for observational Strong lensing time delays Fréderic Courbin1 cosmology, where the Universe seems Georges Meylan1 fairly well described by the Big Bang Gravitational lensing describes the Christopher S. Kochanek2, 3 theory. This concordance model comes deflection of light by a gravitational Eva Eulaers4 at the price of evoking the existence of the field. In the case of a particularly deep Nicolas Cantale1 still poorly known dark matter and even potential well, the phenomenon can Ana M. Mosquera2, 3 more mysterious dark energy, but has the be spectacular: a luminous source in the Ildar Asfandiyarov5 advantage of describing the large-scale background of a massive galaxy (the Pierre Magain4 Universe using only a handful of parame- gravitational lens) can be “multiply im Hans Van Winckel6 ters. Determining these parameters, and aged” and several distorted images of it Dominique Sluse7 testing our assumptions underlying the are seen around this galaxy. This phe- Rathna K. S. Keerthi8 concordance model, requires cosmologi- nomenon is known as “strong gravita- Chelliah S. Stalin8 cal probes — observations that are sen- tional lensing” and has several interesting Tushar P. Prabhu8 sitive tests to the metrics, content and cosmological applications. One of them Prasenjit Saha9 history of the Universe. They include uses the measurement of the “time Simon Dye10 standard candles like Cepheid variable delays” between the multiple images of a stars or Type I a supernovae, standard gravitationally lensed source. rulers like baryonic acoustic oscillations, 1 Laboratoire d’astrophysique, Ecole or involve the measurement of the cos- As illustrated in Figure 1, the different Polytechnique Fédérale de Lausanne, mic microwave background as well optical paths to the multiple images of a Switzerland as weak gravitational lensing over large gravitationally lensed source differ in 2 Department of Astronomy, The Ohio areas of sky, to name just a few. length. In addition, the photons propa- State University, USA gate through different depths of the 3 Center for Cosmology and Astroparticle In practice, these probes are all sensitive potential well of the lensing galaxy. As a Physics, The Ohio State University, to different combinations of cosmologi- consequence, the time it takes a photon USA cal parameters, hence making it manda- to reach the observer along each path 4 Institut d’astrophysique et de tory to combine them together in the is different. If the lensed source is photo- Géophysique, Université de Liège, best possible way. Comparisons of dif- metrically variable, like a quasar, the Belgium ferent cosmological probes also test their light curves measured for the lensed 5 Ulugh Beg Astronomical Institute, consistency, and allow us to check for image will appear shifted in time by some Academy of Sciences, Tashkent, systematic errors in the measurements. amount called the time delay. This delay Uzbekistan Strong gravitational lensing time delays is directly related to the size of the gravi- 6 Instituut voor Sterrenkunde, Katholieke offer one of these cosmological probes, tational lens system, which essentially Universiteit Leuven, Belgium whose strength has so far been under- depends on the expansion rate of the 7 Argelander-Institut für Astronomie, estimated due to the lack of adequate Universe, that is, the Hubble constant H0 Bonn, Germany data and tools. (Figure 1). This straightforward “time 8 Indian Institute of Astrophysics, Koramangala, Bangalore, India +@QFD4MHUDQRD KNV' K@QFDSHLDCDK@X 9 Institute for Theoretical Physics, University of Zürich, Switzerland 10 School of Physics and Astronomy, University of Nottingham, UK 2NTQBDPT@R@Q COSMOGRAIL is a long-term pro gramme for the photometric monitoring 6S of gravitationally lensed quasars. It makes use of several mediumsize tele .ARDQUDQ scopes to derive long and wellsampled light curves of lensed quasars, in order +DMRF@K@WX to measure the time delays between the 2L@KK4MHUDQRD GHFG' RL@KKSHLDCDK@X quasar images. These delays directly relate to the Hubble constant H , with 0 Figure 1. Schematic showing out any need for secondary distance how the time delay Δt between calibrations. COSMOGRAIL was initiated two images of a gravitationally in 2004, and has now secured almost a lensed source depends on the decade of data, resulting in cosmologi scale of the gravitational lens system, and hence on H0. Illus- 6S cal constraints that are very comple tration inspired by Narayan & mentary to other cosmological probes. Bartelmann (1996). The Messenger 150 – December 2012 49 Astronomical Science Tewes M. et al., COSMOGRAIL delay method” to constrain H0 had already been proposed by Sjur Refsdal in 1964 (Refsdal 1964), well before the first strongly lensed quasar was even discov- ered by Walsh, Carswell and Weymann in 1979 (Walsh et al., 1979). The COSMOGRAIL monitoring programme Obtaining accurate measurements of time delays is challenging, but has re cently proved possible. The COSMOGRAIL programme (COSmological MOnitoring of GRAvItational Lenses) is a long-term photo metric monitoring programme designed to reach this goal. Initiated and led by the Laboratory of Astrophysics of the École Polytechnique Fédérale de Lausanne, it combines the strengths of medium-size telescopes available for long periods of time, with the collecting power Figure 2. The Swiss 1.2-metre Leonhard Euler Tele- Photometry of the deconvolved images and spatial resolution of the ESO Very scope at the La Silla Observatory, used for several allows the use of data spanning a broad long-term observational programmes, from exoplan- Large Telescope (VLT) and the Hubble ets to cosmology. Euler is the main telescope of the range of quality, from 0.8 to 2.0 arcsec- Space Telescope (HST). The medium- COSMOGRAIL collaboration. onds for point source full width at half size telescopes are needed continuously maximum (Tewes et al., 2012b). over several years in order to measure the time delays, while the VLT and HST Light curves and time delay measurement The light curves for the four lensed are needed punctually to obtain deep images of RX J1131−1231 are presented and sharp observations in order to con- With these telescopes, we have gathered in Figure 4, where a time delay of image strain the lens models. Indeed, the meas- almost a decade of photometric data D of about 100 days is striking. With a ured time delays can only be turned into with a temporal sampling of between two closer look, however, the curves do not cosmological inference given accurate and five days depending on the targets. perfectly overlap, even when shifted by lens mass models. As the lensed images of a typical quasar the appropriate delays. Some of the mis- have angular separations on the order matches are conspicuous, on time scales COSMOGRAIL uses several medium-size of an arcsecond, our reduction pipeline of a few weeks to several years. These telescopes to carry out a long-term opti- involves an image deconvolution algor- discrepancies result from quasar micro- cal photometric monitoring of selected ithm that can carry out photon-noise lensed quasars. Our set of monitoring tel- limited photometry of heavily blended escopes includes the Swiss 1.2-metre point sources (e.g., Magain et al., 1998). Figure 3. The quadruply lensed quasar RX Leonhard Euler Telescope at the ESO Figure 3 shows an example of such a J1131−1231, seen from space with HST in a colour La Silla Observatory (shown in Figure 2), deconvolution, in the case of the quadru- composite (left; from Claeskens et al., 2006) and the 2-metre Himalayan Chandra Tele- ply lensed quasar RX J1131−1231. The from the ground in the R-band with the Swiss 1.2- metre Leonhard Euler telescope at La Silla (middle). scope (HCT) in India, the 1.5-metre tele- quasar is at a redshift of z = 0.658 The right panel shows the deconvolution of the Euler scope of the Maidanak Observatory whilst the lensing galaxy is at z = 0.295. data, as used for our photometry. (Uzbekistan), and, with a smaller contri- bution, the 1.2-metre Mercator telescope on La Palma (Canary Islands, Spain). In 2010, COSMOGRAIL joined forces with the group of Christopher S. Kochanek 2 ! (Ohio State University, USA) who employs # the 1.3-metre Small & Moderate Aperture & Research Telescope System (SMARTS) - at the Cerro Tololo Inter-American Obser- " vatory in Chile. The resulting collabora- ǥ tion is now the largest quasar lens moni- $ toring programme worldwide. 'TAAKD2O@BD3DKDRBNOD 2VHRR+DNMG@QC$TKDQ3DKDRBNOD$TKDQCDBNMUNKUDC 50 The Messenger 150 – December 2012 Figure 4. R-band light curves for the four lensed images of the z = 0.658 quasar RX J1131−1231. The 17)l photometry for the 707 epochs is obtained from ! L@F deconvolution photometry. The three colours used " to display the data points correspond to four differ- # L@F UD ent instruments/telescopes. K@SH QD D 3NS@KDONBGR 2, 132 LDONBGR curve are uncorrelated, as each quasar $TKDQDONBGR ,DQB@SNQDONBGR image is magnified by different stars of ,@FMHSTC the lensing galaxy. While quasar microlensing is highly interesting on its own and can be used as ')#l C@X a natural telescope to zoom into the inner parts of quasar accretion discs (for a review, see, e.g., Schmidt & Wambsganss, 2010; Eigenbrod et al., 2008; and also ESO Release eso08471), it also perturbs the time delay measurement. We have therefore devised a set of new numerical UD techniques to measure the time delays @SH DK and to accurately estimate their uncertain- DQ ties, even in the presence of microlensing variability (Tewes et al., 2012a). Applying these techniques to RX J1131−1231, the ,@FMHSTC delays between the three close images A, B and C are compatible with being zero, and we measure the delay of image D to be 91 days, with a fractional uncer- ')#l C@X tainty of only 1.5% (1σ), including systematic errors (Tewes et al., 2012b).

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