We present the continuation of our long-term spectroscopic monitoring of the gravitationally lensed quasar QSO 2237+0305. We investigate the chromatic variations observed in the UV/optical continuum of both quasar images A and B, and compare them with numerical simulations to infer the energy profile of the quasar accretion disk. Our procedure combines the microlensing ray-shooting technique with Bayesian analysis, and derives probability distributions for the source sizes as a function of wavelength. We find that the effective caustic crossing timescale is 4.0 ± 1.0 months. Using a robust prior on the effective transverse velocity, we find that the source responsible for the UV/optical continuum has an energy profile well reproduced by a power-law R ∝ λ ζ with ζ = 1.2 ± 0.3, where R is the source size responsible for the emission at wavelength λ. This is the first accurate, model-independent determination of the energy profile of a quasar accretion disk on such small scales.

Aims. We aim to use microlensing taking place in the lensed quasar QSO 2237+0305 to study the structure of the broad line region (BLR) and measure the size of the region emitting the C iv and C iii] lines. Methods. Based on 39 spectrophotometric monitoring data points obtained between Oct. 2004 and Dec. 2007, we derived lightcurves for the C iv and C iii] emission lines. We used three different techniques to analyse the microlensing signal. Different components of the lines (narrow, broad, and very broad) were identified and studied. We built a library of the simulated microlensing lightcurves that reproduce the signal observed in the continuum and in the lines provided only the source size is changed. A Bayesian analysis scheme is then developed to derive the size of the various components of the BLR. Results. 1. The half-light radius of the region emitting the C iv line is found to be R C iv ∼ 66 The size of the C iv emitting region agrees with the radius-luminosity relationship derived from reverberation mapping. Using the virial theorem, we derive the mass of the black hole in QSO 2237+0305 to be M BH ∼ 10 8.3±0.3 M . 3. We find that the C iv and C iii] lines are produced in at least 2 spatially distinct regions, the most compact one giving rise to the broadest component of the line. The broad and narrow line profiles are slightly different for C iv and C iii]. 4. Our analysis suggests a different structure for the C iv and Fe ii+iii emitting regions, with the latter produced in the inner part of the BLR or in a less extended emitting region than C iv.Key words. gravitational lensing: micro -gravitational lensing: strong -quasars: general -quasars: emission linesquasars: individual: QSO 2237+0305 -line: formation IntroductionWe know that quasars and active galactic nucleii (AGN) are powered by matter accreted onto a supermassive black hole. The accretion of material in the direct vicinity of the central black hole releases most of the quasar energy in the form of powerlaw continuum emission. Ionised gas surrounds the central accretion disc and gives rise to broad emission lines, which are used as footprints that allow the identification and classification of quasars. Our knowledge of the kinematics and physical conditions prevailing in the BLR gas remain elusive, especially because the nuclear region of quasars is still spatially unresolved with existing instrumentation.Current insights into the BLR come from various kind of studies: empirical modelling of the line shape with kinematical models, use of photo-ionisation codes to reproduce the observed flux ratios between spectral lines, spectropolarimetric observations, statistical study of the width and asymmetry of the lines, use of the principal component analysis technique, and velocity resolved reverberation mapping (e.g. Boroson & Green 1992;Sulentic et al. 2000;Smith et al. 2005;Marziani et al. 2006;Zamfir et al. 2008;Gaskell 2009Gaskell , 2010bBentz et al. 2010). Despite the development and many successes of these methods, as briefly summarise...

Abstract. We use numerical simulations to test a broad range of plausible observational strategies designed to measure the time delay between the images of gravitationally lensed quasars. Artificial quasar light curves are created along with Monte-Carlo simulations in order to determine the best temporal sampling to adopt when monitoring the photometric variations of systems with time delays between 5 and 120 days, i.e., always shorter than the visibility window across the year. Few and realistic assumptions are necessary on the quasar photometric variations (peak-to-peak amplitude and time-scale of the variations) and on the accuracy of the individual photometric points. The output of the simulations is the (statistical) relative error made on the time delay measurement, as a function of 1-the object visibility over the year; 2-the temporal sampling of the light curves; and 3-the time delay. Also investigated is the effect of long term microlensing variations which must be below the 5% level (either intrinsically or by subtraction) if the goal is to measure time delays with an accuracy of 1-2%. However, while microlensing increases the random error on the time delay, it does not significantly increase the systematic error, which is always a factor 5 to 10 smaller than the random error. Finally, it is shown that, when the time delay is comparable to the visibility window of the object, a logarithmic sampling can significantly improve the time delay determination. All results are presented in the form of compact plots to be used to optimize the observational strategy of future monitoring programs.Key words. gravitational lensing -cosmological parameters -cosmology: observations Measuring time delaysMeasuring time delays in gravitationally lensed quasars is difficult, but not as difficult as it first appeared in the late eighties when the first monitoring programs were started. Obtaining regular observing time on telescopes in good sites was (and is still) not easy and the small angular separations between the quasar images require to perform accurate photometry of blended objects, sometimes with several quasar images plus the lensing galaxy within the seeing disk. COSMOGRAILThe COSMOGRAIL project (COSmological MOnitoring of GRAvItational Lenses), started in April 2004, addresses both issues of carrying out photometry of faint blended sources and of obtaining well sampled light curves of lensed quasars.The project involves 5 telescopes: (1) the Swiss 1.2 m Euler telescope located at La Silla, Chile; (2) the Swiss-Belgian 1.2 m Mercator telescope, located in the Canaria islands (La Palma, Spain); (3) the 2 m robotic telescope of the Liverpool University (UK), also located at La Palma; (4) the 1.5 m telescope of Maidanak observatory in Uzbekistan; and (5) the 2 m Himalayan Chandra Telescope (HCT).All 5 telescopes, and others that will join the collaboration, are used in order to follow the photometric variations of most known gravitationally lensed quasars that are suitable for a determination of H 0 . The sample of...

We present the continuation of our long-term spectroscopic monitoring of the gravitationally lensed quasar QSO 2237+0305. We investigate the chromatic variations observed in the UV/optical continuum of both quasar images A and B, and compare them with numerical simulations to infer the energy profile of the quasar accretion disk. Our procedure combines the microlensing ray-shooting technique with Bayesian analysis, and derives probability distributions for the source sizes as a function of wavelength. We find that the effective caustic crossing timescale is 4.0 ± 1.0 months. Using a robust prior on the effective transverse velocity, we find that the source responsible for the UV/optical continuum has an energy profile well reproduced by a power-law R ∝ λ ζ with ζ = 1.2 ± 0.3, where R is the source size responsible for the emission at wavelength λ. This is the first accurate, model-independent determination of the energy profile of a quasar accretion disk on such small scales.

Aims. To provide the observational constraints required to use the gravitationally lensed quasar SDSS J0924+0219 for the determination of H 0 from the time delay method. We measure here the redshift of the lensing galaxy, we show the spectral variability of the source, and we resolve the lensed host galaxy of the source. Methods. We present our VLT/FORS1 deep spectroscopic observations of the lensed quasar SDSS J0924+0219, as well as archival HST/NICMOS and ACS images of the same object. The two-epoch spectra, obtained in the Multi Object Spectroscopy (MOS) mode, allow for very accurate flux calibration and spatial deconvolution. This strategy provides spectra for the lensing galaxy and for the quasar images A and B, free of any mutual light contamination. We deconvolve the HST images as well, which reveal a double Einstein ring. The mass distributions in the lens, reconstructed in several ways, are compared. Results. We determine the redshift of the lensing galaxy in SDSS J0924+0219: z lens = 0.394 ± 0.001. Only slight spectral variability is seen in the continuum of quasar images A and B, while the C III], Mg II and Fe II emission lines display obvious changes. The flux ratio between the quasar images A and B is the same in the emission lines and in the continuum. One of the Einstein rings found using deconvolution corresponds to the lensed quasar host galaxy at z = 1.524 and a second bluer one, is the image either of a star-forming region in the host galaxy, or of another unrelated lower redshift object. A broad range of lens models give a satisfactory fit to the data. However, they predict very different time delays, making SDSS J0924+0219 an object of particular interest for photometric monitoring. In addition, the lens models reconstructed using exclusively the constraints from the Einstein rings, or using exclusively the astrometry of the quasar images, are not compatible. This suggests that multipole-like structures play an important role in SDSS J0924+0219.

Aims. Our aim is to measure the time delay between the two gravitationally lensed images of the z qso = 1.547 quasar SDSS J1650+4251, in order to estimate the Hubble constant H 0 . Methods. Our measurement is based on R-band light curves with 57 epochs obtained at Maidanak Observatory, in Uzbekistan, from May 2004 to September 2005. The photometry is performed using simultaneous deconvolution of the data, which provides the individual light curves of the otherwise blended quasar images. The time delay is determined from the light curves using two very different numerical techniques, i.e., polynomial fitting and direct cross-correlation. The time delay is converted into H 0 following analytical modeling of the potential well. Results. Our best estimate of the time delay is ∆t = 49.5 ± 1.9 days, i.e., we reach a 3.8% accuracy. The R-band flux ratio between the quasar images, corrected for the time delay and for slow microlensing, is F A /F B = 6.2 ± 5%. Conclusions. The accuracy reached on the time delay allows us to discriminate well between families of lens models. As for most other multiply imaged quasars, only models of the lensing galaxy that have a de Vaucouleurs mass profile plus external shear give a Hubble constant compatible with the current most popular value (H 0 = 72 ± 8 km s −1 Mpc −1 ). A more realistic singular isothermal sphere model plus external shear gives H 0 = 51.7 +4.0 −3.0 km s −1 Mpc −1 .

Abstract. We describe a new project aiming at measuring time delays for most known lensed quasars, from optical light curves obtained with five (almost) dedicated 1-2 m telescopes in the Northern and Southern hemispheres. The goal is to evaluate the Hubble constant H 0 with a precision below 2%. We present here numerical simulations in order to define the optimal temporal sampling in our observations as a function of typical quasar variations, object visibility, and for a given accuracy on the individual photometric points. It is also emphasized that the ongoing effort to obtain deep imaging using both space and ground based facilities must be continued, as illustrated by the comparison of HST and VLT near-IR images of the "cloverleaf": H 1413+117.

Aims. The knowledge of the redshift of a lensing galaxy that produces multiple images of a background quasar is essential to any subsequent modeling, whether related to the determination of the Hubble constant H 0 or to the mass profile of the lensing galaxy. We present the results of our ongoing spectroscopic observations of gravitationally lensed quasars in order to measure the redshift of their lensing galaxies. We report on the determination of the lens redshift in seven gravitationally lensed systems. Methods. Our deep VLT/FORS1 spectra are spatially deconvolved in order to separate the spectrum of the lensing galaxies from the glare of the much brighter quasar images. Our observing strategy involves observations in Multi-Object-Spectroscopy (MOS) mode which allows the simultaneous observation of the target and of several crucial PSF and flux calibration stars. The advantage of this method over traditional long-slit observations is that it allows a much more reliable extraction and flux calibration of the spectra. Results. We obtain the first reliable spectra of the lensing galaxies in six lensed quasars: FBQ 0951+2635 (z lens = 0.260), BRI 0952−0115 (z lens = 0.632), HE 2149−2745 (z lens = 0.603), Q 0142−100 (z lens = 0.491), SDSS J0246−0825 (z lens = 0.723), and SDSS J0806+2006 (z lens = 0.573). The last three redshifts also correspond to the Mg ii doublet seen in absorption in the quasar spectra at the lens redshift. Our spectroscopic redshifts of HE 2149−2745 and FBQ 0951+2635 are higher than previously reported, which means that H 0 estimates from these two systems must be revised to higher values. Finally, we reanalyse the blue side of our previously published spectra of Q 1355−2257 and find Mg ii in absorption at z = 0.702, confirming our previous redshift estimate.The spectra of all lenses are typical of early-type galaxies.