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Register a new userFlux and color variations of the doubly imaged quasar UM673
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With the aim of characterizing the flux and color variations of the multiple components of the gravitationally lensed quasar UM673 as a function of time, we have performed multi-epoch and multi-band photometric observations with the Danish 1.54m telescope at the La Silla Observatory. The observations were carried out in the VRi spectral bands during four seasons (2008--2011). We reduced the data using the PSF (Point Spread Function) photometric technique as well as aperture photometry. Our results show for the brightest lensed component some significant decrease in flux between the first two seasons (+0.09/+0.11/+0.05 mag) and a subsequent increase during the following ones (-0.11/-0.11/-0.10 mag) in the V/R/i spectral bands, respectively. Comparing our results with previous studies, we find smaller color variations between these seasons as compared with previous ones. We also separate the contribution of the lensing galaxy from that of the fainter and close lensed component.
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We present the results of two-band CCD photometric monitoring of the gravitationally lensed quasar Q 0142-100 (UM 673).The data, obtained at ESO-La Silla with the 1.54 m Danish telescope in the Gunn i-band (October 1998 - September 1999) and in the Johnson V-band (October 1998 to December 2001), were analyzed using three different photometric methods. The light-curves obtained with all methods show variations, with a peak-to-peak amplitude of 0.14 magnitude in $V$. Although it was not possible to measure the time delay between the two lensed QSO images, the brighter component displays possible evidence for microlensing: it becomes bluer as it gets brighter, as expected under the assumption of differential magnification of a quasar accretion disk
We present evidence for ultraviolet/optical microlensing in the gravitationally lensed quasar Q0957+561. We combine new measurements from our optical monitoring campaign at the United States Naval Observatory, Flagstaff (USNO) with measurements from the literature and find that the time-delay-corrected r-band flux ratio m_A - m_B has increased by ~0.1 magnitudes over a period of five years beginning in the fall of 2005. We apply our Monte Carlo microlensing analysis procedure to the composite light curves, obtaining a measurement of the optical accretion disk size, log {(r_s/cm)[cos(i)/0.5]^{1/2}} = 16.2^{+0.5}_{-0.6}, that is consistent with the quasar accretion disk size - black hole mass relation.
We report the discovery of four doubly imaged quasar lenses. All the four systems are selected as lensed quasar candidates from the Sloan Digital Sky Survey data. We confirm their lensing hypothesis with additional imaging and spectroscopic follow-up observations. The discovered lenses are SDSS J0743+2457 with the source redshift z_s=2.165, the lens redshift z_l=0.381, and the image separation theta=1.034, SDSS J1128+2402 with z_s=1.608 and theta=0.844, SDSS J1405+0959 with z_s=1.810, z_l~0.66, and theta=1.978, and SDSS J1515+1511 with z_s=2.054, z_l=0.742, and theta=1.989. It is difficult to estimate the lens redshift of SDSS J1128+2402 from the current data. Two of the four systems (SDSS J1405+0959 and SDSS J1515+1511) are included in our final statistical lens sample to derive constraints on dark energy and the evolution of massive galaxies.
We study brightness variations in the double lensed quasar UM673 (Q0142-100) with the aim of measuring the time delay between its two images. In the paper we combine our previously published observational data of UM673 obtained during the 2003 - 2005 seasons at the Maidanak Observatory with archival and recently observed Maidanak and CTIO UM673 data. We analyze the V, R and I-band light curves of the A and B images of UM673, which cover ten observational seasons from August 2001 to November 2010. We also analyze the time evolution of the difference in magnitudes between images A and B of UM673 over more than ten years. We find that the quasar exhibits both short-term (with amplitude of sim 0.1 mag in the R band) and high-amplitude (sim 0.3 mag) long-term variability on timescales of about several months and several years, respectively. These brightness variations are used to constrain the time delay between the images of UM673. From cross-correlation analysis of the A and B quasar light curves and error analysis we measure the mean time delay and its error of 89 pm11 days. Given the input time delay of 88 days, the most probable value of the delay that can be recovered from light curves with the same statistical properties as the observed R-band light curves of UM673 is 95{+5/-16}{+14/-29} days (68 and 95 % confidence intervals). Analysis of the V - I color variations and V, R and I-band magnitude differences of the quasar images does not show clear evidence of the microlensing variations between 1998 and 2010.
We present a blind time-delay strong lensing (TDSL) cosmographic analysis of the doubly imaged quasar SDSS 1206+4332. We combine the relative time delay between the quasar images, Hubble Space Telescope imaging, the Keck stellar velocity dispersion of the lensing galaxy, and wide-field photometric and spectroscopic data of the field to constrain two angular diameter distance relations. The combined analysis is performed by forward modelling the individual data sets through a Bayesian hierarchical framework, and it is kept blind until the very end to prevent experimenter bias. After unblinding, the inferred distances imply a Hubble constant $H_0 = 68.8^{+5.4}_{-5.1}$ kms$^{-1}$Mpc$^{-1}$, assuming a flat Lambda cold dark matter cosmology with uniform prior on $Omega_{rm m}$ in [0.05, 0.5]. The precision of our cosmographic measurement with the doubly imaged quasar SDSS 1206+4332 is comparable with those of quadruply imaged quasars and opens the path to perform on selected doubles the same analysis as anticipated for quads. Our analysis is based on a completely independent lensing code than our previous three H0LiCOW systems and the new measurement is fully consistent with those. We provide the analysis scripts paired with the publicly available software to facilitate independent analysis. The consistency between blind measurements with independent codes provides an important sanity check on lens modelling systematics. By combining the likelihoods of the four systems under the same prior, we obtain $H_0 = 72.5^{+2.1}_{-2.3}$kms$^{-1}$Mpc$^{-1}$. This measurement is independent of the distance ladder and other cosmological probes.
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