Search

We present an algorithm for selecting gravitational lens candidates from amongst Sloan Digital Sky Survey (SDSS) quasars. In median Early Data Release (EDR) conditions, the algorithm allows for the recovery of pairs of equal flux point sources down to separations of $\sim 0{\farcs}7$ or with flux ratios up to $\sim$ 10:1 at a separation of $1\farcs5$. The algorithm also recovers a wide variety of plausible quad geometries. We also present a method for determining the selection function of this algorithm through the use of simulated SDSS images and introduce a method for calibrating our simulated images through truth-testing with real SDSS data. Finally, we apply our algorithm and selection function to SDSS quasars from the EDR to get an upper bound on the lensing fraction. We find 13 candidates among 5120 z $>$ 0.6 SDSS quasars, implying an observed lensing fraction of not more than 4 $\times 10^{-3}$. There is one likely lens system in our final sample, implying an observed lensing fraction of not less than $3 \times 10^{-5}$ (95% confidence levels).

We report follow-up observations of two gravitational lens candidates identified in the Sloan Digital Sky Survey (SDSS) dataset. We have confirmed that SDSS J102111.02+491330.4 is a previously unknown gravitationally lensed quasar. This lens system exhibits two images of a $z = 1.72$ quasar, with an image separation of $1{\farcs}14 \pm 0.04$. Optical and near-IR imaging of the system reveals the presence of the lensing galaxy between the two quasar images. Observations of SDSS J112012.12+671116.0 indicate that it is more likely a binary quasar than a gravitational lens. This system has two quasars at a redshift of $z = 1.49$, with an angular separation of $1{\farcs}49 \pm 0.02$. However, the two quasars have markedly different SEDs and no lens galaxy is apparent in optical and near-IR images of this system. We also present a list of 31 SDSS lens candidates which follow-up observations have confirmed are \textit{not} gravitational lenses., 26 pages, 8 figures. Accepted for publication in AJ

We consider the possibility that future wide-field time-domain optical imaging surveys may be able to discover gravitationally lensed quasar pairs through serendipitous measurements of their time delays. We discuss the merits such a discovery technique would have relative to conventional lens searches. Using simulated quasar lightcurves, we demonstrate that in a survey which observes objects several times each lunar cycle over the course of five years, it is possible to improve the efficiency of a gravitational lens search by 2-3 orders of magnitude through the use of time delay selection. In the most advantageous scenario considered, we are able to improve efficiency by a factor of 1000 with no loss of completeness. In the least advantageous scenario, we are able to improve efficiency by a factor of 110 while reducing completeness by a factor of 9. We show that window function effects associated with the length of the observing season are more important than the total number of datapoints in determining the effectiveness of this method. We also qualitatively discuss several complications which might be relevant to a real time delay search., Comment: 22 pages, 7 figures. Submitted to ApJ

We report the discovery of SDSSJ115517.35+634622.0, a previously unknown gravitationally lensed quasar. The lens system exhibits two images of a $z = 2.89$ quasar, with an image separation of $1{\farcs}832 \pm 0.007$ . Near-IR imaging of the system reveals the presence of the lensing galaxy between the two quasar images. Based on absorption features seen in the Sloan Digital Sky Survey (SDSS) spectrum, we determine a lens galaxy redshift of $z = 0.1756$. The lens is rather unusual in that one of the quasar images is only $0{\farcs}22\pm0{\farcs}07$ ($\sim 0.1 R_{\rm eff}$) from the center of the lens galaxy and photometric modeling indicates that this image is significantly brighter than predicted by a SIS model. This system was discovered in the course of an ongoing search for strongly lensed quasars in the dataset from the SDSS., 18 pages, 6 figures. Accepted for publication in AJ

We report the discovery of a gravitationally lensed quasar resulting from our survey for lenses in the southern sky. Radio images of PMN J1632-0033 with the VLA and ATCA exhibit two compact, flat-spectrum components with separation 1.47" and flux density ratio 13.2. Images with the HST reveal the optical counterparts to the radio components and also the lens galaxy. An optical spectrum of the bright component, obtained with the first Magellan telescope, reveals quasar emission lines at redshift 3.42. Deeper radio images with MERLIN and the VLBA reveal a faint third radio component located near the center of the lens galaxy, which is either a third image of the background quasar or faint emission from the lens galaxy., Comment: 21 pp., including 4 figures; thoroughly revised in light of new MERLIN/HST data; accepted for publication in AJ

We selected Cepheids from the Optical Gravitational Lensing Experiment database for the Magellanic Clouds in the period range of 101.1/=P/=101.4 days. There were 33 objects in the LMC and 35 in the SMC. We find that the median amplitude of Cepheids in the LMC is 18% larger than in the SMC, a 4 sigma effect. For a sample of 42 galactic Cepheids in the same period range, the median amplitude is 7% larger than in the LMC, suggesting that the higher the metal content the larger the amplitude. This implies that the period-flux amplitude relation is not universal and cannot be used to measure distances accurately, unless properly calibrated.