Abstract Details
| Name: S. Rathna Kumar Affiliation: Physical Research Laboratory Conference ID: ASI2016_587 Title : Determination of $H_0$ through monitoring of gravitationally lensed quasars Authors and Co-Authors : Thesis Advisor: C. S. Stalin (Indian Institute of Astrophysics) Abstract Type : Oral Abstract Category : Thesis Abstract : The Hubble constant at the present epoch ($H_0$), the current expansion rate of the Universe, is an important cosmological parameter on which all extragalactic distances and the age of the universe depend. Multiple approaches are needed to measure its value in order to be able to identify and eliminate unknown systematics present in any one approach. Strong gravitational lensing offers one method, which is based on measuring the differences in light travel-time between the multiple images of a variable background source which is strongly lensed by a galaxy which lies in close proximity to its line of sight. The time delays between the images of the background source can be measured by monitoring their brightness variations and comparing the variability features in their light curves. Converting measured time delays into $H_0$ also requires knowledge of the mass structure of the lensing galaxy as well as the source and lens redshifts. A sample of lensed quasars were photometrically monitored using the 2-m Himalayan Chandra Telescope as part of the COSMOGRAIL campaign, which aims to measure the time delays of most known lensed quasars in both hemispheres, using a network of medium-size optical telescopes. Since the angular separations between the images of lensed quasars are comparable to typical seeing values obtained in ground based telescopes, we made use of MCS deconvolution algorithm to carry out accurate photometry of the individual quasar images. Measurement of time delays between the light curves of quasar images is non-trivial due to the irregular temporal sampling of the light curves arising from telescope scheduling and weather constraints, the presence of seasonal gaps in the light curves spanning several months during non-visibility periods of the object, and the presence of uncorrelated brightness variations in the light curves that could arise due to microlensing of the quasar macro-images by stars in the lensing galaxy. Taking all these factors into account, we devised a technique for time delay measurement called “difference-smoothingâ€. The thesis presented the measurements of time delays of two doubly lensed quasars for the first time. We also analyzed the publicly available light curves of the 24 lensed quasars for which time delays had been reported in the literature in a homogeneous manner using the difference-smoothing technique and identified those systems for which the light curves were of sufficiently good quality to enable reliable measurement of time delays. Of those systems, we further selected a sub-sample for pixellated lens modeling using the publicly available PixeLens code to infer the value of $H_0$. |