Further Investigation of the Time Delay, Magnification Ratios, and Variability in the Gravitational Lens 0218+357

Abstract

High-precision VLA flux density measurements for the lensed images of 0218+357 yield a time delay of 10.1 days (95% confidence). This is consistent with independent measurements carried out at the same epoch by Biggs and colleagues in 1999, lending confidence to the robustness of the time delay measurement. However, since both measurements make use of the same features in the light curves, it is possible that the effects of unmodeled processes, such as scintillation or microlensing, are biasing both time delay measurements in the same way. Our time delay estimates result in confidence intervals that are somewhat larger than those of Biggs et al., probably because we adopt a more general model of the source variability, allowing for constant and variable components. When considered in relation to the lens mass model of Biggs et al., our best-fit time delay implies a Hubble constant of H0 = 71 km s-1 Mpc-1 for ?0 = 1 and ?0 = 0 (95% confidence; filled beam). This confidence interval for H0 does not reflect systematic error, which may be substantial, as a result of uncertainty in the position of the lens galaxy. We also measure the flux ratio of the variable components of 0218+357, a measurement of a small region that should more closely represent the true lens magnification ratio. We find ratios of 3.2 (95% confidence; 8 GHz) and 4.3 (15 GHz). Unlike the reported flux ratios on scales of 01, these ratios are not significantly different. We investigate the significance of apparent differences in the variability properties of the two images of the background active galactic nucleus. We conclude that the differences are not significant and that time series much longer than our 100 day time series will be required to investigate propagation effects in this way.