A revised lens time delay for JVAS B0218+357 from a reanalysis of VLA monitoring data

Abstract

We have reanalysed the 1996/1997 VLA monitoring data of the gravitational lens system JVAS B0218+357 to produce improved total flux density and polarization variability curves at 15, 8.4 and 5 GHz. This has been done using improved calibration techniques, accurate subtraction of the emission from the Einstein ring and careful correction of various systematic effects, especially an offset in polarization position angle that is hour-angle dependent. The variations in total and polarized flux density give the best constraints and we determine a combined delay estimate of $11.3 \pm 0.2$ d (1$σ$). This is consistent with the $γ$-ray value recently derived using the Fermi Gamma-ray Space Telescope and thus we find no evidence for a positional shift between the radio and $γ$-ray emitting regions. Combined with the previously published lens model found using LensClean, the new delay gives a value for the Hubble constant of $H_0 = 72.9 \pm 2.6$ km s$^{-1}$ Mpc$^{-1}$ (1$σ$).