Time delay for the gravitational lens system B0218+357

Measurement of the time delay between multiple images of a gravitational le ns system is potentially an accurate method of determining the Hubble const ant over cosmological distances. One of the most promising candidates for a n application of this technique is the system B0218+357, which was found in the Jodrell Bank/VLA Astrometric Survey (JVAS). This system consists of tw o images of a compact radio source, separated by 335 milliarcsec, and an Ei nstein ring which can provide a strong constraint on the mass distribution in the lens. We present here the results of a three-month VLA monitoring ca mpaign at two frequencies. The data are of high quality, and both images sh ow clear variations in total flux density, percentage polarization and pola rization position angle at both frequencies. The time delay between the var iations in the two images has been calculated using a chi-squared minimizat ion to be 10.5 +/- 0.4 d at 95 per cent confidence, with the error being de rived from Monte Carlo simulations of the light curves. Although mass model ling of the system is at a preliminary stage, taking the lensing galaxy to be a singular isothermal ellipsoid and using the new value for the time del ay, we obtain a value for the Hubble constant of 69(-19)(+13) km s(-1) Mpc( -1), again at 95 per cent confidence.