Measuring gravitational lens time delays using low-resolution radio monitoring observations

Obtaining lensing time-delay measurements requires long-term monitoring campaigns with a high enough resolution (<1 arcsec) to separate the multiple images. In the radio, a limited number of high-resolution interferometer arrays make these observations difficult to schedule. To overcome this problem, we propose a technique for measuring gravitational time delays which relies on monitoring the total flux density with low-resolution but high-sensitivity radio telescopes to follow the variation of the brighter image. This is then used to trigger high-resolution observations in optimal numbers which then reveal the variation in the fainter image. We present simulations to assess the efficiency of this method together with a pilot project observing radio lens systems with the Westerbork Synthesis Radio Telescope to trigger Very Large Array observations. This new method is promising for measuring time delays because it uses relatively small amounts of time on high-resolution telescopes. This will be important because instruments that have high sensitivity but limited resolution, together with an optimum usage of follow-up high-resolution observations from appropriate radio telescopes may in the future be useful for gravitational lensing time-delay measurements by means of this new method.