2017ApJ...850..102G

This paper investigates the use of gravitational time delays of macro-lenses to constrain a possible photon mass. The time delay between the two compact images of a source of massive photons is computed. Explicit expressions are given for Schwarzschild and singular isothermal lenses. In the latter case, the time delay is very insensitive to the photon mass. Modeling lens galaxies by a singular isothermal model and a central supermassive black hole, the photon mass-dependent part of the time delay between the compact images is shown to be proportional to the mass of the black hole. The sensitivity of time delays to the photon mass is illustrated by a bound obtained from three active galactic nuclei, which have measurements in several passbands. The bound obtained is comparable to the limit with the deflection of radio waves by the Sun.