Microlensing variability of the gravitationally lensed quasar Q0957+561 A,B

The brightness of the two image components of the first discovered gravitational lens, Q0957+561 A,B has been monitored nightly for three years, and occasionally over the 15 years since its discovery in 1979. Historical data give a record back to 1903, so the brightness has been sampled on timescales from 1 day to 90 years. Large amplitude microlensing predicted for an unresolved source has not been observed. Instead, we find microlensing to be limited to 0.3 mag on the longest timescales, and 0.05 mag on timescales of months. We interpret the low amplitude of the observed long-term microlensing in terms of a statistical theory and conclude that the quasar has a luminous source 6 times larger than the Einstein ring for a half-solar mass microlens, or a quasar luminous source size of 0.3 light years. We discuss some implications of the more rapid microlensing features also observed.