A NEW MICROLENSING EVENT IN THE DOUBLY IMAGED QUASAR Q 0957+561

We present evidence for ultraviolet/optical microlensing in the gravitationally lensed quasar Q 0957+561. We combine new measurements from our optical monitoring campaign at the United States Naval Observatory, Flagstaff, with measurements from the literature and find that the time-delay-corrected r-band flux ratio m(A) - m(B) has increased by similar to 0.1 mag over a period of five years beginning in the fall of 2005. We apply our Monte Carlo microlensing analysis procedure to the composite light curves, obtaining a measurement of the optical accretion disk size, log{(r(s)/cm)[ cos(i)/0.5](1/2)} = 16.2 +/- 0.5, that is consistent with the quasar accretion disk size-black hole mass relation.