A measurement of the transverse velocity of Q2237+0305

Determination of microlensing parameters in the gravitationally lensed quasar Q2237+0305 from the statistics of high magnification events will require monitoring for more than 100 years (Wambsganss, Paczynski & Schneider 1990). However we show that the effective transverse velocity of the lensing galaxy can be determined on a more realistic time-scale through consideration of the distribution of light-curve derivatives. The 10 years of existing monitoring data for Q2237+0305 is analysed. This data displays strong evidence for microlensing that is not associated with a high magnification event. An upper limit of vt < 500 km sec −1 is obtained for the galactic transverse velocity which is smaller than previously assumed values.