A gravitational microlensing determination of continuum source size in Q2237+0305

Following the detection of a gravitational microlensing high-magnification event (HME) in Q2237+0305A, attempts have been made to place limits on the dimensions of the quasar continuum source. The analyses have studied either the observed event magnitude or the event duration. The latter approach ha s been hampered by lack of knowledge about the transverse velocity of the l ensing galaxy. We obtain both upper and lower statistical limits on the siz e of the continuum source from the observed HME using determinations of tra nsverse velocity obtained from the published monitoring data. Our calculati ons take account of the caustic orientation as well as the component of the caustic velocity that results from stellar proper motions. Our determinati on of the source size relies on an estimated duration of 52 d for the HME, and so will be refined when more HMEs are observed. We find that the upper and lower limits on the magnified region of the R-band continuum source are 6x10(15) and 2x10(13) cm, respectively (99 per cent confidence). Through c onsideration of the joint probability for source size and mean microlens ma ss we find that the mean mass lies between similar to 0.01 and similar to 1 M. (95 per cent confidence).