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 has been hampered by lack of knowledge about the transverse velocity of the lensing galaxy. We obtain both upper and lower statistical limits on the size of the continuum source from the observed HME using determinations of transverse velocity obtained from the published monitoring data. Our calculations take account of the caustic orientation as well as the component of the caustic velocity that results from stellar proper motions. Our determination 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 6 × 10¹⁵ and 2 × 10¹³ cm, respectively (99 per cent confidence). Through consideration of the joint probability for source size and mean microlens mass we find that the mean mass lies between ∼0.01 and ∼1 M_⊙ (95 per cent confidence).