Simultaneous multifrequency observations of the BL Lac MS 0205.7+3509

Radio and optical observations of the possible microlensed BL Lac source MS 0205.7+3509 were obtained simultaneously with ASCA x-ray measurements in F ebruary 1997. A single power law model, with a photon index of 2.61, is an adequate fit to the ASCA data, once hydrogenic absorption in excess of the Galactic value is permitted, confirming a previous ROSAT measurement. On th e basis of our simultaneous data we have determined MS 0205.7+3509 to be a typical x-ray selected BL Lac, with alpha(xox) = -0.82. There is no indicat ion of an inverse Compton (IC) component in the ASCA spectrum up to 10 keV. No evidence for variability on hour-long timescales is present in either t he x-ray or the optical data. We discuss these results in the context of a gravitational microlensing scenario for MS 0205.7+3509.