MS 0205.7+3509: A Microlensed BL Lacertae Candidate

In the course of an extensive, deep imaging survey of BL Lacertae objects made at the Canada-France-Hawaii 3.6 m Telescope (CFHT), the BL Lac object MS 0205.7+3509 was found to be significantly decentered (≈1") with respect to its "host" galaxy. In addition, this host is a spiral and, at a tentative z = 0.318, is significantly underluminous with respect to typical BL Lac object hosts. The ROSAT PSPC soft X-ray spectrum shows evidence for an absorbing column well in excess of Galactic at log N_H = 21.2±0.1. Taken together, this evidence suggests but does not conclusively prove that this BL Lac object is background to the spiral through which it is viewed. But no multiple images are found at radio or optical wavelengths suggesting that the foreground galaxy has a very large core radius and/or low surface brightness. Since this makes the fourth such BL Lac object with these general characteristics, a gravitational lensing scenario which creates the observable features of a BL Lac object through microlensing of a background quasar without creating multiple images should be investigated theoretically in some detail.