Spectroscopic monitoring of the BL Lac object AO 0235+164

Aims. Spectroscopic monitoring of BL Lac objects is a difficult task that nonetheless can provide important information on the different components of the active galactic nucleus. Methods. We performed optical spectroscopic monitoring of the BL Lac object AO 0235+ 164 ( z = 0.94) with the VLT and TNG telescopes from Aug. 2003 to Dec. 2004, during an extended WEBT campaign. The flux of this source is both contaminated and absorbed by a foreground galactic system at z = 0.524, the stars of which can act as gravitational micro- lenses. Results. In this period the object was in an optically faint, though variable state, and a broad Mg II emission line was visible at all epochs. The spectroscopic analysis reveals an overall variation in the Mg II line flux of a factor 1.9, while the corresponding continuum flux density changed by a factor 4.3. Most likely, the photoionising radiation can be identified with the emission component that was earlier recognised to be present as a UV- soft- X- ray bump in the source spectral energy distribution and that is visible in the optical domain only in very faint optical states. We estimate an upper limit to the broad line region ( BLR) size of a few light months from the historical minimum brightness level; from this we infer the maximum amplification of the Mg II line predicted by the microlensing scenario. Conclusions. Unless we have strongly overestimated the size of the BLR, only very massive stars could significantly magnify the broad Mg II emission line, but the time scale of variations due to these ( rare) events would be of several years. In contrast, the continuum flux, coming from much smaller emission regions in the jet, could be affected by microlensing from the more plausible MACHO deflectors, with variability time scales of the order of some months.