Simultaneous soft X-ray and GeV gamma-ray observations of BL Lacertae object AO 0235+164

We present data collected during a simultaneous GeV gamma-ray (Compton Gamma Ray Observatory EGRET) and X-ray (ASCA) observation of BL Lac object AO 0235+164 (z = 0.94); we also present ROSAT PSPC X-ray data, obtained 6 months earlier. The X-ray spectra from both ROSAT and ASCA confirm a substantial photoelectric absorption beyond the value expected from our Galaxy, which is most likely caused by heavy elements in the inerstellar medium of an intervening galaxy at z = 0.524; it is possible to measure the elemental abundances in it module the hydrogen spin temperature T-S. By comparing the absorption in our data to the 21 cm absorption, we can limit TS to at most similar to 700 K. The ROSAT data imply a rapid (similar to 3 day) doubling of the soft X-ray flux and a subsequent quasi-exponential decay, whereas no variability is apparent in the ASCA or EGRET data. We detect a significant spectral change of the BL Lac X-ray continuum between the ROSAT and ASCA observations; ASCA data show that the spectrum is substantially harder (with the energy power-law index alpha similar to 1 vs. similar to 2) and fainter than in the ROSAT data. The simplest interpretation of this spectral variability is that the ROSAT data are dominated by a bright, soft tail of a synchrotron component, which disappeared during the ASCA observation, uncovering a harder Compton component. The EGRET data imply a lower GeV flux than in the previous observations, with no apparent change of the GeV spectrum. The ratio of the gamma-ray to the X-ray flux [in E x F(E)] for this radio-discovered object is roughly 30, which is very substantially higher than that inferred for X-ray-discovered BL Lac objects, suggesting an important difference in the structure between the two subclasses of BL Lac objects.