SIMULTANEOUS SOFT-X-RAY AND GEV GAMMA-RAY OBSERVATIONS OF BL-LACERTAEOBJECT 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 o bject AO 0235+164 (z = 0.94); we also present ROSAT PSPC X-ray data, o btained 6 months earlier. The X-ray spectra from both ROSAT and ASCA c onfirm a substantial photoelectric absorption beyond the value expecte d from our Galaxy, which is most likely caused by heavy elements in th e inerstellar medium of an intervening galaxy at z = 0.524; it is poss ible to measure the elemental abundances in it module the hydrogen spi n temperature T-S. By comparing the absorption in our data to the 21 c m absorption, we can limit TS to at most similar to 700 K. The ROSAT d ata imply a rapid (similar to 3 day) doubling of the soft X-ray flux a nd a subsequent quasi-exponential decay, whereas no variability is app arent in the ASCA or EGRET data. We detect a significant spectral chan ge of the BL Lac X-ray continuum between the ROSAT and ASCA observatio ns; ASCA data show that the spectrum is substantially harder (with the energy power-law index alpha similar to 1 vs. similar to 2) and faint er than in the ROSAT data. The simplest interpretation of this spectra l variability is that the ROSAT data are dominated by a bright, soft t ail of a synchrotron component, which disappeared during the ASCA obse rvation, uncovering a harder Compton component. The EGRET data imply a lower GeV flux than in the previous observations, with no apparent ch ange 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 i s very substantially higher than that inferred for X-ray-discovered BL Lac objects, suggesting an important difference in the structure betw een the two subclasses of BL Lac objects.