Search For X-Ray-Emission From Ao0235+164

THE BL Lacertae object, AO0235 + 164, is one of the best documented cases of the inverse-Compton scattering problem, that is, the brightness temperature of the emitting region, as implied by rapid variability and cosmological redshift distance, is well in excess of 1012 K. If the variable emission is produced by incoherent synchrotron radiation, then at these brightness temperatures the electrons would preferentially Compton scatter from their own synchrotron-emitted photons producing an excessive X-ray flux and subsequent cooling. In late-1975 AO0235 + 164 underwent an enormous outburst at radio1,2, infrared and optical3 wavelengths, for which Ledden et al.2 deduced a brightness temperature ≳1015 K at 8 GHz. This source is also an active variable at low frequencies4,5 (≲1 GHz). We describe here our search for X-ray emission from AO0235 + 164.