Multifrequency observations of the BL Lacertae object 0735 +178.

Four simultaneous spectra covering the radio through the X-ray regimes were made between 1979 October and 1981 March of the variable BL Lac object 0735+178. In each spectrum, the IR-UV synchrotron continuum dominates the total observed power (10⁴⁷ ergs s^-1) and presumably becomes opaque between 10¹¹ and 10¹³ Hz. Extensive nonsimultaneous observations were made over a longer period of time in order to study long- and short-term variability at X-ray, optical, and radio frequencies. These data show that the rapid and dramatic variations evident at infrared and optical wavelengths are absent at radio and X-ray frequencies. These observations support a picture where the IR-UV flux emanates from a small region, while the X-rays are produced by the inverse Compton process in the radio-emitting region. A quantitative model for this picture is presented.