Development of a spectral break in the nonthermal emission of AO 0235+164

Results are reported for braod-band photometry of the BL Lac object AO 0235+164 carried out over the spectral range from 0.36 to 3.5 microns in December 1975 and in October and November 1976. It is found that the continuum of this object steepened at visible wavelengths between December 1975 and October 1976, but maintained a relatively constant slope in the near-IR despite a factor-of-ten decrease in flux from the maximum level. Visible-wavelength data are cited which appear to suggest that the steepening of the visible spectrum occurred in less than 300 days, while the near-IR level may have been nearly constant. It is shown that the observed change in the spectral shape of the visible-wavelength continuum cannot be explained in terms of a 'composite' model consisting of an unchanging galaxy and a varying nonthermal source with constant spectral index. Two other general classes of models are considered: intrinsic variability and extrinsic modulation.