The recent high state of the BL Lacertae object AO 0235 and cross-correlations between optical and radio bands

We present new optical (B, V, R, I) and radio (at 14.5, 8.5, and 4.8 GHz) o bservations of the gamma -ray- loud blazar AO 0235 + 164 obtained during th e high state of 1997 December-1998 January. The data were combined with his torical light curves from the literature to study correlated optical and ra dio variations over a time span of more than 20 years. Flux variability wit h large and energy-dependent amplitude is observed at both wave bands, with the source varying over all timescales sampled (years-months-days), in agr eement with previous reports. We have performed a cross-correlation analysi s of optical and radio light curves applying various detailed statistical m ethods. The principal results of our analysis can be summarized as follows : (1) we find that the optical and radio variations exhibit correlated flux changes at their "average" level, stressing the conclusion that the same e mission mechanism is responsible for the radiation in the two bands (i.e., synchrotron emission from shocked plasma in the jet). However, as previousl y reported, a few strong flares at optical do not have obvious counterparts at longer wavelengths, possibly indicating that an additional component is present in the optical (e.g., microlensing), or, alternatively, rapid cool ing of the synchrotron particles in a radiative shock. (2) Periodic variati ons are observed at radio frequencies (14.5 and 8.0 GHz) with a pattern rep eating every similar to5.8 years, as indicated by the Lomb-Scargle periodog ram. This is the first report for periodicity at radio wavelengths for this source; future continuous monitoring is needed to confirm this result. (3) Through the analysis of B-V and R-I slopes, we observe large spectral vari ations, with a "bimodal" behavior. In the first state, the emission is cons istent with a variable power law all across the sampled optical region (fro m R to V bands); in the second state, the R-I slope is constant while the B -V slope varies, i.e., the continuum has various degrees of curvature at th e shorter wavelengths. In general, the power-law slope is not correlated wi th the flux of the source. However, there is an indication that when the so urce is in the first state, the spectrum becomes softer as the source brigh tens.