The Recent High State of the BL Lacertae Object AO 0235 and Cross‐Correlations between Optical and Radio Bands

Abstract

We present new optical (B, V , R, I) and radio (at 14.5, 8.5, and 4.8 GHz) observations of the c-rayÈ loud blazar AO 0235]164 obtained during the high state of 1997 DecemberÈ1998 January. The data were combined with historical light curves from the literature to study correlated optical and radio variations over a time span of more than 20 years. Flux variability with large and energy-dependent amplitude is observed at both wave bands, with the source varying over all timescales sampled (yearsmonths-days), in agreement with previous reports. We have performed a cross-correlation analysis of optical and radio light curves applying various detailed statistical methods. The principal results of our analysis can be summarized as follows : (1) we Ðnd that the optical and radio variations exhibit correlated Ñux changes at their "" average ÏÏ level, stressing the conclusion that the same emission mechanism is responsible for the radiation in the two bands (i.e., synchrotron emission from shocked plasma in the jet). However, as previously reported, a few strong Ñares 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 cooling of the synchrotron particles in a radiative shock. (2) Periodic variations are observed at radio frequencies (14.5 and 8.0 GHz) with a pattern repeating every D5.8 years, as indicated by the Lomb-Scargle periodogram. This is the Ðrst report for periodicity at radio wavelengths for this source ; future continuous monitoring is needed to conÐrm this result. (3) Through the analysis of B[V and R[I slopes, we observe large spectral variations, with a "" bimodal ÏÏ behavior. In the Ðrst state, the emission is consistent with a variable power law all across the sampled optical region (from 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 the shorter wavelengths. In general, the power-law slope is not correlated with the Ñux of the source. However, there is an indication that when the source is in the Ðrst state, the spectrum becomes softer as the source brightens.

A multifrequency analysis of radio variability of blazars

“…A possible periodicity of ∼5.7 years in the radio light-curve of the BL Lac object AO 0235 + 16 has been reported by Roy et al (2000) and Raiteri et al (2001). Indications of a periodic or quasi-periodic component with a time-scale of 5.5-6 years (close to that of AO 0235 + 16) of the radio emission of the BL Lac object S5 0716+714 were found by Raiteri et al (2003) In this paper we present a new investigation of radio light curves of a sample of blazars, exploiting more effective techniques than the Periodogram method commonly used (in various versions) for periodicity analysis.…”

Multicolor Optical Monitoring of the Quasar 3C 273 from 2005 to 2016

“…The ZDCF estimates the cross-correlation function (CCF) in the case of non-uniformly sampled light curves, and deals with sparsely and unequally sampled light curves better than both the interpolated cross-correlation function (ICCF) and the discrete correlation function (DCF; Alexander 1997; Edelson et al 1996;Giveon et al 1999;Roy et al 2000). The ZDCF attempts to correct the biases that affect the original DCF by using equal-population binning.…”

Multi-Color Optical Monitoring of MRK 501 From 2010 to 2015

“…Alexander et al 1997). It has been shown in practice that the calculation of the ZDCF is more robust than that of the ICCF and the DCF when applied to sparsely and unequally sampled light curves (e.g., Edelson et al 1996;Giveon et al 1999;Roy et al 2000). The ZDCF is used in our analysis.…”