On the interpretation of the correlated optical and radio outbursts in AO0235+164

Abstract

A detailed analysis of the spectral properties of the correlated optical and radio outbursts occurred in the BL Lac object AO02235+164 is presented, taking the outburst in Oct.–Nov. 1975 as an example. The observed features can be interpreted reasonably in the frame of relativistic shock-jet models; (1) The background jet spectrum is determined and its structure is analysed on the basis of Königl's jet model. The distributions of electron density and magnetic field along the jet and other parameters are discussed. (2) With the background jet spectrum subtracted, the spectrum of the outburst is determined. It is shown that it can be represented by a uniform synchrotron radiation source with the optically thin spectral index α ∼ 0.25, the break frequency v b 3 × 10 12Hz and the high frequency cutoff v ̃ h ∼ 2 × 10 14 Hz . (3) The adiabatic acceleration of particles and amplification of magnetic field, and the parameters of the emitting regions (i.e. the post-shock regions) are discussed. (4) The simultaneity of the optical and radio outbursts is considered to be due to the optical thinness of the emitting region. The optically thin spectral index in radio band ( α ∼ 0.25) is smaller than that of the background jet spectrum ( α s2 ≅ 0.5) at similar frequencies (∼ 10 12 Hz >), and this can be used to explain the much larger amplitude of variations in optical band. (5) It is suggested that the difference between the spike profile of optical outburst and the broad profile of radio outburst (as an example, for the correlated optical radio outburst in 1979) could be explained by shock propagating through irregularities of energetic electrons and magnetic field along the jet, that is, the scale of the optical emission region along the jet might be much smaller than that of the radio emission region. In summary, the results given here show that the correlated optical-radio outbursts in AO0235+164 may be intrinsic to the source, not due to gravitational micro-lensing effects.