LONG-TERM VARIABILITY OF RADIO-BRIGHT BL LACERTAE OBJECTS

Abstract

Radio-bright BL Lacertae objects (BLOs) are typically very variable and exhibit prominent flaring. We use a sample of 24 BLOs, regularly monitored at Metsähovi Radio Observatory, to get a clear idea of their flaring behavior in the radio domain and to find possible commonalities in their variability patterns. Our goal was to compare the results given by computational timescales and the observed variability parameters determined directly from the flux curves. Also, we wanted to find out if the BLO flares adhere to the generalized shock model, which gives a schematic explanation for the physical process giving rise to the variability. We use long-term monitoring data from 4.8, 8, 14.5, 22, 37, 90, and 230 GHz, obtained mainly from the University of Michigan and Metsähovi Radio Observatories. The structure function, discrete correlation function, and Lomb–Scargle periodogram timescales, calculated in a previous study, are analyzed in more detail. Also, we determine flare durations, rise and decay times, and absolute and relative peak fluxes from the monitoring data. We find that radio-bright BLOs demonstrate a wide range of variability behavior, and few common denominators can be found. BLOs include sources with fast and strong variability, such as OJ 287, PKS 1749+096, and BL Lac, but also sources with more rolling fluctuations such as PKS 0735+178. The most extreme flares can last for up to 13 years or have peak fluxes of approximately 12 Jy in the observer's frame. When the Doppler boosting effect is taken into account, the peak flux of a flare does not depend on the duration of the flare. A rough analysis of the time lags and peak flux evolution indicates that, typically, BLO flares in the mm–cm wavelengths are high peaking, i.e., are in the adiabatic stage. Thus, the results concur with the generalized shock model, which assigns shocks traveling in the jet as the main cause for active galactic nucleus variability. Comparing the computational timescales and the parameters obtained from the flux curve analysis (i.e., rise and decay times and intervals of the flares) reveals that they do have a significant correlation, albeit with large scatter.