In its nearly regular cycle of outbursts the quasar OJ 287 is due for another outburst season in 2006-2010. The prediction for the exact timing depends on the adopted model. In the processing binary ...model of Lehto and Valtonen the timing depends on the time delay between the impact on the primary disk and the time when the impacted gas becomes optically thin. The time delay in turn depends on the properties of the accretion disk, the accretion rate, and the viscosity parameter a, which are not exactly known. We study the flexibility in timing provided by the uncertainties. In order to fix the model, two methods are used: the wobble of the jet, induced by the secondary, and the timing of the 1956 outburst, which has not been previously used. As a result, rather definite dates for the outbursts are obtained, which are different from a straightforward extrapolation of the past light curve. A new optical light curve with many new historical as well as recent points of observation have been put together and has been analyzed in order to reach these conclusions. Also, the high-frequency radio observations are found to agree with the jet wobble picture.
Long term monitoring results from 2001 to mid 2004 of quasar observations at 22 and 37 GHz done at the Metsähovi radio observatory are presented. Approximately 10 000 observations are published here.
BL Lacertae has been the target of several observing campaigns by the Whole Earth Blazar Telescope (WEBT) collaboration and is one of the best studied blazars at all accessible wavelengths. A recent ...analysis of the optical and radio variability indicates that part of the radio variability is correlated with the optical light curve. Here we present an analysis of a huge VLBI data set including 108 images at 15, 22, and 43 GHz obtained between 1995 and 2004. The aim of this study is to identify the different components contributing to the single-dish radio light curves. We obtain separate radio light curves for the VLBI core and jet and show that the radio spectral index of single-dish observations can be used to trace the core variability. Cross-correlation of the radio spectral index with the optical light curve indicates that the optical variations lead the radio by about 100 days at 15 GHz. By fitting the radio time lags vs. frequency, we find that the power law is steeper than expected for a freely expanding conical jet in equipartition with energy density decreasing as the square of the distance down the jet as in the Koenigl model. The analysis of the historical data back to 1968 reveals that during a time range of 16 years the optical variability was reduced and its correlation with the radio emission was suppressed. There is a section of the compact radio jet where the emission is weak such that flares propagating down the jet are bright first in the core region with a secondary increase in flux about 1.0 mas from the core. This illustrates the importance of direct imaging to the interpretation of multi- wavelength light curves that can be affected by several distinct components at any given time. We discuss how the complex behaviour of the light curves and correlations can be understood within the framework of a precessing helical jet model.
We have combined new data from our observing campaigns and data from the literature to construct the radio continuum spectra for a sample of mostly quasar-type high peaking gigahertz-peaked spectrum ...(GPS) source candidates. We have also studied the spectra and variability of so called “bona fide” GPS sources and other inverted-spectrum sources from the literature. For many of our sample sources we now have data spanning over two decades, enabling us to study their long term behaviour. Based on our earlier results we expected to find several new high peaking GPS sources. Instead we found out that even most of the “bona fide” GPS sources cease to adhere to the generic GPS source properties when using these well-sampled long term data sets. In our sample of 35 inverted-spectrum sources from the literature only five seem to be consistent with the GPS properties, and even out of these sources two are too sparsely sampled to firmly make conclusions about their variability. Thirteen of the “bona fide” GPS sources exhibit pronounced activity, which diverges from the low variability expected from these sources. None of our new candidates turned out to have both a convex spectrum and little to no variability, but there is one variable source with a consistently convex spectrum. All the rest have flat spectra, but the upper envelope of the spectrum is clearly convex for four extremely variable sources. Similar continuum spectra with a flat lower envelope and a convex upper envelope are observed for eight previously identified inverted-spectrum sources. According to this study the genuine quasar-type GPS sources are rare but there is a large number of highly variable sources that can have a convex spectrum peaking at high radio frequencies (up to ca. 100 GHz) during flares. Many of the GPS sources from the literature have too easily been classified as GPS sources based on too sparse data, and studying the long term variability is essential for identifying the sources with consistently convex continuum spectra.
Context. The quasar B0605−085 (OH 010) shows a hint for probable periodical variability in the radio total flux-density light curves. Aims. We study the possible periodicity of B0605−085 in the total ...flux-density, spectra, and opacity changes in order to compare it with jet kinematics on parsec scales. Methods. We have analyzed archival total flux-density variabilities at ten frequencies (408 MHz, 4.8 GHz, 6.7 GHz, 8 GHz, 10.7 GHz, 14.5 GHz, 22 GHz, 37 GHz, 90 GHz, and 230 GHz) together with the archival high-resolution very long baseline interferometry data at 15 GHz from the MOJAVE monitoring campaign. Using the Fourier transform and discrete autocorrelation methods we have searched for periods in the total flux-density light curves. In addition, spectral evolution and changes of the opacity have been analyzed. Results. We found a period in multi-frequency total flux-density light curves of 7.9 ± 0.5 yrs. Moreover, a quasi-stationary jet component C1 follows a prominent helical path on a similar timescale of eight years. We have also found that the average instantaneous speeds of the jet components show a clear helical pattern along the jet with a characteristic scale of 3 mas. Taking into account average speeds of jet components, this scale corresponds to a timescale of about 7.7 years. Jet precession can explain the helical path of the quasi-stationary jet component C1 and the periodical modulation of the total flux-density light curves. We have fitted a precession model to the trajectory of the jet component C1, with a viewing angle φ0 = 2.6° ± 2.2°, aperture angle of the precession cone Ω = 23.9° ± 1.9° and fixed precession period (in the observers frame) P = 7.9 yrs.
We have carried out a multifrequency analysis of the radio variability of blazars, exploiting the data obtained during the extensive monitoring programs carried out at the University of Michigan ...Radio Astronomy Observatory (UMRAO, at 4.8, 8, and 14.5 GHz) and at the Metsähovi Radio Observatory (22 and 37 GHz). Two different techniques detect, in the Metsähovi light curves, evidence of periodicity at both frequencies for 5 sources (0224+671, 0945+408, 1226+023, 2200+420, and 2251+158). For the last three sources, consistent periods are found also at the three UMRAO frequencies and the Scargle (1982) method yields an extremely low false-alarm probability. On the other hand, the 22 and 37 GHz periodicities of 0224+671 and 0945+408 (which were less extensively monitored at Metsähovi and for which we get a significant false-alarm probability) are not confirmed by the UMRAO database, where some indications of ill-defined periods of about a factor of two longer are retrieved. We have also investigated the variability index, the structure function, and the distribution of intensity variations of the most extensively monitored sources. We find a statistically significant difference in the distribution of the variability index for BL Lac objects compared to flat-spectrum radio quasars (FSRQs), in the sense that the former objects are more variable. For both populations the variability index steadily increases with increasing frequency. The distribution of intensity variations also broadens with increasing frequency, and approaches a log-normal shape at the highest frequencies. We find that variability enhances by 20–30% the high frequency counts of extragalactic radio-sources at bright flux densities, such as those of the WMAP and Planck surveys. In all objects with detected periodicity we find evidence for the existence of impulsive signals superimposed on the periodic component.
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