Search

Context. Since 2001, the radio quasar 3C 454.3has undergone a period of high optical activity, culminating in the brightest optical state ever observed, during the 2004-2005 outburst. The Whole Earth Blazar Telescope (WEBT) consortium has carried out several multifrequency campaigns to follow the source behaviour.

Aims. We have calculated variability Doppler boosting factors, Lorentz factors, and viewing angles for a large sample of sources by using total flux density observations at 22 and 37 GHz and VLBI data.

Aims. We have studied the characteristic timescales of 80 AGNs at 22, 37 and 90 GHz examining the properties of the wavelet method and comparing them to traditional Fourier-based methods commonly used in astronomy.

Context. The blazar sequence is a scenario in which the bolometric luminosity of the blazar governs the appearance of its spectral energy distribution. The most prominent result is the significant negative correlation between the synchrotron peak frequencies and the synchrotron peak luminosities of the blazar population.

Context.The blazar AO 0235+164was claimed to show a quasi-periodic behaviour in the radio and optical bands in the past, with the main outbursts repeating every 5–6 years. However, the predicted 2004 outburst did not occur, and further analysis suggested a longer time scale, according to which the next event would have occurred in the 2006–2007 observing season. Moreover, an extra emission component contributing to the UV and soft X-ray flux was detected, whose nature is not yet clear. An optical outburst was observed in late 2006–early 2007, which triggered a Whole Earth Blazar Telescope (WEBT) campaign as well as target of opportunity (ToO) observations by the Swift satellite.

Context.The quasar-type blazar 3C 454.3was observed to undergo an unprecedented optical outburst in spring 2005, affecting the source brightness from the near-IR to the X-ray frequencies. This was first followed by a millimetric and then by a radio outburst, which peaked in February 2006.

Aims.We present a study of variability time scales in a large sample of Active Galactic Nuclei at several frequencies between 4.8 and 230 GHz. We investigate the differences of various AGN types and frequencies and correlate the measured time scales with physical parameters such as the luminosity and the Lorentz factor. Our sample consists of both high and low polarization quasars, BL Lacertae objects and radio galaxies. The basis of this work is the 22 GHz, 37 GHz and 87 GHz monitoring data from the Metsähovi Radio Observatory spanning over 25 years. In addition, we used higher 90 GHz and 230 GHz frequency data obtained with the SEST-telescope between 1987 and 2003. Further lower frequency data at 4.8 GHz, 8 GHz and 14.5 GHz from the University of Michigan monitoring programme have been used.

Context.Some intra-day variable, compact extra-galactic radio sources show brightness temperatures severely exceeding 1012K, the limit set by catastrophic inverse-Compton (IC) cooling in sources of incoherent synchrotron radiation. The violation of the IC limit, actually possible under non-stationary conditions, would lead to IC avalanches in the soft-γ-ray energy band during transient periods.

Context. Since 2001, the radio quasar 3C 454.3 has undergone a period of high optical activity, culminating in the brightest optical state ever observed, during the 2004-2005 outburst. The Whole Earth Blazar Telescope (WEBT) consortium has carried out several multifrequency campaigns to follow the source behaviour.Aims. The GLAST-AGILE Support Program (GASP) was born from the WEBT to provide long-term continuous optical-to-radio monitoring of a sample of ?-loud blazars, during the operation of the AGILE and GLAST (now known as Fermi GST) ?-ray satellites. The main aim is to shed light on the mechanisms producing the high-energy radiation, through correlation analysis with the low-energy emission. Thus, since 2008 the monitoring task on 3C?454.3 passed from the WEBT to the GASP, while both AGILE and Fermi detected strong ?-ray emission from the source.Methods. We present the main results obtained by the GASP at optical, mm, and radio frequencies in the 2008-2009 season, and compare them with the WEBT?results from previous years.Results. An optical outburst was observed to peak in mid July?2008, when Fermi detected the brightest ?-ray levels. A?contemporaneous mm outburst maintained its brightness for a longer time, until the cm emission also reached the maximum levels. The behaviour compared in the three bands suggests that the variable relative brightness of the different-frequency outbursts may be due to the changing orientation of a curved inhomogeneous jet. The optical light curve is very well sampled during the entire season, which is also well covered by the various AGILE and Fermi observing periods. The relevant cross-correlation studies will be very important in constraining high-energy emission models.

This paper is a summary of the work that our group has done and recently published in several papers on longterm radio variability of GPS sources. We have studied the longterm up to 30 years variability of GPS sources and candidates, with emphasis on the highfrequency radio domain. Our data sets show that only a relatively small number of these sources retain their convex spectra when they are monitored densely and for long periods of time. The current GPS samples are especially contaminated by small, beamed blazartype sources. Also the remaining population with consistently convex GPStype spectra seems to be heterogeneous, falling into several subpopulations when their observed properties are used for clustering them through a selforganizing map © 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim

Context. The blazar sequence is a scenario in which the bolometric luminosity of the blazar governs the appearance of its spectral energy distribution. The most prominent result is the significant negative correlation between the synchrotron peak frequencies and the synchrotron peak luminosities of the blazar population.Aims. Observational studies of the blazar sequence have, in general, neglected the effect of Doppler boosting. We study the dependence of both the synchrotron peak frequency and luminosity with Doppler-corrected quantities.Methods. We determine the spectral energy distributions of 135 radio-bright AGN and find the best-fit parabolic function for the distribution to quantify their synchrotron emission. The corresponding measurements of synchrotron peak luminosities and frequencies are Doppler-corrected with a new set of Doppler factors calculated from variability data. The relevant correlations for the blazar sequence are determined for these intrinsic quantities.Results. The Doppler factor depends strongly on the synchrotron peak frequency, the lower energy sources being more boosted. Applying the Doppler correction to the peak frequencies and luminosities annuls the negative correlation between the two quantities, which becomes positive. For BL Lacertae objects, the positive correlation is particularly strong.Conclusions. The blazar sequence, when defined as the anticorrelation between the peak frequency and luminosity of the synchrotron component of the spectral energy distribution, disappears when the intrinsic, Doppler-corrected values are used. It is an observational phenomenon created by variable Doppler boosting across the synchrotron peak frequency range.

Aims. We have studied the flare characteristics of 55?AGN at 8?different frequency bands between 4.8 and 230?GHz. Our extensive database enables us to study the various observational properties of flares in these sources and compare our results with theoretical models.Methods. We visually extracted 159 individual flares from the flux density curves and calculated different parameters, such as the peak flux density and duration, in all the frequency bands. The selection of flares is based on the 22 and 37?GHz data from Mets?hovi Radio Observatory and 90 and 230?GHz data from the SEST?telescope. Additional lower frequency?4.8, 8, and 14.5?GHz data are from the University of Michigan Radio Observatory. We also calculated variability indices and compared them with earlier studies.Results. The observations seem to adhere well to the shock model, but there is still large scatter in the data. Especially the time delays between different frequency bands are difficult to study due to the incomplete sampling of the higher frequencies. The average duration of the flares is 2.5?years at 22 and 37?GHz, which shows that long-term monitoring is essential for understanding the typical behaviour in these sources. It also seems that the energy release in a flare is independent of the duration of the flare.

Context. Gigahertz-peaked spectrum (GPS) sources and high frequency peakers (HFPs) are among the smallest of active galactic nuclei currently believed to represent the earliest phases in the evolution of extragalactic radio sources. Recently there has been evidence of contamination by other types of radio sources among the GPS and HFP samples, but the confirmed GPS sources or HFPs also seem to form a very heterogeneous population.Aims. We study the statistical clustering of the GPS sources and the HFPs by taking as many source parameters as possible to find homogeneous groups among the sources. We expect the clustering to give us insight into the physical parameters that play a role in different source populations.Methods. We have collected a sample of 206 GPS sources and HFPs from the literature and gathered a massive database of various source properties, such as the redshift, the size, the polarization, the magnitudes, and the properties of the radio continuum. To visualize and to cluster these multidimensional data we used self-organising maps (SOM), which are neural networks trained by an unsupervised algorithm. We have classified the sources with an auxiliary classification to trace the locations of different types of radio continuum spectra on the map. Results. The sources form distinctive clusters on the map, which is supported by the accordant organisation of the non-numerical parameters not used in the analysis, such as the radio morphology and the optical identification. Our results confirm that the blazars contaminating the GPS and the HFP samples are physically different from the genuine GPS sources and HFPs, and they should be excluded from the samples. The genuine GPS sources form various clusters, which indicates the existence of different subpopulations, besides the expected galaxy-quasar dualism.

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.

Aims.Although the Fermi mission has increased our knowledge of gamma-ray AGN, many questions remain, such as the site of gamma-ray production, the emission mechanism, and the factors that govern the strength of the emission. Using data from a high radio band, 37 GHz, uncontaminated by other radiation components besides the jet emission, we study these questions with averaged flux densities over the the first year of Fermi operations.

We report on the extreme γ-ray activity from the flat spectrum radio quasar (FSRQ) PKS 1510−089 observed by the AGILE satellite in March 2009. In the same period a radio-to-optical monitoring of the source was provided by the GASP–WEBT and REM facilities. In the radio band we made use also of multi-epoch 15-GHz Very Long Baseline Array data from the MOJAVE Program to get information on the parsec-scale structure. Moreover, several Swift target of opportunity observations were triggered, adding important information on the source behaviour from optical/UV to hard X-rays. We paid particular attention to the calibration of the Swift/UVOT data to make it suitable to the blazars spectra. Simultaneous observations from radio to γrays allowed us to study in detail the correlation among the emission variability at differentfrequencies and to investigate the mechanisms at work during this high activity state of the source.

Aims.In a previous study we suggested that the broad-band emission and variability properties of BL Lacertaecan be accounted for by a double synchrotron emission component with relatedinverse-Compton emission from the jet, plus thermal radiation from the accretion disc. Here we investigate the matter with further data extending over a wider energy range.

Aims. We investigate the dependence of γ-ray brightness of blazars on intrinsic properties of their parsec-scale radio jets and the implication for relativistic beaming.Methods. By combining apparent jet speeds derived from high-resolution VLBA images from the MOJAVE program with millimetre-wavelength flux density monitoring data from Metsähovi Radio Observatory, we estimate the jet Doppler factors, Lorentz factors, and viewing angles for a sample of 62 blazars. We study the trends in these quantities between the sources which were detected in γ-rays by the FermiLarge Area Telescope (LAT) during its first three months of science operations and those which were not detected.Results. The LAT-detected blazars have on average higher Doppler factors than non-LAT-detected blazars, as has been implied indirectly in several earlier studies. We find statistically significant differences in the viewing angle distributions between γ-ray bright and weak sources. Most interestingly, γ-ray bright blazars have a distribution of comoving frame viewing angles that is significantly narrower than that of γ-ray weak blazars and centred roughly perpendicular to the jet axis. The lack of γ-ray bright blazars at large comoving frame viewing angles can be explained by relativistic beaming of γ-rays, while the apparent lack of γ-ray bright blazars at small comoving frame viewing angles, if confirmed with larger samples, may suggest an intrinsic anisotropy or Lorentz factor dependence of the γ-ray emission.

Aims. We investigate the dependence of γ-ray brightness of blazars on intrinsic properties of their parsec-scale radio jets and the implication for relativistic beaming.