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. Methods. We decomposed the flux curves into exponential flares and determined the variability brightness temperatures of the fastest flares. By assuming the same intrinsic brightness temperature for each source, we calculated the Doppler boosting factors for 87 sources. In addition we used new apparent jet speed data to calculate the Lorentz factors and viewing angles for 67 sources. Results. We find that all quasars in our sample are Doppler-boosted and that the Doppler boosting factors of BL Lacertae objects are lower than of quasars. The new Lorentz factors are about twice as high as in earlier studies, which is mainly due to higher apparent speeds in our analyses. The jets of BL Lacertae objects are slower than of quasars. There are some extreme sources with very high derived Lorentz factors of the order of a hundred. These high Lorentz factors could be real. It is also possible that the sources exhibit such rapid flares that the fast variations have remained undetected in monitoring programmes, or else the sources have a complicated jet structure that is not amenable to our simple analysis. Almost all the sources are seen in a small viewing angle of less than 20 degrees. Our results follow the predictions of basic unification schemes for AGN.

We present results from monitoring the multi-waveband flux, linear polarization, and parsec-scale structure of the quasar PKS 1510−089, concentrating on -2eight major γ-ray flares that occurred during the interval 2009.0-2009.5. The γ-ray peaks were essentially simultaneous with maxima at optical wavelengths, although the flux ratio of the two wavebands varied by an order of magnitude. The optical polarization vector rotated by 720 • during a 5-day period encompassing six of these flares. This culminated in a very bright, ∼ 1 day, optical and γ-ray flare as a bright knot of emission passed through the highest-intensity, stationary feature (the "core") seen in 43 GHz Very Long Baseline Array images. The knot continued to propagate down the jet at an apparent speed of 22c and emit strongly at γ-ray energies as a months-long X-ray/radio outburst intensified. We interpret these events as the result of the knot following a spiral path through a mainly toroidal magnetic field pattern in the acceleration and collimation zone of the jet, after which it passes through a standing shock in the 43 GHz core and then continues downstream. In this picture, the rapid γ-ray flares result from scattering of infrared seed photons from a relatively slow sheath of the jet as well as from optical synchrotron radiation in the faster spine. The 2006-2009.7 radio and X-ray flux variations are correlated at very high significance; we conclude that the X-rays are mainly from inverse Compton scattering of infrared seed photons by 20-40 MeV electrons.

We report on the γ -ray activity of the high-synchrotron-peaked BL Lacertae object Markarian 421 (Mrk 421) during the first 1.5 years of Fermi operation, from 2008 August 5 to 2010 March 12. We find that the Large Area Telescope (LAT) γ -ray spectrum above 0.3 GeV can be well described by a power-law function with photon index Γ = 1.78 ± 0.02 and average photon flux F (>0.3 GeV) = (7.23 ± 0.16) × 10 −8 ph cm −2 s −1 . Over this time period, the Fermi-LAT spectrum above 0.3 GeV was evaluated on seven-day-long time intervals, showing significant variations in the photon flux (up to a factor ∼3 from the minimum to the maximum flux) but mild spectral variations. The variability amplitude at X-ray frequencies measured by RXTE/ASM and Swift/BAT is substantially larger than that in γ -rays measured by Fermi-LAT, and these two energy ranges are not significantly correlated. We also present the first results from the 4.5 month long multifrequency campaign on Mrk 421, which included the VLBA, Swift, RXTE, MAGIC, the F-GAMMA, GASP-WEBT, and other collaborations and instruments that provided excellent temporal and energy coverage of the source throughout the entire campaign During this campaign, Mrk 421 showed a low activity at all wavebands. The extensive multi-instrument (radio to TeV) data set provides an unprecedented, complete look at the quiescent spectral energy distribution (SED) for this source. The broadband SED was reproduced with a leptonic (one-zone synchrotron self-Compton) and a hadronic model (synchrotron proton blazar). Both frameworks are able to describe the average SED reasonably well, implying comparable jet powers but very different characteristics for the blazar emission site.

We present results from a parsec-scale jet kinematics study of 409 bright radio-loud AGNs based on 15 GHz VLBA data obtained between 1994 August 31 and 2016 December 26 as part of the 2cm VLBA survey and MOJAVE programs. We tracked 1744 individual bright features in 382 jets over at least five epochs. A majority (59%) of the best-sampled jet features showed evidence of accelerated motion at the > 3σ level. Although most features within a jet typically have speeds within ∼ 40% of a characteristic median value, we identified 55 features in 42 jets that had unusually slow pattern speeds, nearly all of which lie within 4 pc (100 pc deprojected) of the core feature. Our results combined with other speeds from the literature indicate a strong correlation between apparent jet speed and synchrotron peak frequency, with the highest jet speeds being found only in low-peaked AGNs. Using Monte Carlo simulations, we find best fit parent population parameters for a complete sample of 174 quasars above 1.5 Jy at 15 GHz. Acceptable fits are found with a jet population that has a simple unbeamed power law luminosity function incorporating pure luminosity evolution, and a power law Lorentz factor distribution ranging from 1.25 to 50 with slope −1.4 ± 0.2. The parent jets of the brightest radio quasars have a space density of 261 ± 19 Gpc −3 and unbeamed 15 GHz luminosities above ∼ 10 24.5 W Hz −1 , consistent with FR II class radio galaxies.