Synchrotron emission from blazar jets - energy distributions and radio variability

This thesis studies the synchrotron radiation of blazars from two perspectives: the spectral energy distributions, and radio emission and its variability. The first gives insight to the stable continuum emission originating in blazar jets, while the latter tells about the ephemeral events occuring in the jet flow, causing an outburst of radiation. The main goal is to establish the typical range of synchrotron emission properties for the fragmented blazar population, and use those data to test for any correlation between the wavelength of maximum synchrotron energy and source luminosity. We determined the synchrotron peak frequencies and luminosities for two samples of blazars using a parabolic fit to archival data. The first was a very large sample of BL Lacertae objects (BLOs) and the second a complete sample of northern 1 Jy active galactic nuclei (AGN), for which we studied especially the Doppler-corrected properties. The range of synchrotron peak frequencies varied from the infrared to X-ray domain, and their distribution was smooth. Neither sample exhibited anti-correlation between the peak frequency and peak luminosity, contrary to the so-called blazar sequence scenario. In fact, when the Doppler-correction is properly applied, the two quantities have a positive correlation. This result was unexpected, but helps solve some inconsistencies in previous blazar research. It will also shed light on the relationship between the spectral energy distributions (SED) and the physics of the nucleus, as well as allows us to link the SED shape to other fundamental jet parameters, such as the jet speed and viewing angle. To find the limits of the typical BLO radio behaviour, we observed a sample of almost 400 BL Lacertae objects in Metsähovi Radio Observatory for almost 4 years. Typically, BLOs are faint at 37 GHz, as only a third of the sample was detected at S ⁄ N > 4. However, there are also very bright and variable objects, which were studied in two separate works. Their radio flux curves were examined at several frequencies and their flaring behaviour determined, both individually and with respect to other subgroups of AGN. When the variable radio emission of blazars is considered, the range of typical behaviour is very large. The bright sources have intense flares of up to 50 Jy. The average flare duration is 2.5 years at 22 and 37 GHz

Long-term variability of radio-bright BL Lacertae objects

Radio-bright BL Lacertae objects (BLOs) are typically variable and exhibit prominent flaring. We use a sample of 24 BLOs to get a clear idea of their flaring behavior and to find possible commonalities in their variability patterns. Our goal was to compare the results given by computational time scales 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. We use long-term monitoring data from 4.8, 8, 14.5, 22, 37, 90 and 230 GHz. The structure function, discrete correlation function and Lomb-Scargle periodogram time scales, calculated in a previous study, are analyzed in more detail. We determine flare durations, rise and decay times, absolute and relative peak fluxes from the monitoring data. We find that BLOs demonstrate a wide range of variability behavior. 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 like 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 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.Comment: Published in the Astronomical Journa

37 GHz observations of a large sample of BL Lacertae objects

We present 37 GHz data obtained at Metsahovi Radio Observatory in 2001 December - 2005 April for a large sample of BL Lacertae objects. We also report the mean variability indices and radio spectral indices in frequency intervals 5 - 37 GHz and 37 - 90 GHz. Approximately 34 % of the sample was detected at 37 GHz, 136 BL Lacertae objects in all. A large majority of the detected sources were low-energy BL Lacs (LBLs). The variability index values of the sample were diverse, the mean fractional variability of the sample being \Delta S_2 = 0.31. The spectral indices also varied widely, but the average radio spectrum of the sample sources is flat. Our observations show that many of the high-energy BL Lacs (HBL), which are usually considered radio-quiet, can at times be detected at 37 GHz.Comment: 12 pages, 5 figures + 5 tables. Published in Astronomical Journa

On the Location of the Gamma-ray Emission in the 2008 Outburst in the BL Lacertae Object AO 0235+164 through Observations across the Electromagnetic Spectrum

We present observations of a major outburst at centimeter, millimeter, optical, X-ray, and gamma-ray wavelengths of the BL Lacertae object AO 0235+164. We analyze the timing of multi-waveband variations in the flux and linear polarization, as well as changes in Very Long Baseline Array (VLBA) images at 7mm with 0.15 milliarcsecond resolution. The association of the events at different wavebands is confirmed at high statistical significance by probability arguments and Monte-Carlo simulations. A series of sharp peaks in optical linear polarization, as well as a pronounced maximum in the 7 mm polarization of a superluminal jet knot, indicate rapid fluctuations in the degree of ordering of the magnetic field. These results lead us to conclude that the outburst occurred in the jet both in the quasi-stationary "core" and in the superluminal knot, both parsecs downstream of the supermassive black hole. We interpret the outburst as a consequence of the propagation of a disturbance, elongated along the line of sight by light-travel time delays, that passes through a standing recollimation shock in the core and propagates down the jet to create the superluminal knot. The multi-wavelength light curves vary together on long time-scales (months/years), but the correspondence is poorer on shorter time-scales. This, as well as the variability of the polarization and the dual location of the outburst, agrees with the expectations of a multi-zone emission model in which turbulence plays a major role in modulating the synchrotron and inverse Compton fluxes.Comment: Accepted for Publication in the Astrophysical Journal Letters. 7 pages (including 5 figures). Minor corrections with regard to previous version, as proposed by the refere

37 GHz observations of a large sample of BL Lacertae objects

ABSTRACT We present 37 GHz data obtained at Metsähovi Radio Observatory from 2001 December to 2005 April for a large sample of BL Lacertae objects. We also report the mean variability indices and radio spectral indices in frequency intervals 5Y37 and 37Y90 GHz. Approximately 34% of the sample was detected at 37 GHz, 136 BL Lacertae objects in all. A large majority of the detected sources were low-energy BL Lac objects. The variability index values of the sample were diverse, the mean fractional variability of the sample being ÁS 2 ¼ 0:31. The spectral indices also varied widely, but the average radio spectrum of the sample sources is flat. Our observations show that many of the highenergy BL Lac objects, which are usually considered radio-quiet, can at times be detected at 37 GHz

The connection between the parsec-scale radio jet and gamma-ray flares in the blazar 1156+295

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