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 in 2008. 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 7mm 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 at >12 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.
Astron.Astrophys. 450 (2006) 667-680 We study the polarization of the SiO maser emission in a representative
sample of evolved stars in order to derive an estimate of the strength of the
magnetic ...field, and thus determine the influence of this magnetic field on
evolved stars. We made simultaneous spectroscopic measurements of the 4 Stokes
parameters, from which we derived the circular and linear polarization levels.
The observations were made with the IF polarimeter installed at the IRAM 30m
telescope. A discussion of the existing SiO maser models is developed in the
light of our observations. Under the Zeeman splitting hypothesis, we derive an
estimate of the strength of the magnetic field. The averaged magnetic field
varies between 0 and 20 Gauss, with a mean value of 3.5 Gauss, and follows a
1/r law throughout the circumstellar envelope. As a consequence, the magnetic
field may play the role of a shaping, or perhaps collimating agent of the
circumstellar envelopes in evolved objects.
We locate the gamma-ray and lower frequency emission in flares of the BL Lac object AO 0235+164 at >12pc in the jet of the source from the central engine. We employ time-dependent ...multi-spectral-range flux and linear polarization monitoring observations, as well as ultra-high resolution (~0.15 milliarcsecond) imaging of the jet structure at lambda=7mm. The time coincidence in the end of 2008 of the propagation of the brightest superluminal feature detected in AO 0235+164 (Qs) with an extreme multi-spectral-range (gamma-ray to radio) outburst, and an extremely high optical and 7mm (for Qs) polarization degree provides strong evidence supporting that all these events are related. This is confirmed at high significance by probability arguments and Monte-Carlo simulations. These simulations show the unambiguous correlation of the gamma-ray flaring state in the end of 2008 with those in the optical, millimeter, and radio regime, as well as the connection of a prominent X-ray flare in October 2008, and of a series of optical linear polarization peaks, with the set of events in the end of 2008. The observations are interpreted as the propagation of an extended moving perturbation through a re-collimation structure at the end of the jet's acceleration and collimation zone.
We analyze the multifrequency behavior of the quasar 3C 454.3 during three prominent \gamma-ray outbursts: 2009 Autumn, 2010 Spring, and 2010 Autumn. The data reveal a repeating pattern, including a ...triple flare structure, in the properties of each \gamma-ray outburst, which implies similar mechanism(s) and location for all three events. The multi-frequency behavior indicates that the lower frequency events are co-spatial with the \gamma-ray outbursts, although the \gamma-ray emission varies on the shortest timescales. We determine that the variability from UV to IR wavelengths during an outburst results from a single synchrotron component whose properties do not change significantly over the different outbursts. Despite a general increase in the degree of optical linear polarization during an outburst, the polarization drops significantly at the peak of the \gamma-ray event, which suggests that both shocks and turbulent processes are involved. We detect two disturbances (knots) with superluminal apparent speeds in the parsec-scale jet associated with the outbursts in 2009 Autumn and 2010 Autumn. The kinematic properties of the knots can explain the difference in amplitudes of the \gamma-ray events, while their millimeter-wave polarization is related to the optical polarization during the outbursts. We interpret the multi-frequency behavior within models involving either a system of standing conical shocks or magnetic reconnection events located in the parsec-scale millimeter-wave core of the jet. We argue that \gamma-ray outbursts with variability timescales as short as ~ 3 hr can occur on parsec scales if flares take place in localized regions such as turbulent cells.
We present a survey of the polarization properties of mm-wavelength methanol masers, comprising both classes, and transitions from 84.5 to 157.0 GHz. Linear polarization is found in more than half of ...the observed objects, and circular polarization is tentatively detected in two sources. Class I and Class II CH_3OH masers show similar polarization properties.The largest linear polarization is found in the 133 GHz Class I maser towards L379 (39.5 %), and in the 157 GHz Class II maser towards G9.62+0.19 (36.7 %). The spectral profiles of the polarization angle of Class I masers are mostly flat, except for two sources showing a linear slope. Since the mm-line methanol masers are expected to be weakly (or not) saturated, we suggest that the stronger fractional polarizations found by us are enhanced by anisotropic pumping and radiative losses. In NGC 7538, we find, for both maser classes, a good agreement between our polarization angles, and those measured for the submillimeter dust continuum. This can be taken as evidence for magnetic alignment of dust grains. It is also possible that an unsaturated maser with equally populated magnetic substates simply amplifies polarized continuum seed radiation. For Class II masers, the polarization properties of the various velocity components towards a given source with detectable polarization are quite homogeneous. A possible explanation is discussed. Since methanol is non-paramagnetic, the circular polarization of the unsaturated maser emission can only be due to variations of the angle between the magnetic field and the line of sight along the maser propagation path.
We combine time-dependent multi-waveband flux and linear polarization observations with sub-milliarcsecond-scale polarimetric images at lambda=7mm of the BL Lacertae-type blazar OJ287 to locate the ...gamma-ray emission in prominent flares in the jet of the source >14pc from the central engine. We demonstrate a highly significant correlation between the strongest gamma-ray and millimeter-wave flares through Monte-Carlo simulations. The two reported gamma-ray peaks occurred near the beginning of two major mm-wave outbursts, each of which is associated with a linear polarization maximum at millimeter wavelengths. Our Very Long Baseline Array observations indicate that the two mm-wave flares originated in the second of two features in the jet that are separated by >14 pc. The simultaneity of the peak of the higher-amplitude gamma-ray flare and the maximum in polarization of the second jet feature implies that the gamma-ray and mm-wave flares are co-spatial and occur >14 pc from the central engine. We also associate two optical flares, accompanied by sharp polarization peaks, with the two gamma-ray events. The multi-waveband behavior is most easily explained if the gamma-rays arise from synchrotron self-Compton scattering of optical photons from the flares. We propose that flares are triggered by interaction of moving plasma blobs with a standing shock. The gamma-ray and optical emission is quenched by inverse Compton losses as synchrotron photons from the newly shocked plasma cross the emission region. The mm-wave polarization is high at the onset of a flare, but decreases as the electrons emitting at these wavelengths penetrate less polarized regions.
We study the polarization of the SiO maser emission in a representative sample of evolved stars in order to derive an estimate of the strength of the magnetic field, and thus determine the influence ...of this magnetic field on evolved stars. We made simultaneous spectroscopic measurements of the 4 Stokes parameters, from which we derived the circular and linear polarization levels. The observations were made with the IF polarimeter installed at the IRAM 30m telescope. A discussion of the existing SiO maser models is developed in the light of our observations. Under the Zeeman splitting hypothesis, we derive an estimate of the strength of the magnetic field. The averaged magnetic field varies between 0 and 20 Gauss, with a mean value of 3.5 Gauss, and follows a 1/r law throughout the circumstellar envelope. As a consequence, the magnetic field may play the role of a shaping, or perhaps collimating agent of the circumstellar envelopes in evolved objects.
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.
We analyze the behavior of the parsec-scale jet of the quasar 3C~454.3 during pronounced flaring activity in 2005-2008. Three major disturbances propagated down the jet along different trajectories ...with Lorentz factors \(\Gamma>\)10. The disturbances show a clear connection with millimeter-wave outbursts, in 2005 May/June, 2007 July, and 2007 December. High-amplitude optical events in the \(R\)-band light curve precede peaks of the millimeter-wave outbursts by 15-50 days. Each optical outburst is accompanied by an increase in X-ray activity. We associate the optical outbursts with propagation of the superluminal knots and derive the location of sites of energy dissipation in the form of radiation. The most prominent and long-lasting of these, in 2005 May, occurred closer to the black hole, while the outbursts with a shorter duration in 2005 Autumn and in 2007 might be connected with the passage of a disturbance through the millimeter-wave core of the jet. The optical outbursts, which coincide with the passage of superluminal radio knots through the core, are accompanied by systematic rotation of the position angle of optical linear polarization. Such rotation appears to be a common feature during the early stages of flares in blazars. We find correlations between optical variations and those at X-ray and \(\gamma\)-ray energies. We conclude that the emergence of a superluminal knot from the core yields a series of optical and high-energy outbursts, and that the mm-wave core lies at the end of the jet's acceleration and collimation zone.
We present multi-wavelength studies of the radio galaxy 3C 120 and the blazar CTA 102 during unprecedented gamma-ray flares for both sources. In both studies the analysis of gamma-ray data has been ...compared with a series of 43 GHz VLBA images from the VLBA-BU-BLAZAR program, providing the necessary spatial resolution to probe the parsec scale jet evolution during the high energy events. To extend the radio dataset for 3C 120 we also used 15 GHz VLBA data from the MOJAVE sample. These two objects which represent very different classes of AGN, have similar properties during the gamma-ray events. The gamma-ray flares are associated with the passage of a new superluminal component through the mm VLBI core, but not all ejections of new components lead to gamma-ray events. In both sources gamma-ray events occurred only when the new components are moving in a direction closer to our line of sight. We locate the g-ray dissipation zone a short distance from the radio core but outside of the broad line region, suggesting synchrotron self-Compton scattering as the probable mechanism for the gamma-ray production.
