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Esko Valtaoja - One of the best experts on this subject based on the ideXlab platform.
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Total Flux density radio observations as a tool for understanding AGN behaviour
Astrophysics and Space Science, 2007Co-Authors: Anne Lähteenmäki, Merja Tornikoski, Talvikki Hovatta, Esko ValtaojaAbstract:A wealth of information on the properties of jets in Active Galactic Nuclei (AGNs) can be derived from Total Flux density observations at high radio frequencies. This includes, for example, the Doppler factor, the Lorentz factor, and the viewing angle of the jet. We have earlier calculated these parameters for a sample of ∼80 sources of different AGN types using almost 20 years of 22 and 37 GHz data from Metsahovi Radio Observatory. We have now gathered data for an additional ten years, and studied the long term characteristic variability time scales of a large sample of AGNs using the first order structure function, the discrete autocorrelation function and the Lomb-Scargle periodogram. Some of the results will be presented in this paper. We also stress the importance of long term observations of AGNs, the main reason for this being misinterpretations of source properties due to insufficient time coverage. Only a few observing epochs will too easily lead to incorrect conclusions about variability, continuum spectra, and the general detectability of the source, not to mention the exclusion of interesting objects from further studies. This is particularly important when considering, for example, the Planck satellite for which the quality of the main mission product, the accurate cosmic microwave background anisotropy maps, depends heavily on the elimination of foreground sources such as AGNs.
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Connections between millimetre continuum variations and VLBI structure in 27 AGN
Astronomy & Astrophysics, 2002Co-Authors: Tuomas Savolainen, Esko Valtaoja, Kaj Wiik, Svetlana G. Jorstad, Alan P. MarscherAbstract:We compare Total Flux density variations in 27-ray blazars with structural changes in their parsec-scale jets using multi-epoch VLBA observations at 22 and 43 GHz together with data from the Metsahovi quasar monitoring program at 22 and 37 GHz. There is a clear connection between Total Flux density outbursts and VLBI components emerging into the jet. For essentially every new moving VLBI component, there is a coincident Total Flux density flare, with evolution similar to that of the component. Furthermore, extrapolated ejection times of the new VLBI components correspond to the beginnings of associated flares. Our results suggest that it is possible to explain all the radio variations as shocks propagating down the jet. A large fraction of the shocks grow and decay within the innermost few tenths of a milliarcsecond and therefore we see them only as "core flares" in the VLBI images. However, with present data we cannot exclude the possibility that the core itself also brightens (and thus contributes to the flare) as a shock passes through it.
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Total Flux Density Variations in Extragalactic Radio Sources. III. Doppler Boosting Factors, Lorentz Factors, and Viewing Angles for Active Galactic Nuclei
The Astrophysical Journal, 1999Co-Authors: Anne Lähteenmäki, Esko ValtaojaAbstract:We have calculated Doppler boosting factors, Dvar, for a sample of active galactic nuclei (AGNs) using Total Flux density variation monitoring data at 22 and 37 GHz. We argue that this method is more accurate than the other commonly used methods based on the synchrotron self-Compton X-ray Flux or equipartition of energy. We compare our Doppler factors with other results and conclude that even if the average Dvar for a class of sources is very similar to all others, the variability Doppler factors for individual objects are more accurate and reliable. An important application of precise Doppler factors is presented, namely, calculating the Lorentz factors, Γ, and the viewing angles, θ, of relativistic outflows inAGNs. We find that high-polarization quasars have the greatest Doppler boosting, while low-polarizationquasars and BL Lac objects are less boosted. The two groups of quasars show different characteristics because of different combinations of the Lorentz factor and viewing angle, rather than either a different Γ or θ alone.
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Total Flux Density Variations in Extragalactic Radio Sources. II. Determining the Limiting Brightness Temperature for Synchrotron Sources
The Astrophysical Journal, 1999Co-Authors: Anne Lähteenmäki, Esko Valtaoja, K. WiikAbstract:The maximum intrinsic brightness temperature Tb, lim for powerful synchrotron-emitting radio sources is usually assumed to be ≈1012 K, limited by the inverse Compton catastrophe. A lower value of ≈5×1010 K, based on the equipartition brightness temperature, has been suggested by Readhead on the basis of Tb, obs distributions derived from VLBI observations. We present two new methods for estimating Tb, lim in extragalactic radio sources by using Total Flux density variations. A reasonable estimate of the value of Tb, lim for a source can be obtained by comparing the Doppler boosting factors derived from Total Flux density variations at 22 and 37 GHz with traditional estimates based on the radio and synchrotron self-Compton (SSC) X-ray Fluxes. Another independent estimate of Tb, lim is obtained by comparing the brightness temperatures derived from variability data with the values calculated from VLBI observations. Using several data sets, we find that both methods yield a value of ≤1011 K, in accordance with the equipartition brightness temperature limit proposed by Readhead.
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Total Flux Density Variations in Extragalactic Radio Sources. I. Decomposition of Variations into Exponential Flares
The Astrophysical Journal Supplement Series, 1999Co-Authors: Esko Valtaoja, Anne Lähteenmäki, Harri Teräsranta, Markku LainelaAbstract:We show that 22 and 37 GHz Total Flux density variations in compact extragalactic radio sources can to a good accuracy be modeled by superposition of a small number of flare components. Both the rise and the decay of these flares are exponential, with a characteristic decay timescale 1.3 times longer than the rise timescale. The properties of the individual model flares derived from these Flux decompositions are in agreement with data obtained from VLBI observations of the corresponding new shock components. The Total Flux density decompositions can be used to search correlations between radio and other regimes, to calibrate and to interpret VLBI observations, and to derive physical parameters of the shocks. In particular, the associated brightness temperatures of the flares can be used to estimate the amount of Doppler boosting in each source and, using additional VLBI data, to derive the intrinsic brightness temperatures, the Lorentz factors and the viewing angles of the sources.
M F Aller - One of the best experts on this subject based on the ideXlab platform.
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3C120: Total Flux variations and evolution of the very-long-baseline interferometry structure
Astronomical & Astrophysical Transactions, 2006Co-Authors: A. E. Volvach, H D Aller, Alexander B. Pushkarev, M F AllerAbstract:Variability research results on the radio source 3C120 are presented using the data from international geodetic very-long-baseline interferometry (VLBI) observations at 2 and 8 GHz, the ongoing monitoring programme within the frequency range 4.8–36 GHz being carried out at the Crimean Astrophysical Observatory (Ukraine) and the Radio Astronomy Observatory of Michigan University (USA). The joint analysis of integral Flux variations and milliarcsecond structures allowed us to detect flares at high frequencies accompanied by the appearance of new VLBI components seen at centimetre wavelengths. It is found that the Flux variations of 3C120 at different frequencies are almost simultaneous, and the outburst in 1998 is accompanied by the birth of a new superluminal component. An activity cycle scenario based on the Flux variation and structure analysis is discussed.
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pearson readhead survey sources ii the long term centimeter band Total Flux and linear polarization properties of a complete radio sample
The Astrophysical Journal, 2003Co-Authors: M F Aller, H D Aller, P A HughesAbstract:Using UMRAO centimeter-band Total Flux density and linear polarization monitoring observations of the complete Pearson-Readhead extragalactic source sample obtained between 1984 August and 2001 March, we identify the range of variability in extragalactic objects as functions of optical and radio morphological classification and relate Total Flux density variations to structural changes in published coeval VLBI maps in selected objects. As expected, variability is common in flat- or inverted-spectrum ( ≤ 0.5) core-dominated QSOs and BL Lac objects. Unexpectedly, we find Flux variations in several steep-spectrum sample members, including the commonly adopted Flux standard 3C 147. Such variations are characteristically several-year rises or declines or infrequent outbursts, requiring long-term observations for detection: we attribute them to the brightening of weak core components, a change that is suppressed by contributions from extended structure in all but the strongest events, and identify a wavelength dependence for the amplitude of this variability consistent with the presence of opacity in some portions of the jet flow. One morphological class of steep-spectrum objects, the compact symmetric objects (CSOs), characteristically shows only low-level variability. We examine the statistical relation between fractional polarization and radio class based on the data at 14.5 and 4.8 GHz. The blazars typically exhibit flat-to-inverted polarization spectra, a behavior attributed to opacity effects. Among the steep-spectrum objects, the lobe-dominated FR I galaxies have steep fractional polarization spectra, while the FR II galaxies exhibit fractional polarization spectra ranging from inverted to steep, with no identifiable common property that accounts for the range in behavior. For the CSO/gigahertz-peaked spectrum sources, we verify that the fractional polarizations at 4.8 GHz are only of the order of a few tenths of a percent, but at 14.5 GHz we find significantly higher polarizations, ranging from 1% to 3%; this frequency dependence supports a scenario invoking Faraday depolarization by a circumnuclear torus. We have identified preferred orientations of the electric vector of the polarized emission (EVPA) at 14.5 and 4.8 GHz in roughly half of the objects and compared these with orientations of the flow direction indicated by VLBI morphology. When comparing the distributions of the orientation offsets for the BL Lac objects and the QSOs, we find differences in both range and mean value, in support of intrinsic class differences. In the shock-in-jet scenario, we attribute this to the allowed range of obliquities of shocks developing in the flow relative to the flow direction: in the BL Lac objects the shocks are nearly transverse to the flow direction, while in the QSOs they include a broader range of obliquities and can be at large angles to it. The fact that we find long-term stability in EVPA over many events implies that a dominant magnetic field orientation persists; in the core-dominated objects, with small contribution from the underlying quiescent jet, this plausibly suggests that the magnetic field has a long-term memory, with subsequent shock events exhibiting similar EVPA orientation, or, alternatively, the presence of a standing shock in the core. We have looked for systematic, monotonic changes in EVPA, which might be expected in the emission from a precessing jet, a model currently invoked for some AGNs; none were identified. Further, we carried out a Scargle periodogram analysis of the Total Flux density observations, but found no strong evidence for periodicity in any of the sample sources. The only well-established case in support of both jet precession and periodic variability remains the non-sample member OJ 287.
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pearson readhead survey sources ii the longterm centimeter band Total Flux and linear polarization properties of a complete radio sample
arXiv: Astrophysics, 2002Co-Authors: M F Aller, H D Aller, P A HughesAbstract:Using UMRAO centimeter-band Total Flux density and linear polarization monitoring observations of the complete Pearson-Readhead extragalactic source sample obtained between August 1984 and March 2001, we identify the range of variability in extragalactic objects as functions of optical and radio morphological classification, and relate Total Flux density variations to structural changes in published coeval VLBI maps in selected objects. We have identified preferred orientations of the electric vector of the polarized emission at 14.5 and 4.8 GHz in roughly half of the objects, and compared these with orientations of the flow direction indicated by VLBI morphology. When comparing the distributions of the orientation offsets for the BL Lacs and for the QSOs, we find differences in both range and mean value, in support of intrinsic class differences. In the shock-in-jet scenario, we attribute this to the allowed range of obliquities of shocks developing in the flow relative to the flow direction. The fact that we find longterm stability in EVPA over many events implies that a dominant magnetic field orientation persists; in the core-dominated objects, with small contribution from the underlying quiescent jet, this plausibly suggests that the magnetic field has a longterm memory, with subsequent shock events exhibiting similar EVPA orientation, or, alternatively, the presence of a standing shock in the core. We have looked for systematic, monotonic changes in EVPA which might be expected in the emission from a precessing jet, a model currently invoked for some AGNs; none were identified. Further, we carried out a Scargle periodogram analysis of the Total Flux density observations, but found no strong evidence for periodicity in any of the sample sources.
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Centimeter-band Variability in GPS Sources
arXiv: Astrophysics, 2002Co-Authors: M F Aller, Philip A. Hughes, H D Aller, Richard M. PlotkinAbstract:Monitoring results are presented for the Stangellini 1 Jy GHz-peaked-spectrum source sample, illustrating that several members exhibit variability in Total Flux and/or linear polarization over timescales of order a decade. The variability occurs while the spectrum, based on the integrated Fluxes, remains steep and characteristic of a transparent source. Total Flux variability is unexpected in view of recent VLBI observations indicating no or hidden cores in several members. However, both the variability, and the detection of circular polarization in one class member, argue for the presence of opacity in some portion of the radio jet.
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centimeter wavelength Total Flux and linear polarization properties of radio loud bl lacertae objects
The Astrophysical Journal, 1999Co-Authors: M F Aller, H D Aller, P A Hughes, G. E. LatimerAbstract:We present results from a long-term program to quantify the range of behavior of the centimeter-wavelength Total Flux and linear polarization variability properties of a sample of 41 radio-loud BL Lac objects using weekly to trimonthly observations with the University of Michigan 26 m telescope operating at 14.5, 8.0, and 4.8 GHz; these observations are used to identify class-dependent differences between these BL Lacs and QSOs in the Pearson-Readhead sample. As a group, the BL Lacs are found to be more highly variable in Total Flux density than the QSOs. These changes are often nearly simultaneous and of comparable amplitude at 14.5 and 4.8 GHz, which contrasts with the behavior in the QSOs and supports the existence of class-dependent differences in opacity within the parsec-scale jet flows. Structure-function analyses of the Flux observations quantify that a characteristic timescale is identifiable in only one-third of the BL Lacs and that in the majority of the program sources the activity is uncorrelated within the timescales probed. The time-averaged fractional linear polarizations are only on the order of a few percent and are consistent with the presence of tangled magnetic fields within the emitting regions. In many sources a preferred long-term orientation of the electric vector position angle is present. When compared with the very long baseline interferometry structural axis, no preferred position angle difference is identified; the derived distribution resembles that known for core components from very long baseline polarimetry measurements. The polarized Flux typically exhibits variability with timescales of months to a few years and exhibits the signature of a propagating shock during several resolved outbursts. The Flux and polarization variability indicate that the source emission is predominately due to evolving source components and supports the occurrence of more frequent shock formation in BL Lac parsec-scale flows than in QSO jets, where the magnetic field topology even during outbursts is similar to that of the underlying quiescent flow. The differences that we find in variability behavior and polarization between BL Lacs and QSOs can be explained by differences in stability between the jet flows found by recent studies of relativistic hydrodynamic flows.
P A Hughes - One of the best experts on this subject based on the ideXlab platform.
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pearson readhead survey sources ii the long term centimeter band Total Flux and linear polarization properties of a complete radio sample
The Astrophysical Journal, 2003Co-Authors: M F Aller, H D Aller, P A HughesAbstract:Using UMRAO centimeter-band Total Flux density and linear polarization monitoring observations of the complete Pearson-Readhead extragalactic source sample obtained between 1984 August and 2001 March, we identify the range of variability in extragalactic objects as functions of optical and radio morphological classification and relate Total Flux density variations to structural changes in published coeval VLBI maps in selected objects. As expected, variability is common in flat- or inverted-spectrum ( ≤ 0.5) core-dominated QSOs and BL Lac objects. Unexpectedly, we find Flux variations in several steep-spectrum sample members, including the commonly adopted Flux standard 3C 147. Such variations are characteristically several-year rises or declines or infrequent outbursts, requiring long-term observations for detection: we attribute them to the brightening of weak core components, a change that is suppressed by contributions from extended structure in all but the strongest events, and identify a wavelength dependence for the amplitude of this variability consistent with the presence of opacity in some portions of the jet flow. One morphological class of steep-spectrum objects, the compact symmetric objects (CSOs), characteristically shows only low-level variability. We examine the statistical relation between fractional polarization and radio class based on the data at 14.5 and 4.8 GHz. The blazars typically exhibit flat-to-inverted polarization spectra, a behavior attributed to opacity effects. Among the steep-spectrum objects, the lobe-dominated FR I galaxies have steep fractional polarization spectra, while the FR II galaxies exhibit fractional polarization spectra ranging from inverted to steep, with no identifiable common property that accounts for the range in behavior. For the CSO/gigahertz-peaked spectrum sources, we verify that the fractional polarizations at 4.8 GHz are only of the order of a few tenths of a percent, but at 14.5 GHz we find significantly higher polarizations, ranging from 1% to 3%; this frequency dependence supports a scenario invoking Faraday depolarization by a circumnuclear torus. We have identified preferred orientations of the electric vector of the polarized emission (EVPA) at 14.5 and 4.8 GHz in roughly half of the objects and compared these with orientations of the flow direction indicated by VLBI morphology. When comparing the distributions of the orientation offsets for the BL Lac objects and the QSOs, we find differences in both range and mean value, in support of intrinsic class differences. In the shock-in-jet scenario, we attribute this to the allowed range of obliquities of shocks developing in the flow relative to the flow direction: in the BL Lac objects the shocks are nearly transverse to the flow direction, while in the QSOs they include a broader range of obliquities and can be at large angles to it. The fact that we find long-term stability in EVPA over many events implies that a dominant magnetic field orientation persists; in the core-dominated objects, with small contribution from the underlying quiescent jet, this plausibly suggests that the magnetic field has a long-term memory, with subsequent shock events exhibiting similar EVPA orientation, or, alternatively, the presence of a standing shock in the core. We have looked for systematic, monotonic changes in EVPA, which might be expected in the emission from a precessing jet, a model currently invoked for some AGNs; none were identified. Further, we carried out a Scargle periodogram analysis of the Total Flux density observations, but found no strong evidence for periodicity in any of the sample sources. The only well-established case in support of both jet precession and periodic variability remains the non-sample member OJ 287.
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pearson readhead survey sources ii the longterm centimeter band Total Flux and linear polarization properties of a complete radio sample
arXiv: Astrophysics, 2002Co-Authors: M F Aller, H D Aller, P A HughesAbstract:Using UMRAO centimeter-band Total Flux density and linear polarization monitoring observations of the complete Pearson-Readhead extragalactic source sample obtained between August 1984 and March 2001, we identify the range of variability in extragalactic objects as functions of optical and radio morphological classification, and relate Total Flux density variations to structural changes in published coeval VLBI maps in selected objects. We have identified preferred orientations of the electric vector of the polarized emission at 14.5 and 4.8 GHz in roughly half of the objects, and compared these with orientations of the flow direction indicated by VLBI morphology. When comparing the distributions of the orientation offsets for the BL Lacs and for the QSOs, we find differences in both range and mean value, in support of intrinsic class differences. In the shock-in-jet scenario, we attribute this to the allowed range of obliquities of shocks developing in the flow relative to the flow direction. The fact that we find longterm stability in EVPA over many events implies that a dominant magnetic field orientation persists; in the core-dominated objects, with small contribution from the underlying quiescent jet, this plausibly suggests that the magnetic field has a longterm memory, with subsequent shock events exhibiting similar EVPA orientation, or, alternatively, the presence of a standing shock in the core. We have looked for systematic, monotonic changes in EVPA which might be expected in the emission from a precessing jet, a model currently invoked for some AGNs; none were identified. Further, we carried out a Scargle periodogram analysis of the Total Flux density observations, but found no strong evidence for periodicity in any of the sample sources.
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centimeter wavelength Total Flux and linear polarization properties of radio loud bl lacertae objects
The Astrophysical Journal, 1999Co-Authors: M F Aller, H D Aller, P A Hughes, G. E. LatimerAbstract:We present results from a long-term program to quantify the range of behavior of the centimeter-wavelength Total Flux and linear polarization variability properties of a sample of 41 radio-loud BL Lac objects using weekly to trimonthly observations with the University of Michigan 26 m telescope operating at 14.5, 8.0, and 4.8 GHz; these observations are used to identify class-dependent differences between these BL Lacs and QSOs in the Pearson-Readhead sample. As a group, the BL Lacs are found to be more highly variable in Total Flux density than the QSOs. These changes are often nearly simultaneous and of comparable amplitude at 14.5 and 4.8 GHz, which contrasts with the behavior in the QSOs and supports the existence of class-dependent differences in opacity within the parsec-scale jet flows. Structure-function analyses of the Flux observations quantify that a characteristic timescale is identifiable in only one-third of the BL Lacs and that in the majority of the program sources the activity is uncorrelated within the timescales probed. The time-averaged fractional linear polarizations are only on the order of a few percent and are consistent with the presence of tangled magnetic fields within the emitting regions. In many sources a preferred long-term orientation of the electric vector position angle is present. When compared with the very long baseline interferometry structural axis, no preferred position angle difference is identified; the derived distribution resembles that known for core components from very long baseline polarimetry measurements. The polarized Flux typically exhibits variability with timescales of months to a few years and exhibits the signature of a propagating shock during several resolved outbursts. The Flux and polarization variability indicate that the source emission is predominately due to evolving source components and supports the occurrence of more frequent shock formation in BL Lac parsec-scale flows than in QSO jets, where the magnetic field topology even during outbursts is similar to that of the underlying quiescent flow. The differences that we find in variability behavior and polarization between BL Lacs and QSOs can be explained by differences in stability between the jet flows found by recent studies of relativistic hydrodynamic flows.
G. E. Latimer - One of the best experts on this subject based on the ideXlab platform.
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Cm-Wavelength Total Flux and Linear Polarization Properties of Radio-Loud BL Lacertae Objects
The Astrophysical Journal, 1999Co-Authors: Margo F. Aller, Hugh D. Aller, Philip A. Hughes, G. E. LatimerAbstract:Results from a long-term program to quantify the range of behavior of the cm-wavelength Total Flux and linear polarization variability properties of a sample of 41 radio-loud BL Lac objects using weekly to tri-monthly observations with the University of Michigan 26-m telescope operating at 14.5, 8.0, and 4.8 GHz are presented; these observations are used to identify class-dependent differences between these BL Lacs and QSOs in the Pearson-Readhead sample. The BL Lacs are found to be more highly variable in Total Flux density than the QSOs, exhibiting changes that are often nearly-simultaneous and of comparable amplitude at 14.5 and 4.8 GHz in contrast to the behavior in the QSOs and supporting the existence of class-dependent differences in opacity within the parsec-scale jet flows. Structure function analyses of the Flux observations quantify that a characteristic timescale is identifiable in only 1/3 of the BL Lacs. The time-averaged fractional linear polarizations are only on the order of a few percent and are consistent with the presence of tangled magnetic fields within the emitting regions. In many sources a preferred long-term orientation of the EVPA is present; when compared with the VLBI structural axis, no preferred position angle difference is identified. The polarized Flux typically exhibits variability with timescales of months to a few years and shows the signature of a propagating shock during several resolved outbursts. The observations indicate that the source emission is predominately due to evolving source components and support the occurrence of more frequent shock formation in BL Lac parsec-scale flows than in QSO jets. The differences in variability behavior and polarization between BL Lacs and QSOs can be explained by differences in jet stability.
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centimeter wavelength Total Flux and linear polarization properties of radio loud bl lacertae objects
The Astrophysical Journal, 1999Co-Authors: M F Aller, H D Aller, P A Hughes, G. E. LatimerAbstract:We present results from a long-term program to quantify the range of behavior of the centimeter-wavelength Total Flux and linear polarization variability properties of a sample of 41 radio-loud BL Lac objects using weekly to trimonthly observations with the University of Michigan 26 m telescope operating at 14.5, 8.0, and 4.8 GHz; these observations are used to identify class-dependent differences between these BL Lacs and QSOs in the Pearson-Readhead sample. As a group, the BL Lacs are found to be more highly variable in Total Flux density than the QSOs. These changes are often nearly simultaneous and of comparable amplitude at 14.5 and 4.8 GHz, which contrasts with the behavior in the QSOs and supports the existence of class-dependent differences in opacity within the parsec-scale jet flows. Structure-function analyses of the Flux observations quantify that a characteristic timescale is identifiable in only one-third of the BL Lacs and that in the majority of the program sources the activity is uncorrelated within the timescales probed. The time-averaged fractional linear polarizations are only on the order of a few percent and are consistent with the presence of tangled magnetic fields within the emitting regions. In many sources a preferred long-term orientation of the electric vector position angle is present. When compared with the very long baseline interferometry structural axis, no preferred position angle difference is identified; the derived distribution resembles that known for core components from very long baseline polarimetry measurements. The polarized Flux typically exhibits variability with timescales of months to a few years and exhibits the signature of a propagating shock during several resolved outbursts. The Flux and polarization variability indicate that the source emission is predominately due to evolving source components and supports the occurrence of more frequent shock formation in BL Lac parsec-scale flows than in QSO jets, where the magnetic field topology even during outbursts is similar to that of the underlying quiescent flow. The differences that we find in variability behavior and polarization between BL Lacs and QSOs can be explained by differences in stability between the jet flows found by recent studies of relativistic hydrodynamic flows.
H D Aller - One of the best experts on this subject based on the ideXlab platform.
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3C120: Total Flux variations and evolution of the very-long-baseline interferometry structure
Astronomical & Astrophysical Transactions, 2006Co-Authors: A. E. Volvach, H D Aller, Alexander B. Pushkarev, M F AllerAbstract:Variability research results on the radio source 3C120 are presented using the data from international geodetic very-long-baseline interferometry (VLBI) observations at 2 and 8 GHz, the ongoing monitoring programme within the frequency range 4.8–36 GHz being carried out at the Crimean Astrophysical Observatory (Ukraine) and the Radio Astronomy Observatory of Michigan University (USA). The joint analysis of integral Flux variations and milliarcsecond structures allowed us to detect flares at high frequencies accompanied by the appearance of new VLBI components seen at centimetre wavelengths. It is found that the Flux variations of 3C120 at different frequencies are almost simultaneous, and the outburst in 1998 is accompanied by the birth of a new superluminal component. An activity cycle scenario based on the Flux variation and structure analysis is discussed.
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pearson readhead survey sources ii the long term centimeter band Total Flux and linear polarization properties of a complete radio sample
The Astrophysical Journal, 2003Co-Authors: M F Aller, H D Aller, P A HughesAbstract:Using UMRAO centimeter-band Total Flux density and linear polarization monitoring observations of the complete Pearson-Readhead extragalactic source sample obtained between 1984 August and 2001 March, we identify the range of variability in extragalactic objects as functions of optical and radio morphological classification and relate Total Flux density variations to structural changes in published coeval VLBI maps in selected objects. As expected, variability is common in flat- or inverted-spectrum ( ≤ 0.5) core-dominated QSOs and BL Lac objects. Unexpectedly, we find Flux variations in several steep-spectrum sample members, including the commonly adopted Flux standard 3C 147. Such variations are characteristically several-year rises or declines or infrequent outbursts, requiring long-term observations for detection: we attribute them to the brightening of weak core components, a change that is suppressed by contributions from extended structure in all but the strongest events, and identify a wavelength dependence for the amplitude of this variability consistent with the presence of opacity in some portions of the jet flow. One morphological class of steep-spectrum objects, the compact symmetric objects (CSOs), characteristically shows only low-level variability. We examine the statistical relation between fractional polarization and radio class based on the data at 14.5 and 4.8 GHz. The blazars typically exhibit flat-to-inverted polarization spectra, a behavior attributed to opacity effects. Among the steep-spectrum objects, the lobe-dominated FR I galaxies have steep fractional polarization spectra, while the FR II galaxies exhibit fractional polarization spectra ranging from inverted to steep, with no identifiable common property that accounts for the range in behavior. For the CSO/gigahertz-peaked spectrum sources, we verify that the fractional polarizations at 4.8 GHz are only of the order of a few tenths of a percent, but at 14.5 GHz we find significantly higher polarizations, ranging from 1% to 3%; this frequency dependence supports a scenario invoking Faraday depolarization by a circumnuclear torus. We have identified preferred orientations of the electric vector of the polarized emission (EVPA) at 14.5 and 4.8 GHz in roughly half of the objects and compared these with orientations of the flow direction indicated by VLBI morphology. When comparing the distributions of the orientation offsets for the BL Lac objects and the QSOs, we find differences in both range and mean value, in support of intrinsic class differences. In the shock-in-jet scenario, we attribute this to the allowed range of obliquities of shocks developing in the flow relative to the flow direction: in the BL Lac objects the shocks are nearly transverse to the flow direction, while in the QSOs they include a broader range of obliquities and can be at large angles to it. The fact that we find long-term stability in EVPA over many events implies that a dominant magnetic field orientation persists; in the core-dominated objects, with small contribution from the underlying quiescent jet, this plausibly suggests that the magnetic field has a long-term memory, with subsequent shock events exhibiting similar EVPA orientation, or, alternatively, the presence of a standing shock in the core. We have looked for systematic, monotonic changes in EVPA, which might be expected in the emission from a precessing jet, a model currently invoked for some AGNs; none were identified. Further, we carried out a Scargle periodogram analysis of the Total Flux density observations, but found no strong evidence for periodicity in any of the sample sources. The only well-established case in support of both jet precession and periodic variability remains the non-sample member OJ 287.
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pearson readhead survey sources ii the longterm centimeter band Total Flux and linear polarization properties of a complete radio sample
arXiv: Astrophysics, 2002Co-Authors: M F Aller, H D Aller, P A HughesAbstract:Using UMRAO centimeter-band Total Flux density and linear polarization monitoring observations of the complete Pearson-Readhead extragalactic source sample obtained between August 1984 and March 2001, we identify the range of variability in extragalactic objects as functions of optical and radio morphological classification, and relate Total Flux density variations to structural changes in published coeval VLBI maps in selected objects. We have identified preferred orientations of the electric vector of the polarized emission at 14.5 and 4.8 GHz in roughly half of the objects, and compared these with orientations of the flow direction indicated by VLBI morphology. When comparing the distributions of the orientation offsets for the BL Lacs and for the QSOs, we find differences in both range and mean value, in support of intrinsic class differences. In the shock-in-jet scenario, we attribute this to the allowed range of obliquities of shocks developing in the flow relative to the flow direction. The fact that we find longterm stability in EVPA over many events implies that a dominant magnetic field orientation persists; in the core-dominated objects, with small contribution from the underlying quiescent jet, this plausibly suggests that the magnetic field has a longterm memory, with subsequent shock events exhibiting similar EVPA orientation, or, alternatively, the presence of a standing shock in the core. We have looked for systematic, monotonic changes in EVPA which might be expected in the emission from a precessing jet, a model currently invoked for some AGNs; none were identified. Further, we carried out a Scargle periodogram analysis of the Total Flux density observations, but found no strong evidence for periodicity in any of the sample sources.
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Centimeter-band Variability in GPS Sources
arXiv: Astrophysics, 2002Co-Authors: M F Aller, Philip A. Hughes, H D Aller, Richard M. PlotkinAbstract:Monitoring results are presented for the Stangellini 1 Jy GHz-peaked-spectrum source sample, illustrating that several members exhibit variability in Total Flux and/or linear polarization over timescales of order a decade. The variability occurs while the spectrum, based on the integrated Fluxes, remains steep and characteristic of a transparent source. Total Flux variability is unexpected in view of recent VLBI observations indicating no or hidden cores in several members. However, both the variability, and the detection of circular polarization in one class member, argue for the presence of opacity in some portion of the radio jet.
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centimeter wavelength Total Flux and linear polarization properties of radio loud bl lacertae objects
The Astrophysical Journal, 1999Co-Authors: M F Aller, H D Aller, P A Hughes, G. E. LatimerAbstract:We present results from a long-term program to quantify the range of behavior of the centimeter-wavelength Total Flux and linear polarization variability properties of a sample of 41 radio-loud BL Lac objects using weekly to trimonthly observations with the University of Michigan 26 m telescope operating at 14.5, 8.0, and 4.8 GHz; these observations are used to identify class-dependent differences between these BL Lacs and QSOs in the Pearson-Readhead sample. As a group, the BL Lacs are found to be more highly variable in Total Flux density than the QSOs. These changes are often nearly simultaneous and of comparable amplitude at 14.5 and 4.8 GHz, which contrasts with the behavior in the QSOs and supports the existence of class-dependent differences in opacity within the parsec-scale jet flows. Structure-function analyses of the Flux observations quantify that a characteristic timescale is identifiable in only one-third of the BL Lacs and that in the majority of the program sources the activity is uncorrelated within the timescales probed. The time-averaged fractional linear polarizations are only on the order of a few percent and are consistent with the presence of tangled magnetic fields within the emitting regions. In many sources a preferred long-term orientation of the electric vector position angle is present. When compared with the very long baseline interferometry structural axis, no preferred position angle difference is identified; the derived distribution resembles that known for core components from very long baseline polarimetry measurements. The polarized Flux typically exhibits variability with timescales of months to a few years and exhibits the signature of a propagating shock during several resolved outbursts. The Flux and polarization variability indicate that the source emission is predominately due to evolving source components and supports the occurrence of more frequent shock formation in BL Lac parsec-scale flows than in QSO jets, where the magnetic field topology even during outbursts is similar to that of the underlying quiescent flow. The differences that we find in variability behavior and polarization between BL Lacs and QSOs can be explained by differences in stability between the jet flows found by recent studies of relativistic hydrodynamic flows.
