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Search Results: 1 - 10 of 369398 matches for " D. C. Gabuzda "
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Surprising Evolution of the Parsec-scale Faraday Rotation Gradients in the Jet of the BL Lac Object B1803+784
M. Mahmud,D. C. Gabuzda,V. Bezrukovs
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15013.x
Abstract: Several multi-frequency polarization studies have shown the presence of systematic Faraday Rotation gradients across the parsec-scale jets of Active Galactic Nuclei (AGN), taken to be due to the systematic variation of the line-of-sight component of a helical magnetic (B) field across the jet. Other studies have confirmed the presence and sense of these gradients in several sources, thus providing evidence that these gradients persist over time and over large distances from the core. However, we find surprising new evidence for a reversal in the direction of the Faraday Rotation gradient across the jet of B1803+784, for which multi-frequency polarization observations are available at four epochs. At our three epochs and the epoch of Zavala & Taylor (2003), we observe transverse Rotation Measure (RM) gradients across the jet, consistent with the presence of a helical magnetic field wrapped around the jet. However, we also observe a "flip" in the direction of the gradient between June 2000 and August 2002. Although the origins of this phenomena are not entirely clear, possibly explanations include (i) the sense of rotation of the central supermassive black hole and accretion disc has remained the same, but the dominant magnetic pole facing the Earth has changed from North to South; (ii) a change in the direction of the azimuthal B field component as a result of torsional oscillations of the jet; and (iii) a change in the relative contributions to the observed rotation measures of the "inner" and "outer" helical fields in a magnetic-tower model. Although we cannot entirely rule out the possibility that the observed changes in the RM distribution are associated instead with changes in the thermal-electron distribution in the vicinity of the jet, we argue that this explanation is unlikely.
Pc-scale study of Radio galaxies & BL Lacs
P. Kharb,P. Shastri,D. C. Gabuzda
Physics , 2003, DOI: 10.1016/S1387-6473(03)00065-4
Abstract: We study two aspects of the differences between Active Galactic Nuclei (AGN) of the two Fanaroff-Riley types in order to investigate the causes of the F-R divide. We (a) contrast the properties of the optical cores in beamed and unbeamed AGN of the two types, incorporating Hubble Space Telescope measurements of the unbeamed objects, and (b) contrast the nuclear magnetic field geometry of the beamed AGN of the two types using Very Long Baseline Polarimetry.
Magnetic Field Geometry in "Red" and "Blue" BL Lacs
P. Kharb,D. C. Gabuzda,P. Shastri
Physics , 2003, DOI: 10.1023/B:ASTR.0000004993.84117.d4
Abstract: We compare the systematics of the magnetic field geometry in the "red" low-energy peaked BL Lacs (LBLs) and "blue" high-energy peaked BL Lacs (HBLs) using VLBI polarimetric images. The LBLs are primarily "radio--selected" BL Lacs and the HBLs are primarily "X-ray selected". In contrast to the LBLs, which show predominantly transverse jet magnetic fields, the HBLs show predominantly longitudinal fields. Thus, while the SED peaks of core-dominated quasars, LBLs and HBLs form a sequence of increasing frequency, the magnetic field geometry does not follow an analogous sequence. We briefly investigate possible connections between the observed parsec-scale magnetic field structures and circular polarization measurements in the literature on various spatial scales.
Hints of High Core Faraday Rotations from a Joint Analysis of VLBA and Optical Polarization Data
J. C. Algaba,D. C. Gabuzda,P. S. Smith
Physics , 2009,
Abstract: Although the continua of radio-loud Active Galactic Nuclei (AGN) are typically dominated by synchrotron radiation over virtually the entire spectrum, it is not clear whether the radio and higher-frequency emission originate in the same or different parts of the jet. Several different radio--optical correlations based on polarization data have been found recently, suggesting that the optical and radio polarization may be closely related, and that the corresponding emission regions may be cospatial (Gabuzda et. al2006, Jorstad et al. 2007, D'Arcangelo et al. 2007) Our joint analysis of optical and VLBA polarization data for a sample of about 40 AGNs shows that, after correction for the inferred VLBA core Faraday rotations, most BL Lac objects and some quasars have aligned VLBA-core and optical polarizations, although many quasars also show no obvious relationship between their VLBA-core and optical polarization angles. This may indicate that not all AGNs have cospatial regions of optical and radio emission in their jets. However, another possibility is that some of the 7mm-2cm VLBA cores have Faraday rotations of the order of several tens of thousand of rad/m^2, which were not properly fit by our three-frequency data due to n*pi ambiguities in the observed polarization angles, leading to incorrect subtraction of the effects of the core Faraday rotation, and so incorrect "zero-wavelength" radio polarization angles. The possibility of such high core Faraday rotations is supported by the results of the parsec-scale Faraday-rotation studies of Zavala & Taylor (2004) and Jorstad et al. (2007).
Search for correlations between the optical and radio polarization of AGNs II: VLBA polarization data at 12+15+22+24+43 GHz
J. C. Algaba,D. C. Gabuzda,P. S. Smith
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.20061.x
Abstract: Previous research showed most BL Lac objects and some quasars have aligned VLBI-core and optical polarizations, although some of the AGNs also showed no obvious relationship between VLBI-core and optical polarization angles. This may indicate that some AGNs have co-spatial regions of optical and radio emission, while others do not. Another possibility is that some of the VLBI cores had Faraday rotations of several tens of thousand of rad/m^2, which were not properly fit using the three-frequency data due to n\pi ambiguities in the observed polarization angles, leading to incorrect subtraction of the effects of the core Faraday rotation, and so incorrect intrinsic radio polarization angles \chi_0. We obtained additional 12+15+22+24+43GHz plus optical observations for 8 of 40 AGNs previously considered. Our results indicate that, although some VLBI radio cores have comparatively high rotation measures, this alone cannot explain the misalignments found between the radio core and optical VLBI polarization angles, \Delta\chi = |\chi_opt - \chi_0|. Comparison between \Delta\chi and (i) the orientation of \chi_0 relative to the jet direction, (ii) the degree of polarization of the core, (iii) a depolarization factor, (iv) the core rotation measures and (v) the core magnetic fields 1pc from the jet base do not yield evidence for any correlations between these properties. There is some evidence that the maximum observed \Delta\chi tends to decrease as the core-region magnetic field increases, suggesting that misalignments in \Delta\chi could be associated in part with relatively low core magnetic fields. Thus, although the overall distribution of \Delta\chi for all 40 sources in our sample does show a significant peak at \Delta\chi\sim0, it remains unclear what distinguishes these AGN cores from those showing appreciable misalignment between optical and VLBI-core polarization position angles.
The Radio Structure of High-Energy Peaked BL Lacertae Objects
T. A. Rector,D. C. Gabuzda,J. T. Stocke
Physics , 2003, DOI: 10.1086/367802
Abstract: We present VLA and first-epoch VLBA observations that are part of a program to study the parsec-scale radio structure of a sample of fifteen high-energy-peaked BL Lacs (HBLs). The sample was chosen to span the range of logarithmic X-ray to radio flux ratios observed in HBLs. As this is only the first epoch of observations, proper motions of jet components are not yet available; thus we consider only the structure and alignment of the parsec- and kiloparsec-scale jets. Like most low-energy-peaked BL Lacs (LBLs), our HBL sample shows parsec-scale, core-jet morphologies and compact, complex kiloparsec-scale morphologies. Some objects also show evidence for bending of the jet 10-20pc from the core, suggesting interaction of the jet with the surrounding medium. Whereas LBLs show a wide distribution of parsec- to kpc-scale jet misalignment angles, there is weak evidence that the jets in HBLs are more well-aligned, suggesting that HBL jets are either intrinsically straighter or are seen further off-axis than LBL jets.
Magnetic Fields in Astrophysical Jets: From Launch to Termination
R. E. Pudritz,M. J. Hardcastle,D. C. Gabuzda
Physics , 2012, DOI: 10.1007/s11214-012-9895-z
Abstract: Long-lived, stable jets are observed in a wide variety of systems, from protostars, through Galactic compact objects to active galactic nuclei (AGN). Magnetic fields play a central role in launching, accelerating, and collimating the jets through various media. The termination of jets in molecular clouds or the interstellar medium deposits enormous amounts of mechanical energy and momentum, and their interactions with the external medium, as well, in many cases, as the radiation processes by which they are observed, are intimately connected with the magnetic fields they carry. This review focuses on the properties and structures of magnetic fields in long-lived jets, from their launch from rotating magnetized young stars, black holes, and their accretion discs, to termination and beyond. We compare the results of theory, numerical simulations, and observations of these diverse systems and address similarities and differences between relativistic and non-relativistic jets in protostellar versus AGN systems. On the observational side, we focus primarily on jets driven by AGN because of the strong observational constraints on their magnetic field properties, and we discuss the links between the physics of these jets on all scales.
Analysing the Transverse Structure of the Relativistic Jets of AGN
E. G. Murphy,T. V. Cawthorne,D. C. Gabuzda
Physics , 2013, DOI: 10.1093/mnras/sts561
Abstract: This paper describes a method of fitting total intensity and polarization profiles in VLBI images of astrophysical jets to profiles predicted by a theoretical model. As an example, the method is used to fit profiles of the jet in the Active Galactic Nucleus Mrk501 with profiles predicted by a model in which a cylindrical jet of synchrotron plasma is threaded by a magnetic field with helical and disordered components. This fitting yields model Stokes Q profiles that agree with the observed profiles to within the 1-2 \sigma uncertainties; the I model and observed profiles are overall not in such good agreement, with the model I profiles being generally more symmetrical than the observed profiles. Consistent fitting results are obtained for profiles derived from 6cm VLBI images at two distances from the core, and also for profiles obtained for different wavelengths at a single location in the VLBI jet. The most striking success of the model is its ability to reproduce the spine-sheath polarization structure observed across the jet. Using the derived viewing angle in the jet rest frame, \delta' approximately 83 degrees, together with a superluminal speed reported in the literature, \beta apparent = 3.3, yields a solution for the viewing angle and velocity of the jet in the observer's frame \delta degrees and \beta approximately 0.96. Although these results for Mrk501 must be considered tentative, the combined analysis of polarization profiles and apparent component speeds holds promise as a means of further elucidating the magnetic field structures and other parameters of parsec-scale AGN jets.
Unusual Radio Properties of the BL Lac Object 0820+225
D. C. Gabuzda,A. B. Pushkarev,N. N. Garnich
Physics , 2001, DOI: 10.1046/j.1365-8711.2001.04446.x
Abstract: We present the results of simultaneous VLBA polarisation observations of the BL Lacertae object 0820+225 at 5, 8, and 15 GHz, together with earlier images at 5 GHz. This source has an unusually rich total intensity and polarisation structure compared to other objects with comparable redshifts. The magnetic field in the inner part of the complex and highly twisted VLBI jet is transverse, as is typical of BL Lacertae objects, but becomes roughly longitudinal further from the core, possibly due to shear. Although the integrated rotation measure of 0820+225 is modest, the rotation-measure distribution on parsec scales is non-uniform, and clearly shows regions where the rotation measure is substantially higher than the integrated value.
Analysis of 6cm VLBI polarization observations of a complete sample of northern BL Lacertae Objects
D. C. Gabuzda,A. B. Pushkarev,T. V. Cawthorne
Physics , 2003, DOI: 10.1046/j.1365-8711.2000.03932.x
Abstract: The results of VLBI total intensity (I) and linear polarization (P) observations at 6cm are presented for ten radio bright BL Lacertae objects. These images complete first-epoch polarization observations for the 1-Jy sample of northern BL Lacertae objects defined by Kuhr and Schmidt. Estimates of superluminal speeds are presented for several sources, bringing the total number of sources in the sample for which such estimates are available to 16. Second epoch observations currently being reduced should yield speed estimates for VLBI features in essentially all the sources in the sample. The jet magnetic fields of these BL Lacertae objects are usually transverse to the local jet direction, but a sizeable minority (about 30%) have VLBI jet components with longitudinal magnetic fields. This may suggest that the conditions in the VLBI jets of BL Lacertae objects are favorable for the formation of relativistic shocks; alternatively, it may be that the toroidal component of the intrinsic jet magnetic field is characteristically dominant in these sources.
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