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Parsec-scale properties of a complete sample of radio galaxies  [PDF]
T. Venturi,G. Giovannini,L. Feretti,W. D. Cotton,L. Lara
Physics , 2000,
Abstract: We report the most important results on the parsec-scale properties of a complete sample of radio galaxies carried out at radio frequencies with the Global VLBI array and with the VLBA. Relativistic parsec-scale jets are common both in FRI and FRII radio galaxies, and their orientation to the line of sight is in agreement with the expectations from the unified schemes for radio loud AGNs. Proper motion has been detected in a few FRI galaxies in the sample. FRI and FRII radio galaxies exhibit very similar properties on the parsec-scale. Finally a few radio sources in the sample show evidence of velocity structure in the parsec-scale jet, i.e. a central spine with high Lorentz factor $\gamma$ and slower layers.
MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VIII. Faraday rotation in parsec-scale AGN jets  [PDF]
T. Hovatta,M. L. Lister,M. F. Aller,H. D. Aller,D. C. Homan,Y. Y. Kovalev,A. B. Pushkarev,T. Savolainen
Physics , 2012, DOI: 10.1088/0004-6256/144/4/105
Abstract: We report observations of Faraday rotation measures (RMs) for a sample of 191 extragalactic radio jets observed within the MOJAVE program. Multifrequency VLBA observations were carried out over twelve epochs in 2006 at four frequencies between 8 and 15 GHz. We detect parsec-scale Faraday RMs in 149 sources and find the quasars to have larger RMs on average than BL Lac objects. The median core RMs are significantly higher than in the jet components. This is especially true for quasars where we detect a significant negative correlation between the magnitude of the RM and the de-projected distance from the core. We perform detailed simulations of the observational errors of total intensity, polarization and Faraday rotation, and concentrate on the errors of transverse Faraday RM gradients in unresolved jets. Our simulations show that the finite image restoring beam size has a significant effect on the observed RM gradients, and spurious gradients can occur due to noise in the data if the jet is less than two beams wide in polarization. We detect significant transverse RM gradients in four sources (0923+392, 1226+023, 2230+114 and 2251+158). In 1226+023 the RM is for the first time seen to change sign from positive to negative over the transverse cuts, which supports the presence of a helical magnetic field in the jet. In this source we also detect variations in the jet RM over a time scale of three months, which are difficult to explain with external Faraday screens and suggest internal Faraday rotation. By comparing fractional polarization changes in jet components between the four frequency bands to depolarization models we find that an external purely random Faraday screen viewed through only a few lines of sight can explain most of our polarization observations but in some sources, such as 1226+023 and 2251+158, internal Faraday rotation is needed.
On the deceleration of FRI jets: mass loading by stellar-winds  [PDF]
Manel Perucho,José María Martí,Robert A. Laing,Philip E. Hardee
Physics , 2014, DOI: 10.1093/mnras/stu676
Abstract: Jets in low-luminosity radio galaxies are known to decelerate from relativistic speeds on parsec scales to mildly or sub-relativistic speeds on kiloparsec scales. Several mechanisms have been proposed to explain this effect, including strong reconfinement shocks and the growth of instabilities (both leading to boundary-layer entrainment) and mass loading from stellar winds or molecular clouds. We have performed a series of axisymmetric simulations of the early evolution of jets in a realistic ambient medium to probe the effects of mass loading from stellar winds using the code Ratpenat. We study the evolution of Fanaroff-Riley Class I (FRI) jets, with kinetic powers L_j \sim 1.e41-1.e44 erg/s, within the first 2 kpc of their evolution, where deceleration by stellar mass loading should be most effective. Mass entrainment rates consistent with present models of stellar mass loss in elliptical galaxies produce deceleration and effective decollimation of weak FRI jets within the first kiloparsec. However, powerful FRI jets are not decelerated significantly. In those cases where the mass loading is important, the jets show larger opening angles and decollimate at smaller distances, but the overall structure and dynamics of the bow-shock are similar to those of unloaded jets with the same power and thrust. According to our results, the flaring observed on kpc scales is initiated by mass loading in the weaker FRI jets and by reconfinement shocks or the growth of instabilities in the more powerful jets. The final mechanism of decollimation and deceleration is always the development of disruptive pinching modes.
VLBA and Chandra Observations of Jets in FRI radio galaxies: Constraints on Jet Evolution  [PDF]
P. Kharb,C. O'Dea,A. Tilak,S. Baum,E. Haynes,J. Noel-Storr,C. Fallon,K. Christiansen
Physics , 2012, DOI: 10.1088/0004-637X/754/1/1
Abstract: (ABRIDGED) We present here the results from new Very Long Baseline Array observations at 1.6 and 5 GHz of 19 galaxies of a complete sample of 21 UGC FRI radio galaxies. New Chandra data of two sources, viz., UGC00408 and UGC08433, are combined with the Chandra archival data of 13 sources. The 5 GHz observations of ten "core-jet" sources are polarization-sensitive, while the 1.6 GHz observations constitute second epoch total intensity observations of nine "core-only" sources. Polarized emission is detected in the jets of seven sources at 5 GHz, but the cores are essentially unpolarized, except in M87. Polarization is detected at the jet edges in several sources, and the inferred magnetic field is primarily aligned with the jet direction. This could be indicative of magnetic field "shearing" due to jet-medium interaction, or the presence of helical magnetic fields. The jet peak intensity $I_\nu$ falls with distance $d$ from the core, following the relation, $I_\nu\propto d^a$, where $a$ is typically -1.5. Assuming that adiabatic expansion losses are primarily responsible for the jet intensity "dimming", two limiting cases are considered: [1] the jet has a constant speed on parsec-scales and is expanding gradually such that the jet radius $r\propto d^0.4$; this expansion is however unobservable in the laterally unresolved jets at 5 GHz, and [2] the jet is cylindrical and is accelerating on parsec-scales. Accelerating parsec-scale jets are consistent with the phenomenon of "magnetic driving" in Poynting flux dominated jets. Chandra observations of 15 UGC FRIs detect X-ray jets in nine of them. The high frequency of occurrence of X-ray jets in this complete sample suggests that they are a signature of a ubiquitous process in FRI jets.
The jets in 3C 296  [PDF]
M. J. Hardcastle,P. Alexander,G. G. Pooley,J. M. Riley
Physics , 1997, DOI: 10.1093/mnras/288.1.1L
Abstract: We present observations made with the VLA at 1.5 and 8.4 GHz of the nearby FRI radio galaxy 3C296. The most recent models of FRI radio galaxies suggest that substantial deceleration must take place in their jets, with strongly relativistic velocities on parsec scales giving place to at most mildly relativistic velocities on scales of tens of kiloparsecs. The region over which this deceleration takes place is therefore of considerable interest. By considering the side-to-side asymmetries of the jets of 3C296, we constrain the region of strong deceleration in the source. Our observations show evidence that the jets have slow edges surrounding faster central spines. We discuss the implications of our observations for models of the magnetic field structure in these objects.
Are There Rotation Measure Gradients Across AGN Jets?  [PDF]
G. B. Taylor,R. Zavala
Physics , 2010, DOI: 10.1088/2041-8205/722/2/L183
Abstract: We report on multi-frequency polarimetry Very Long Baseline Interferometry observations of active galactic nuclei using the VLBA. These observations are used to construct images of the Faraday Rotation Measure (RM) in J1613+342, Mrk 501, 3C 371, and BL Lac. Despite having resolved the jets in total intensity and polarization for three of these sources no RM gradients are found. This is in contrast to the large fraction of sources with RM gradients now claimed in the literature, and invoked as evidence in support of helical magnetic fields. We propose objective criteria for establishing what constitutes an RM gradient. Furthermore, although we note the absence of simple, monotonic gradients, comparison with simulations could reveal systematic changes in the RM which may be masked by a varying jet orientation.
Parsec-scale Faraday Rotation Measures from General Relativistic MHD Simulations of Active Galactic Nuclei Jets  [PDF]
Avery E. Broderick,Jonathan C. McKinney
Physics , 2010, DOI: 10.1088/0004-637X/725/1/750
Abstract: For the first time it has become possible to compare global 3D general relativistic magnetohydrodynamic (GRMHD) jet formation simulations directly to very-long baseline interferometric multi-frequency polarization observations of the pc-scale structure of active galactic nucleus (AGN) jets. Unlike the jet emission, which requires post hoc modeling of the non-thermal electrons, the Faraday rotation measures (RMs) depend primarily upon simulated quantities and thus provide a robust way in which to confront simulations with observations. We compute RM distributions of 3D global GRMHD jet formation simulations, with which we explore the dependence upon model and observational parameters, emphasizing the signatures of structures generic to the theory of MHD jets. With typical parameters, we find that it is possible to reproduce the observed magnitudes and many of the structures found in AGN jet RMs, including the presence of transverse RM gradients. In our simulations the RMs are generated within a smooth extension of the jet itself, containing ordered toroidally dominated magnetic fields. This results in a particular bilateral morphology that is unlikely to arise due to Faraday rotation in distant foreground clouds. However, critical to efforts to probe the Faraday screen will be resolving the transverse jet structure. Therefore, the RMs of radio cores may not be reliable indicators of the properties of the rotating medium. Finally, we are able to constrain the particle content of the jet, finding that at pc-scales AGN jets are electromagnetically dominated, with roughly 2% of the comoving energy in nonthermal leptons and much less in baryons.
The evolution of the large-scale emission in FRI jets  [PDF]
Pol Bordas,Valentí Bosch-Ramon,Manel Perucho
Physics , 2010, DOI: 10.1111/j.1365-2966.2010.17982.x
Abstract: Recent observations in X-rays and gamma-rays of nearby FRI radio galaxies have raised the question of the origin of the emission detected in the termination structures of their jets. The study of these structures can give information on the conditions for particle acceleration and radiation at the front shocks. In addition, an evolutionary scenario can help to disentangle the origin of the detected X-ray emission in young FRI sources, like some Gigahertz Peaked Spectrum AGNs. This work focuses on the nature and detectability of the radiation seen from the termination regions of evolving FRI jets. We use the results of a relativistic, two-dimensional numerical simulation of the propagation of an FRI jet, coupled with a radiation model, to make predictions for the spectra and lightcurves of the thermal and non-thermal emission at different stages of the FRI evolution. Our results show that under moderate magnetic fields, the synchrotron radiation would be the dominant non-thermal channel, appearing extended in radio and more compact in X-rays, with relatively small flux variations with time. The shocked jet synchrotron emission would dominate the X-ray band, although the shocked ISM/ICM thermal component alone may be significant in old sources. Inverse Compton scattering of CMB photons could yield significant fluxes in the GeV and TeV bands, with a non-negligible X-ray contribution. The IC radiation would present a bigger angular size in X-rays and GeV than in TeV, with fluxes increasing with time. We conclude that the thermal and non-thermal broadband emission from the termination regions of FRI jets could be detectable for sources located up to distances of a few 100 Mpc.
Faraday Rotation Measures in the Parsec-Scale Jets of the Radio Galaxies M87, 3C 111, and 3C 120  [PDF]
R. T. Zavala,G. B. Taylor
Physics , 2002, DOI: 10.1086/339441
Abstract: Parsec-scale Faraday rotation measure (RM) maps are presented for the radio galaxies M87, 3C 111, and 3C 120. These Very Long Baseline Array observations were made at 8, 12, and 15 GHz. M87 has an extreme RM distribution that varies from -4000 radians m^-2 to more than 9000 radians m^-2 across a projected distance of 0.3 pc in its jet. M87 has no polarized flux closer than 17 mas from the core. 3C 111 and 3C 120 both show polarized emission in their cores that is consistent with the expectations of unified schemes for these broad-line radio galaxies. 3C 111 has an RM gradient that increases from -200 radians m^-2 four mas from the core to -750 radians m^-2 on the side of the jet closest to the core. 3C 120 has a more moderate RM distribution in the jet of approximately 100 radians m^-2, but this increases by an order of magnitude in the core.
Studying magnetic fields in several parsec-scale AGN jets using Faraday Rotation  [PDF]
Andrea Reichstein,Denise Gabuzda
Physics , 2011,
Abstract: We present multi-frequency radio observations from the Very Long Baseline Array (VLBA) of selected AGN that seem to have a B-field structure with a central "spine" of B-field orthogonal to the jet and a longitudinal B-field near one or both edges of the jet. Two explanations for this structure have been discussed in the literature: shocks making the central orthogonal field combined with a jet-medium interaction causing the longitudinal "sheath", or both components produced by a helical jet magnetic field. One way to investigate this structure is to look for gradients in the Faraday Rotation across the jet. We will discuss results for 0333+321, 1150+812 and 2037+511 providing evidence for the latter picture.
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