Transverse Rotation Measure Gradients Research Articles

Evolution of Magnetic Field of the Quasar 1604+159 at parsec Scale

We have analyzed the total intensity, spectral index, linear polarization, and rotation measure (RM) distributions at the parsec scale for the quasar 1604+159. The source was observed at 5.0, 8.4, and 15.4 GHz in 2002 and 4.6, 5.1, 6.0, 7.8, 12.2, 15.2, and 43.9 GHz in 2020 with the American Very Long Baseline Array (VLBA). Combining the polarization results of Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments at 15 GHz from 2009 to 2013, we studied the evolution of magnetic field at the parsec scale for the source. We detected a core-jet structure. The jet extends to the distance of ∼25 mas from the core at a direction of ∼66° north by east. The shape of the jet derived from 15 GHz data varies slightly with time and could be described by a straight line. Based on the linear polarization distribution in 2002, we divided the source structure into the central region and the jet region. In the jet region, we find the polarized emission varies with time. The flatter spectral index values and electric vector position angle direction indicate the possible existence of shocks, contributing to the variation of polarization in the jet with time. In the central region, the derived core shift index k r values indicate that the core in 2002 is close to the equipartition case, while it deviated from the case in 2020. The measured magnetic field strength in 2020 is 2 orders of magnitude lower than that in 2002. We detected transverse RM gradients, evidence of a helical magnetic field, in the core. The polarized emission orients in general toward the jet direction in the core. At 15 GHz, in the place close to the jet base, the polarization direction changes significantly with time from perpendicular to parallel to the jet direction. The evolution of RM and magnetic field structure are potential reasons for the observed polarization change. The core ∣RM∣ in 2020 increases with frequency following a power law with index a = 2.7 ± 0.5, suggesting a fast electron density falloff in the medium with distance from the jet base.

Probing the B-fields of AGN jets on kiloparsec scales - NGC 6251

AbstractBy constructing images of the Faraday rotation measure (RM) of large scale astrophysical jets, the line-of-sight magnetic field component and electron density in the region of Farady rotation can be investigated. A significant gradient in the RM transverse to the jet direction may indicate a corresponding gradient in the line-of-sight magnetic field, implying a toroidal or helical magnetic field, which would, in turn, imply the presence of an associated electrical current in the jet. The detection of such large scale gradients can reliably demonstrate that helical or toroidal fields can persist to large distances from the central AGN. We present a kiloparsec-scale Faraday rotation map of NGC 6251 that shows statistically significant transverse RM gradients across its kiloparsec scale jet structure that imply an outward current.

The jets of AGN as giant coaxial cables

Context. The currents carried by the jets of active galactic nuclei (AGNs) can be probed using maps of the Faraday rotation measure (RM), since a jet current will be accompanied by a toroidal magnetic field, which will give rise to a systematic change in the RM across the jet. Aims. The aim of this study is to identify new AGNs displaying statistically significant transverse RM gradients across their parsec-scale jets, in order to determine how often helical magnetic fields occur in AGN jets, and to look for overall patterns in the implied directions for the toroidal field components and jet currents. Methods. We have carried out new analyses of Faraday RM maps derived from previously published 8.1, 8.4, 12.1 and 15.3 GHz data obtained in 2006 on the NRAO Very Long Baseline Array (VLBA). In a number of key ways, our procedures were identical to those of the original authors, but the new imaging and analysis differs from the original methods in several ways: the technique used to match the resolutions at the different frequencies, limits on the widths spanned by the RM gradients analyzed, treatment of core-region RM gradients, approach to estimation of the significances of the gradients analyzed, and inclusion of a supplementary analysis using circular beams with areas equal to those of the corresponding elliptical naturally weighted beams. Results. This new analysis has substantially increased the number of AGNs known to display transverse RM gradients that may reflect the presence of a toroidal magnetic-field component. The collected data on parsec and kiloparsec scales indicate that the current typically flows inward along the jet axis and outward in a more extended region surrounding the jet, typical to the current structure of a co-axial cable, accompanied by a self-consistent system of nested helical magnetic fields, whose toroidal components give rise to the observed transverse Faraday rotation gradients. Conclusions. The new results presented here make it possible for the first time to conclusively demonstrate the existence of a preferred direction for the toroidal magnetic-field components – and therefore of the currents – of AGN jets. Discerning the origin of this current-field system is of cardinal importance for understanding the physical mechanisms leading to the formation of the intrinsic jet magnetic field, which likely plays an important role in the propagation and collimation of the jets; one possibility is the action of a “cosmic battery”.

Evidence for Toroidal B-Field Components in AGN Jets on Kiloparsec Scales

Though helical magnetic fields are generally believed to arise when the jets of Active Galactic Nuclei (AGN) are launched, it is still unclear what role they play (and if they survive) to the largest jet scales. A helical or toroidal B-field may contribute substantially to the collimation of the jet. This B-field structure can be detected in images of the Faraday rotation measure (RM)—a measure of the change in polarisation angle of an electromagnetic wave as it passes through a magneto-ionic medium. The Faraday rotation measure is directly proportional to the line-of-sight magnetic field; therefore a monotonic gradient in the RM transverse to the jet indicates similar behaviour of the line-of-sight B-field component. This type of analysis has mostly been done on parsec scales using VLBI observations at centimetre wavelengths, while relatively few studies have probed decaparsec to kiloparsec scales. The detection of RM gradients with significances of 3 σ or more on such large scales can demonstrate the presence of a toroidal field component, which may be associated with a helical field that has persisted to these distances from the centre of the AGN. We present the results of new Faraday rotation analyses for 2 AGN on kiloparsec scales based on multiwavelength VLA observations, with robust transverse RM gradients detected in both. Furthermore, the direction of the inferred toroidal B-fields on the sky supports previous results indicating a predominance of outward currents in the jets on kiloparsec scales.

Parsec scale Faraday-rotation structure across the jets of nine active galactic nuclei

A number of groups have recently been active in searching for gradients in the ob- served Faraday rotation measure (RM) across jets of Active Galactic Nuclei (AGNs) on various scales and estimating their reliability. Such RM structures provide direct evidence for the presence of an azimuthal magnetic field component, which may be associated with a helical jet magnetic field, as is expected based on the results of many theoretical studies. We present new parsec-scale RM maps of 4 AGNs here, and analyze their transverse RM structures together with those for 5 previously published RM maps. All these jets display transverse RM gradients with significances of at least 3{\sigma}. This is part of an ongoing effort to establish how common transverse RM gradients that may be associated with helical or toroidal magnetic fields are in AGNs on parsec scales.

The Origin and Structure of the Magnetic Fields and Currents of AGN Jets

This paper reviews observational evidence obtained to date about the overall structure of the magnetic fields in the jets of Active Galactic Nuclei (AGN). Because they are sensitive to the line-of-sight magnetic-field component, Faraday rotation observations of AGN jets provide an effective tool for searching for toroidal jet magnetic fields, whose line-of-sight component changes systematically across the jet. Transverse Faraday rotation measure (RM) gradients providing direct evidence for helical/toroidal magnetic fields have been reliably detected in nearly 40 AGN on parsec scales. Helical magnetic fields are believed to form due to the combined action of the rotation of the central black hole and accretion disk, and these observations demonstrate that at least some of this helical field survives to distances well beyond the Very Long Baseline Interferometry (VLBI) core. Observations of reversals in the direction of the transverse RM gradients in a number of AGN provide evidence for a“return”magnetic field forming a nested helical-field structure with oppositely directed azimuthal components in the inner and outer regions of the helical magnetic field. The collected data now provide firm evidence for a predominance of inward jet currents on parsec scales and outward currents on scales greater than a few tens of parsecs. This suggests a global pattern of magnetic fields and currents with an inward current near the jet axis and an outward current farther from the jet axis, with these currents closing in the accretion disk and far out in the radio lobes, forming a self-consistent set of fields and currents together with the implied nested helical-field structure.

Parsec-scale Faraday rotation and polarization of 20 active galactic nuclei jets

We perform polarimetry analysis of 20 active galactic nuclei (AGN) jets using the Very Long Baseline Array (VLBA) at 1.4, 1.6, 2.2, 2.4, 4.6, 5.0, 8.1, 8.4, and 15.4 GHz. The study allowed us to investigate linearly polarized properties of the jets at parsec-scales: distribution of the Faraday rotation measure (RM) and fractional polarization along the jets, Faraday effects and structure of Faraday-corrected polarization images. Wavelength-dependence of the fractional polarization and polarization angle is consistent with external Faraday rotation, while some sources show internal rotation. The RM changes along the jets, systematically increasing its value towards synchrotron self-absorbed cores at shorter wavelengths. The highest core RM reaches 16,900 rad/m^2 in the source rest frame for the quasar 0952+179, suggesting the presence of highly magnetized, dense media in these regions. The typical RM of transparent jet regions has values of an order of a hundred rad/m^2. Significant transverse rotation measure gradients are observed in seven sources. The magnetic field in the Faraday screen has no preferred orientation, and is observed to be random or regular from source to source. Half of the sources show evidence for the helical magnetic fields in their rotating magnetoionic media. At the same time jets themselves contain large-scale, ordered magnetic fields and tend to align its direction with the jet flow. The observed variety of polarized signatures can be explained by a model of spine-sheath jet structure.

Evidence for a toroidal magnetic-field component in 5C 4.114 on kiloparsec scales

A monotonic, statistically significant gradient in the observed Faraday Rotation Measure (RM) across the jet of an Active Galactic Nucleus (AGN) reflects a corresponding gradient in the electron density and/or line-of-sight magnetic (B) field. Such gradients may indicate the presence of a toroidal B field component, possibly associated with a helical jet B field. Although transverse RM gradients have been reported across a number of parsec-scale AGN jets, the same is not true on kiloparsec scales, suggesting that other (e.g. random) B-field components usually dominate on these larger scales. We have identified an extended, monotonic transverse RM gradient across the Northern lobe of a previously published Very Large Array (kiloparsec-scale) RM image of 5C4.114. We reanalyzed these VLA data in order to determine the significance of this RM gradient. The RM gradient across the Northern kiloparsec-scale lobe structure of 5C4.114 has a statistical significance of about 4sigma. There is also a somewhat less prominent monotonic transverse RM gradient across the Southern jet/lobe (significance ~ 3sigma). Other parts of the RM distribution observed across the source are patchy and show no obvious order.This suggests that we are observing a random RM component associated with the foreground material in the cluster in which the radio source is located and through which it is viewed, superposed on a more ordered RM component that arises in the immediate vicinity of the AGN jets. We interpret the transverse RM gradient as reflecting the systematic variations of the line-of-sight component of a helical or toroidal B field associated with the jets of 5C4.114. These results suggest that the helical field that arises due to the joint action of the rotation of the central black hole and its accretion disc and the jet outflow can survive to distances of thousands of parsec from the central engine.

Transverse Faraday-rotation gradients across the jets of 15 active galactic nuclei

The presence of a helical magnetic field threading the jet of an Active Galactic Nucleus (AGN) should give rise to a gradient in the observed Faraday rotation measure (RM) across the jet, due to the associated systematic change in the line-of-sight magnetic field. Reports of observations of transverse RM gradients across AGN jets have appeared in the literature starting from 2002, but concerns were raised about the resolution required for these gradients to be reliable, and there was a lack of a full understanding of the best approach to accurate estimation of the uncertainties of local RM values. These questions have now been resolved by recent Monte Carlo simulations carried out by various groups, enabling both a verification of previously published results and reliable analyses of new data. We consider here RM gradients across the jet structures of 15 AGN, some previously published in the refereed literature but without a correct and complete error analysis, and some published for the first time here, all of which have monotonic transverse RM gradients with significances of at least 3 sigma.

Connecting magnetic towers with Faraday rotation gradients in active galactic nuclei jets

The idea that systematic Faraday Rotation gradients across the parsec-scale jets of AGNs can reveal the presence of helical magnetic (B) fields has been around since the early 1990s. These gradients are taken to be due to the systematic variation of the line of sight-B-field across the jet. We present here the parsec-scale Faraday Rotation distributions for the BL Lac objects 0716+714 and 1749+701, based on polarization data obtained with the Very Long Baseline Array (VLBA) at two wavelengths near each of the 2cm, 4cm and 6cm bands (0716+714) and at four wavelengths in the range 18-22 cm (1749+701). The Rotation Measure (RM) maps for both these sources indicate systematic gradients across their jets, as expected if these jets have helical B fields. The significance of these transverse RM gradients is > 3 sigma in all cases. We present the results of Monte Carlo simulations directly demonstrating the possibility of observing such transverse RM gradients even if the intrinsic jet structure is much narrower than the observing beam. We observe an intriguing new feature in these sources, a reversal in the direction of the gradient in the jet as compared to the gradient in the core region. This provides new evidence to support models in which field lines emerging from the central region of the accretion disk and closing in the outer region of the accretion disk are both "wound up" by the differential rotation of the disk. The net observed RM gradient will essentially be the sum effect of two regions of helical field, one nested inside the other. The direction of the net RM gradient will be determined by whether the inner or outer helix dominates the RM integrated through the jet, and RM gradient reversals will be observed if the inner and outer helical fields dominate in different regions of the jet. This potentially provides new insights about the overall configuration of the jet B fields.

Magnetic fields and polarization in AGN jets

In the literature there is now a wealth of images of AGN in both linear and circular polarization at milliarcsecond resolution of many sources at multiple epochs and wavelengths. This review is a broad overview of our current state of knowledge of the polarization properties of AGN jets, and of what we can infer about the structure of their magnetic fields and the distribution of Faraday rotating material, and also what is premature to infer. We suggest that while many of the observations of transverse rotation measure gradients may be correct, they have in general been observed with neither enough resolution nor sensitivity to be incontrovertible. Fortunately, this could be remedied using the new broad-band receivers on the VLBA. We also argue that while a transverse rotation measure gradient is a strong indicator of a toroidal component of magnetic field, it does not necessarily imply a helical magnetic field. In jets where the dominant field component appears to be parallel to the jet, it more likely consists of sheared loops or compressed random fields that are not vector ordered. Finally, we draw attention to the power of imaging in all four Stokes parameters at multiple frequencies to constrain the three-dimensional magnetic field structure and the particle content of jets and measure fundamental physical properties. The linear polarization of the radio emission from AGN jets (assumed to be synchrotron radiation) tells us about the structure and order of the magnetic field. Especially on parsec scales, these are intimately related to the launching, acceleration and collimation of the jets (see the review by D. Meier, these proceedings). Monitoring reveals the evo- lution of the magnetic field, and the formation and dissi- pation of shocks. Faraday rotation measurements are an important diagnostic of thermal electrons (and lower en- ergy relativistic electrons) inside and outside the jet, and have revealed a torroidal component of the magnetic field in some sources. Observations of circular polarization can tell us about the positron fraction in the relativistic electron population and the vector-ordering of the magnetic field. The latter then permits an estimate of the net magnetic flux in the jet, and hence (because it is a conserved quantity) the magnetic flux at the central black hole-accretion disk system. It should be noted that none of the physical pa- rameters listed in this paragraph can be gleaned from total intensity images. With the advent of the VLBA in 1993, polarimetry at milliarcsecond resolution came of age. Earlier polariza- tion results, and some of the first polarization results from the VLBA, were discussed in the 1997 Annual Reviews article by Zensus (1). A conference celebrating the 10th birthday of the VLBA (2) featured 18 papers devoted to the polarization properties of AGN on parsec scales. These in- cluded observations of rotation measures, transverse gra-

Evidence for Helical Magnetic fields in Kiloparsec-Scale AGN Jets and the Action of a Cosmic Battery

A search for transverse kiloparsec-scale gradients in Faraday rotation-measure (RM) maps of extragalactic radio sources in the literature has yielded 6 AGNs displaying continuous, monotonic RM gradients across their jets, oriented roughly orthogonal to the local jet direction. The most natural interpretation of such transverse RM gradients is that they are caused by the systematic change in the line-of-sight components of helical magnetic fields associated with these jets. All the identified transverse RM gradients increase in the counterclockwise (CCW) direction on the sky relative to the centers of these AGNs. Taken together with the results of Contopoulos et al. [7], who found evidence for a predominance of clockwise (CW) transverse RM gradients across parsec-scale (VLBI) jets, this provides new evidence for preferred orientations of RM gradients due to helical jet magnetic fields, with a reversal from CW in the inner jets to CCW farther from the centers of activity. This can be explained by the "Poynting-Robertson cosmic-battery" mechanism, which can generate helical magnetic fields with a characteristic "twist," which are expelled with the jet outflows. If the Poynting–Robertson battery mechanism is not operating, an alternative mechanism must be identified, which is able to explain the predominance of CW/CCW RM gradients on parsec/kiloparsec scales.

Faraday rotation in the MOJAVE blazars: 3C 273 a case study

Radio polarimetric observations of Active Galactic Nuclei can reveal the magnetic field structure in the parsec-scale jets of these sources. We have observed the γ-ray blazar 3C 273 as part of our multi-frequency survey with the Very Long Baseline Array to study Faraday rotation in a large sample of jets. Our observations re-confirm the transverse rotation measure gradient in 3C 273. For the first time the gradient is seen to cross zero which is further indication for a helical magnetic field and spine-sheath structure in the jet. We believe the difference to previous epochs is due to a different part of the jet being illuminated in our observations.

ROTATION MEASURES ACROSS PARSEC-SCALE JETS OF FANAROFF-RILEY TYPE I RADIO GALAXIES

We present the results of a parsec-scale polarization study of three FRI radio galaxies - 3C66B, 3C78 and 3C264 - obtained with the Very Long Baseline Array at 5, 8 and 15 GHz. Parsec-scale polarization has been detected in a large number of beamed radio-loud active galactic nuclei, but in only a handful of the relatively unbeamed radio galaxies. We report here the detection of parsec-scale polarization at one or more frequencies in all three FRI galaxies studied. We detect Faraday rotation measures of the order of a few hundred rad/m^2 in the nuclear jet regions of 3C78 and 3C264. In 3C66B polarization was detected at 8 GHz only. A transverse rotation measure gradient is observed across the jet of 3C78. The inner-jet magnetic field, corrected for Faraday rotation, is found to be aligned along the jet in both 3C78 and 3C264, although the field becomes orthogonal further from the core in 3C78. The RM values in 3C78 and 3C264 are similar to those previously observed in nearby radio galaxies. The transverse RM gradient in 3C78, the increase in the degree of polarization at the jet edge, the large rotation in the polarization angles due to Faraday rotation and the low depolarization between frequencies, suggests that a layer surrounding the jet with a sufficient number of thermal electrons and threaded by a toroidal or helical magnetic field is a good candidate for the Faraday rotating medium. This suggestion is tentatively supported by Hubble Space Telescope optical polarimetry but needs to be examined in a greater number of sources.

Concurrent 43 and 86 GHz Very Long Baseline Polarimetry of 3C 273

We present submilliarcsecond resolution total intensity and linear polarization VLBI images of 3C 273, using concurrent 43 and 86 GHz data taken with the Very Long Baseline Array in 2002 May. The structure seen in the innermost jet suggest that we have fortuitously caught the jet in the act of changing direction. The polarization images confirm that the core is unpolarized (fractional polarization ≤1%) at 86 GHz, but also show well ordered magnetic fields (m ~ 15%) in the inner jet, at a projected distance of 2.3 pc from the core. In this strongly polarized region, the rotation measure changes across the jet by ~4.2 × 104 rad m-2 over an angular width of about 0.3 mas. If the lack of polarization in the core is also attributed to a Faraday screen, then a rotation measure dispersion ≳5.2 × 104 rad m-2 must be present in or in front of that region. These are among the highest rotation measures reported so far in the nucleus of any active galaxy or quasar, and must occur outside (but probably close to) the radio emitting region. The transverse rotation measure gradient is in the same sense as that observed by Asada and coworkers and by Zavala and Taylor at greater core distances. The magnitude of the transverse gradient decreases rapidly with distance down the jet, and appears to be variable.

Helical magnetic fields associated with the relativistic jets of four BL Lac objects

Evidence has been mounting that many of the transverse jet B fields observed in BL Lac objects on parsec scales represent the dominant toroidal component of the intrinsic jet B fields. Such fields could come about, for example, as a result of the ‘winding up’ of an initial ‘seed’ field with a significant longitudinal component by the rotation of the central accreting object. If this is the case, this should give rise to gradients in the rotation measure across the jets, due to the systematic change in the line-of-sight component of the jet B field. We present evidence for transverse rotation measure gradients in four BL Lac objects, strengthening arguments that the jets of these objects do indeed have toroidal or helical B fields. This underlines the view of the jets as fundamentally electromagnetic structures, and suggests that they may well carry non-zero currents. It also provides a natural means to collimate the jets.