Radiative Processes in Astrophysics. Radio Polarization

Transcription

1 Radiative Processes in Astrophysics. Radio Polarization Cormac Reynolds May 24, 2010

2 Radio Polarization Topics Description of the polarized state Stokes parameters. (TMS 4.8) Why are radio sources polarized? (RL 6.5, L ) Faraday rotation. (L ) Polarization calibration. (TMS 4.8) Bibliography TMS: Interferometry and Synthesis in Radio Astronomy (Second Edition), Thompson, Moran and Swenson. Wiley Interscience RL: Rybicki and Lightman, Radiative Processes in Astrophysics. Wiley Interscience L: Longair, High Energy Astrophysics (Second Edition). Cambridge University Press

3 Describing the Polarized Wave Stokes Parameters The polarized state of a wave can be described in terms of 4 parameters, known as Stokes parameters. I gives the total energy flux or the total intensity Q, U give the linear polarization. V gives the circular polarization. V 0 intrinsically for synchrotron radiation. The complex polarization, P, is then given as P = Q + iu = mie 2iχ = pe 2iχ (1) where m is the fractional polarization, p is the polarized flux density and χ is the polarization position angle (= electric vector position angle, EVPA). We may also note: p p = Q 2 + U 2 + V 2 m = p/i (2) χ = 1 2 tan 1 U Q The Poincaré sphere provides a means for visualising the Stokes parameters. The point P represents a given polarization state and lies on the surface of the sphere whose radius is the total polarized power. 1

4 Relating the Stokes Parameters to Interferometry The elements of radio interferometers are equipped with either linearly or circularly polarized feeds. The correlations formed between the orthogonal modes are related to the Stokes parameters by: Circularly polarized feeds (most common in VLBI) Measure right circular polarization (RCP or R) and left circular polarization (LCP or L). The correlations that can be formed between antennas j and k are R j R k = I + V L j L k = I V R j L k = Q + iu L j R k = Q iu Linearly polarized feeds (e.g. Westerbork, ATCA) Measure two perpendicular modes, p and q p j pk = I + Q cos 2χ + U sin 2χ q j qk = I Q cos 2χ U sin 2χ p j qk = I Q sin 2χ + U cos 2χ + iv q j pk = I Q sin 2χ + U cos 2χ iv (3) (4) 2

5 Why Radio Sources are Polarized The radio emission from most bright radio sources arises from Synchrotron Radiation linearly polarized. The radiation from a single relativistic e gyrating around a magnetic field is elliptically polarized. For an ensemble of e s with a smooth distribution of pitch angles the opposite senses of the elliptical polarization will cancel out resulting in linearly polarized radiation. If the e s have a power law distribution the fractional polarization (m) in the presence of a uniform magnetic field is given by: m = α + 1 (5) α + 5/3 where α is the spectral index of the source. E.g. if α = 0.5 (typical of synchroton emitting sources), m = 0.7. Real radio sources generally have m < 0.1 tangled magnetic fields, cellular depolarization, Faraday depolarization, etc. Higher m can be seen as the result of repolarization. 3

6 Depolarization Radio sources show low polarization compared with simple synchrotron models. Cellular depolarization The magnetic fields in the sources are not uniform but have small scale structures. Consider a source composed of many cells in each of which the magnetic field is randomly orientated (Figure 2). The polarization contribution from each cell adds vectorially. If there are N cells within the telescope beam, m m/ N. Cellular depolarization. Arrows represent the dominant magnetic field direction in the cell. Radio sources can also be affected by Faraday depolarization which we will return to later. 4

7 Repolarization Repolarization by field compression Compression of an initially random magnetic field (e.g. by a shock) modifies components of the field perpendicular to the direction of compression. A random field viewed along the axis of compression will still appear to be random. If a random field is compressed into a plane and is viewed in that plane, it will appear indistinguishable from an ordered field and will produce polarization at the level given by equation 5. If the field is only partially compressed or is viewed at some angle to the plane of compression, then the observed polarization will be less than this. Compression of a cube containing an initially random magnetic field (Laing 1980, MNRAS). 5

8 Repolarization The observed polarization for a compressed, initially random field is given by: m = α + 1 α + 5/3 (1 k 2 ) cos 2 ɛ 2 (1 k 2 ) cos 2 ɛ where the first term on the RHS is the same as equation 5, k is the compression factor (k = 1 full compression) and ɛ is the viewing angle measured in the frame of the source (relativistic aberration is important for AGN jets). Repolarization by field shear Shear layers result from the interaction between the surface of an expanding source and the surrounding medium. Essentially, a component of magnetic field gets stretched out along the direction of motion in a boundary layer resulting in the development of a uniform component of magnetic field, with direction parallel to the direction of motion. (6) 6

9 Faraday Rotation I For synchrotron radiation, the rest-frame polarization position angle can be used to infer the direction of the magnetic field. For optically thin emission, the EVPA is perpendicular to the magnetic field, for optically thick emission it is parallel. However, interpretation of the observed EVPA is complicated by the presence of Faraday rotation. Faraday rotation is a rotation of the angle of polarization and occurs when electromagnetic radiation propagates through an ionised plasma containing cold (thermal) electrons and a magnetic field. Linearly polarized radiation can be decomposed into a superposition of a right circularly polarized (RCP) wave and a left circularly polarized (LCP) wave. Consider 2 orthogonal states of the electric field E x = îe 0 cos(kz ωt) E y = ĵe 0 sin(kz ωt) (7) E(z, t) = E 0 [î cos(kz ωt) + ĵ sin(kz ωt)] LCP.mp4 rcp.mp4 RCP: the electric field vector rotates counter-clockwise when viewed with the wave approaching the observer. LCP: rotates clockwise. 7

10 Faraday Rotation II Similarly one can have a wave described by E(z, t) = E 0 [î cos(kz ωt) ĵ sin(kz ωt)] (8) which is left circularly polarized (LCP). The superposition of an LCP and RCP state gives plane (linear) polarization if the amplitudes of the LCP and RCP states are equal. The resultant of the superposition is which is a plane polarized wave. E(z, t) = (E 0 + E 0 )[î cos(kz ωt)+ (E 0 E 0 )ĵ sin(kz ωt)] E = 2E 0 î cos(kz ωt) As the plane wave propagates through an ionised medium, the force on an e in the gas is e(e + (v B)) where E is the wave electric field and this force will be balanced by the centripetal force of the electron (cf. dipole moment of e ): The e displacement is given by: e(e + Bωr) = m erω 2 (10) r = e m (9) " # 1 ω 2 ± e m Bω E (11) 8

11 Faraday Rotation III The ± in this equation is because the e s rotation can be either in the same or the opposite direction as the LCP and RCP mode respectively. Now, the relative permittivity of the plasma (ɛ) is given by: " # ɛ = 1 nee2 1 ɛ 0 m eω 2 1 ± eb (12) ωm e Noting that the plasma frequency (ν p) is ν 2 p = (nee2 )/(4π 2 ɛ 0 m) (13) and ω = 2πν we find " # ɛ = 1 ν2 p 1 ν 2 1 ± eb ωm e (14) and so the relative permittivity has 2 values depending on the direction of the e s motion. For one of the fundamental modes (RCP and LCP) it will be in the same direction, for the other it will be in the opposite. The wave velocity is given by c/ ɛ. The right and left circularly polarized components of the linearly polarized wave will therefore propagate at different speeds through the plasma as they will experience different refractive 9

12 Faraday Rotation IV indices and this leads to a rotation of the angle of linear polarization. We can calculate the difference in the speeds ( v). v 2 1 v 2 2 = (v 2 + v) 2 v 2 2 2v 2 v 2v 2 v = c 2 j 1 ɛ 1 1 ɛ 2 ff Substituting for ɛ (eqn 14) and assuming eb mω << 1 Since v 2 c and ɛ 1 2v 2 v c2 (ɛ 2 ɛ 1 ) ɛ 2 ` 2v 2 v = 2c2 ν p 2 eb ν mω ɛ 2 v = c2 ` ν p ν ɛ 2 v 2 2 eb mω νp 2 eb v = c ν mω (15) (16) (17) 10

13 Faraday Rotation V Finally substituting ν p (eqn 13) v nee2 4πɛ 0 m eb 2πmν 3 c v neb ν 3 v n ebλ 3 (18) Consider again our 2 circularly polarized modes, now propagating with speeds v 1 and v 2 respectively. In one period of oscillation (τ) the relative phase difference (φ = 2πl ) between the λ two modes is 2π vτ/λ. Since τ = 1 ν = λ, the angle of rotation of the polarization plane (Θ) per c oscillation period is then given by 2Θ = 2π v/c. For a given frequency the total rotation angle (θ) is proportional to the distance travelled by the wave (D) divided by λ. θ = π Z D v ds (19) λ 0 c We know v from eqn 18 and with appropriate choice of units we get Z D θ = λ 2 n eb ds (20) 0 11

14 Faraday Rotation VI where θ is measured in rads, n e is in m 3, λ is in m, B is the line of sight magnetic field in Tesla and D is in parsecs. This equation is normally written as where is the rotation measure in units of rad m 2. θ = RM λ 2 (21) Z D RM = n eb ds (22) 0 Faraday rotation is characterised by a wavelength dependence of the degree of rotation. In the simplest case of a resolved, homogeneous foreground Faraday screen the observed EVPA is related to the rest-frame EVPA by where χ obs is the observed EVPA and χ 0 is the rest-frame EVPA. χ obs = χ 0 + RM λ 2 (23) Faraday rotation can arise as a result of thermal gas either (i) within the volume of material that is emitting the radiation, or (ii) external to the emitting material. The former situation is known as internal Faraday rotation, the latter as external Faraday rotation. 12

15 External Faraday Rotation For a resolved external Faraday screen located between the synchrotron emitting plasma and the observer, equation 23 completely describes the effect of the screen on the source polarization. The situation is somewhat complicated if the Faraday screen is not resolved and there is a distribution of Faraday depths within a single beam of the telescope. Radiation from a source with uniform polarization position angle will pass through different Faraday depths and thus the electric vectors will be rotated by different amounts. Since linear polarization is a complex quantity, the telescope effectively does a vector sum of the polarizations within the beam. Thus, if there are fluctuations in the Faraday depth across the beam then this results in a departure from the simple λ 2 law given by equation 23 and leads to depolarization (a decrease in the percentage polarization with increasing wavelength). 13

16 Internal Faraday Rotation Faraday rotation can occur within the ionised plasma responsible for the observed synchrotron radiation. Since a single electron s contribution to the Faraday rotation is inversely proportional to the square of the electron s effective mass (equation 18), relativistic electrons such as those responsible for synchrotron radiation do not give rise to significant Faraday rotation. However, a population of thermal (cold) electrons within the synchrotron emitting plasma can produce internal Faraday rotation. Internal Faraday rotation can often be distinguished from external Faraday rotation. Internal Faraday rotation tends to induce depolarization and a departure from a simple λ 2 rotation as a result of the different path length through the Faraday screen which is traversed by radiation from the front and back of the source. For realistic geometries, internal Faraday rotation cannot produce a rotation greater than π/2 rads. This constraint is even tighter for a spherical geometry where the maximum rotation is π/4 rads. 14

17 Polarization Calibration in VLBI The polarization calibration of any radio interferometer is in principle the same. However, the high resolution of VLBI introduces some additional practical problems. Nonetheless, most of what follows is equally applicable to connected-element interferometers as to VLBI arrays. In previous lectures you have seen the calibration technique required to produce total intensity maps from interferometer data a priori amplitude calibration, fringe-fitting, self-calibration and deconvolution. The calibration of polarization data is largely similar but with a few extra steps required to produce well-calibrated cross-hand visibilities (i.e. the RL and LR correlations which provide the Stokes Q and U parameters). 15

18 Instrumental Polarization In real observations using dual polarized feeds the signal measured on one polarization is corrupted by leakage from the orthogonal sense. As the typical polarization of radio sources is also only a few percent this must be removed before reliable polarization maps can be made. The instrumental response of an antenna to the radiation field can be written as R = G R [E R e iφ + D R E L e iφ ] L = G L [E L e iφ + D L E R e iφ ] where R and L are the measured RCP and LCP signals respectively, G R and G L are the antenna gains in RCP and LCP respectively, E R and E L are the true RCP and LCP electric fields from the source respectively, D R and D L are the fraction of the orthogonal sense that leaks through to RCP and to LCP respectively, and φ is the parallactic angle (PA). (24) The PA is the angle between the lines on the celestial sphere joining the source to the north celestial pole and the source to the antenna s zenith, which measures the orientation of the feed with respect to the source. Diagram illustrating the parallactic angle (φ) and how it changes as a source moves across the sky. 16

19 Instrumental Polarization The correlations in equation 3 can then be rewritten in terms of the R and L instrumental responses, e.g. R j R k = G Rj G Rk "ERj E Rk e i φ j φ k D Rj E Lj E Rk ei φ j +φ k R j L k = G Rj G Lk "ERj E Lk e i φ j +φ k D Rj E Lj E Lk ei φ j φ k + D Rk E Rj E Lk e i φ j +φ k + D Rj D Rk E Lj E Lk ei φ j φ k # + D Lk E Rj E Rk e i φ j φ k + D Rj D Lk E Lj E Rk ei φ j +φ k #. + + (25) Ignoring terms of higher order than D 2 and cross-hand terms and assuming V = 0 (so that E Rj E Lj ), the ratio of the cross- to parallel-hand terms can be written as e.g. R j L k L j L k = G Rj G Lk G Lj G Lk " ERj E Lk E Lj E Rk e 2iφ j +D Rj + D Lk e2i(φ k φ j )i (26) where the angle brackets indicate quantities that have been time averaged by the correlator. 17

20 Instrumental Polarization The first term inside the square brackets in equation 26 is the source polarization and can be set to zero if unpolarized sources are observed, giving: R j L k L j L = G Rj GLk G k Lj GLk h D Rj + D Lk e2i(φ k φ j )i (27) This is the equation of a circle of radius D Lk centred on D Rj (see Figure 6). PLot file version 1 created 11-NOV :43:03 Imag vs Real for 3C84 1.7G.MULTI.1 FG # 1 IF 1 CHAN 1 STK RL/RR 100 LA - PT ( 5-9 ) PLot file version 2 created 11-NOV :43:09 Imag vs Real for 3C84 1.7G.MULTI.1 FG # 1 IF 1 CHAN 1 STK RL/RR 100 LA - PT ( 5-9 ) Ratio x Ratio x Ratio x Ratio x 1000 The plot on the left shows the cross- to parallel-hand ratio (cf. equation 26) for 2 VLBA antennas at 1.7 GHz, for the bright unpolarized source 3C 84 after the source has been self-calibrated but before the instrumental D-terms have been corrected. The D-terms trace a circle due to rotation with parallactic angle. The plot on the right shows the same but with the D-term correction applied. 18

21 Polarization position angle calibration The complex gains G R and G L of the antennae are determined during self-calibration. This is done relative to a reference antenna whose gain phases are arbitrarily set to zero. Although this has no serious consequences for total intensity mapping (apart from the loss of absolute position information) it results in the introduction of an arbitrary phase offset (equal to the RL phase difference at the reference antenna) to the RL and LR correlations which produces a corresponding rotation of χ in the polarization map. The polarization position angle of connected-element interferometers such as the VLA can easily be calibrated by observations of sources with known integrated EVPA. Unfortunately no such sources are known on VLBI scales. Instead, we observe a source with compact polarization structure simultaneously with the VLBI array and with a single dish/connected-element interferometer. If the source has sufficiently compact polarization structure then all the polarized flux detected by the low resolution instrument (e.g. VLA) should also be present in the VLBI observation. As a result, the integrated polarization position angle in the VLBI observation should be the same as that of the VLA. It is important that the time between the two observations be as short as possible as many compact polarized sources are variable on timescales of days or less. 19

22 Matrix Formulation A more elegant mathematical description of polarimetry is given by the matrix formulation of Hamaker, Bregman and Sault (see TMS 4.8). «The EM field can be described by a complex vector E EL = E R Any linear effect can be described in terms of a 2 2 (Jones) matrix, thus E = J nj n 1... J 1 E An interferometer measures coherencies (E) between the EM field at the individual elements (j, k), giving a visibility matrix V: «V jk = E j E k = ERR E RL E LR E LL The Jones matrices describing all the signal propagation effects are cumulative and may be multiplied to give a single Jones matrix (J j ) for each antenna, so (28) V jk = J j E(Jk E) = J j E E J k (29) The true source coherency (B, or source brightness) is then related to the measured visibility by: and this is known as the Measurement Equation. V jk = J j BJ k (30) 20

23 Matrix Formulation Some Jones matrices describing typical propagation effects are: «exp(iθ) 0 J rotn = 0 exp( iθ) «1 DR J leakage = D L 1 «GR 0 J gain = 0 G L (31) The simple form of the Jones matrices for describing most effects and the fact that polarization effects are intrinsically accounted for makes this a very attractive formalism for describing radio instruments. casa already incorporates a modified version of the Hamaker Measurement Equation but older (and more widely used) packages do not (hence the brief treatment here). 21

24 Where s the Science? A few highlights: Laing-Garrington effect Magnetic field alignments in quasar cores and lobes helical fields? Faraday rotation as a probe of thermal gas content Kinematic constraints for parsec-scale jets Circular polarization has implications for jet composition difficult to measure but may answer some important questions Polarization gives us information about the magnetic field orientation in the sources, the degree of ordering in the magnetic fields and the thermal gas content much of this information cannot be obtained in any other way. Can help us distinguish between thermal and non-thermal emission mechanisms thermal emission is not generally polarized. 22

25 1988 Nature PublishingGroup Laing-Garrington Effect The cause of the one-sidedness in the jets of otherwise symmetrical extragalactic radio sources was a subject of some controversy for many years. The evidence for bulk relativistic motion suggested that the one-sidedness could simply be the result of Doppler boosting of the approaching jet, and Doppler diminution of the receding jet but this was hard to confirm. Alternative models such as the flip-flop model could not be ruled out. Laing, Garrington et al. observed a sample of double-lobed FR II sources with asymmetric jets and showed that the lobe on the jet side was less depolarized than the counterjet side in almost all cases (see the example). Most obvious explanation is that the depolarization is caused by differential Faraday rotation through the ionised medium surrounded the radio source. The side with the jet is closer to us and therefore seen through a smaller depth of depolarizing medium. Best evidence yet that the asymmetries in jet sidedness are due to relativistic Doppler boosting. 23

26 Faraday Rotation Faraday rotation measurements require simultaneous observations at at least 3 frequencies to fit for the λ 2 dependence. High resolution is important because polarization is a complex quantity multiple components within the telescope beam can cancel each other out. Also must be wary of optical depth effects (electric vector to B for optically thin emission, for optically thick 90 rotation). Recent multi-frequency VLBI observations of quasars have revealed non-uniform Faraday rotation on parsec scales. The rotation measures (RM) can be very high close to the core (associated with the active galactic nucleus) but quickly decrease along the jet (see Figure 22). At distances of more than a few tens of parsecs from the core, the RM is generally consistent with the RM of our own Galaxy. 24

27 Faraday Rotation Rotation measure map of the weak emission line blazar BL Lac made using VLBA observations at 5, 8 and 15 GHz. The contours are the 5 GHz total intensity. The RM in the core (northernmost component) is enhanced with respect to the jet. This is a little surprising as BL Lac has very weak emission lines (indicating a lack of thermal gas on scales < 0.1 pc) yet appears to have similar gas content on scales of 1 pc as some strong emission line quasars (Reynolds et al MNRAS 327, 1071). Plot of χ Vs λ 2 for the core and 3 jet components. 25

28 Faraday Rotation Local regions of enhanced Faraday rotation are sometimes seen near bends and bright regions in the jet (see Figure 8). This is evidence that the jet bends and brightness enhancements are caused by an impact with the intergalactic medium, rather than, say, a hydrodynamic instability. Nan et al On the left is total intensity contours with degree of polarization in gray-scale. On the right is a close-up of the region where the jet brightness is enhanced showing the RM. There is a RM gradient across the bright component. Nan et al. A&A 344,

29 Faraday Rotation Faraday rotation can be used to constrain the quantities of thermal gas located close to the core, which may have implications for unified schemes (see the cartoon in Fig 9). However, sources must be compared using observations with the same linear resolution to account for the steep RM gradients near the core. Cartoon model of the environment around an AGN (Taylor 2000, ApJ) 27

30 Faraday Rotation Faraday rotation measurements are important for interpreting magnetic field alignments at all but the highest radio frequencies. RMs of rad m 2 have been measured in quasar cores rotation of > 40 even at 22 GHz. When Faraday rotation is observed, the interpretation depends on whether the Faraday screen is resolved or not. If the Faraday screen is not resolved, a situation similar to the cellular depolarization illustrated earlier occurs. Even with just two regions of differing RM this leads to a departure from a simple λ 2 rotation. For this reason, good agreement with a λ 2 law is often taken as proof that the Faraday screen is resolved and the whole Faraday screen within the telescope beam can be well described by a single RM. 28

31 Faraday Rotation However, over a limited (but sometimes quite large) range in wavelength the departure from a λ 2 law may not be apparent. Figure 10 shows an example of the integrated RM that would be produced by 2 components with very different intrinsic RMs, but which are not resolved from each other. Using observations at 5, 8 and 15 GHz (the most popular for RM measurements) the departure from the λ 2 law would not be detectable even if the EVPA was determined to an accuracy of 3 (which is quite good for VLBI). Beware! EVPA as a function of λ 2 for an unresolved source with 2 components of the Faraday screen. The RM of one component is 550 rad m 2, the RM of the other is 100 rad m 2. The integrated RM is 204 rad m 2. The dotted line is the EVPA derived for the 2 component model. The triangles mark the EVPA at 5, 8 and 15 GHz with an error bar of ±3 and the dashed line is the best fit straight line to the EVPA at these three points. Observations of this component at these 3 frequencies would not detect a departure from a λ 2 law and could lead to the incorrect assumption that the screen was resolved. 29

32 Magnetic Field Alignments in Blazar Jets The rest-frame EVPA is perpendicular to the magnetic field for optically thin synchrotron emission, and parallel for optically thick synchrotron emission. The jets in blazars are optically thin (except very close to the core). Polarization studies suggest that blazars can have magnetic fields which are either aligned with the jet direction or perpendicular to the jet direction. Perpendicular magnetic fields can be given by shock compression (see earlier) or by a toroidal magnetic field. Parallel magnetic fields could be due to a uniform longitudinal magnetic field or a shear interaction with the surrounding medium. The weak-lined objects (BL Lacs) tend to have perpendicular magnetic field directions, the strong-lined objects (OVV quasars) tend to have parallel magnetic fields. Suggests that the quasars are dominated by a uniform longitudinal component of magnetic field, while BL Lacs have a tangled magnetic field which is enhanced by compression due to shocks. 30

33 Magnetic Field Alignments in Blazar Jets Evidence for Helical Magnetic Fields Helical magnetic fields can give both parallel and perpendicular electric vectors depending on the pitch angle of the magnetic field. Figure 11 shows the RM gradient across the jet in 3C 273. The variation in RM across the jet is consistent with a helical magnetic field (in fact it is hard to explain by any other mechanism). The variation in RM is due to changes in the parallel component of the magnetic field (B ) as the magnetic field lines wrap around the jet. Good lateral resolution of a jet is rare, but such signatures may provide the key to determining how common helical magnetic fields are in blazars. Also note that if the B-field helix is wound up by the accretion disk, the orientation of the RM gradient gives the direction of rotation of the accretion disk. The change in RM across the jet is consistent with B changing due to the presence of a helical magnetic field (Asada et al. 2002, PASJ 54, L39). 31

34 Magnetic Field Alignments in Blazar Jets Evidence for Shear Interactions and Differential Speeds VLBI polarization images of by Attridge et al. (1999) show a jet with a two-component magnetic field structure (see attached Figure). The spine of the jet has compact knots with longitudinal electric vectors ( B-field) while the edge of the jet has transverse electric vectors, such as would be expected from a sheath formed by a shear interaction with the surrounding medium. In the simplest picture the sheath is more slowly moving than the spine, and therefore it s beaming cone is different. The jet angle to the line of sight then determines whether the spine or the sheath emission dominates your image. (Attridge et al. 1999, ApJ) Fig. 1a Fig. 1b 32

35 Magnetic Field Alignments in Blazar Jets The Importance of High Resolution Many of the reported misalignments between EVPA and structural position angle can probably be explained by resolution effects. The polarization is often confined to small regions, and the jet structure can change on very small scales. Even if the polarization is well-aligned with the structure, small scale wiggles in the jets, combined with insufficient resolution can smear out the structural changes such that the polarization and structural position angles appear misaligned (e.g. Figure 12). The map on the left is at 5 GHz observed with the VLBA. The contours are of total intensity and the overlaid sticks indicate the polarized intensity and EVPA. The map on the right uses the same data, but with the addition of (v. long) baselines to the orbiting VLBI antenna HALCA. The improved resolution given by the space baselines allow one to see that the electric vectors very neatly follow the bend in the jet with no evidence for misalignments (Gabuzda 1999, NewAR, 43, 695). 33