Using high frequency (12-22 GHz) VLBA observations we confirm the existence of a Faraday rotation measure gradient of {approx}500 rad m{sup -2} mas{sup -1} transverse to the jet axis in the quasar 3C273. The gradient is seen in two epochs spaced roughly six months apart. This stable transverse rotation measure gradient is expected if a helical magnetic field wraps around the jet. The overall order to the magnetic field in the inner projected 40 parsecs is consistent with a helical field. However, we find an unexpected increase in fractional polarization along the edges of the source, contrary to expectations. This high fractional polarization rules out internal Faraday rotation, but is not readily explained by a helical field. After correcting for the rotation measure, the intrinsic magnetic field direction in the jet of 3C273 changes from parallel to nearly perpendicular to the projected jet motion at two locations. If a helical magnetic field causes the observed rotation measure gradient then the synchrotron emitting electrons must be separate from the helical field region. The presence or absence of transverse rotation measure gradients in other sources is also discussed

/Naval Observ., Flagstaff

/NRAO, Socorro /KIPAC, Menlo Park

Taylor, G.B.

Zavala, Robert T.

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Work supported in part by Department of Energy contract DE-AC02-76SF00515Faraday Rotation Measure Gradients from a Helical MagneticField in 3C 273R.T. Zavala1 and G.B. Taylor2,3bzavala@nofs.navy.mil; gtaylor@nrao.eduABSTRACTUsing high frequency (12-22 GHz) VLBA observations we confirm the exis-tence of a Faraday rotation measure gradient of ∼500 rad m−2 mas−1 transverseto the jet axis in the quasar 3C273. The gradient is seen in two epochs spacedroughly six months apart. This stable transverse rotation measure gradient isexpected if a helical magnetic field wraps around the jet. The overall order tothe magnetic field in the inner projected 40 parsecs is consistent with a helicalfield. However, we find an unexpected increase in fractional polarization alongthe edges of the source, contrary to expectations. This high fractional polariza-tion rules out internal Faraday rotation, but is not readily explained by a helicalfield. After correcting for the rotation measure, the intrinsic magnetic field di-rection in the jet of 3C273 changes from parallel to nearly perpendicular to theprojected jet motion at two locations. If a helical magnetic field causes the ob-served rotation measure gradient then the synchrotron emitting electrons mustbe separate from the helical field region. The presence or absence of transverserotation measure gradients in other sources is also discussed.Subject headings: galaxies: active – galaxies: quasars: individual (3C273) –galaxies: jets – radio continuum: galaxies – polarization1United States Naval Observatory, 10391 W. Naval Observatory Rd., Flagstaff, AZ 86001-11492National Radio Astronomy Observatory, P.O. Box O, Socorro, NM 878013Kavli Institute of Particle Astrophysics and Cosmology, Menlo Park, CA 94025SLAC-PUB-10993June 2005astro-ph/0505357KIPAC, SLAC, Stanford Univesity, Stanford, CA  94309Submitted to Astrophys.J.Lett.– 2 –1. IntroductionHow jets are launched by massive black holes remains one of the fundamental unsolvedissues in astrophysics. Many researchers have suggested that magnetic fields are intimatelyinvolved in the collimation of the jet and could determine which sources have prominent jetsand which do not (Meier, Koide, & Uchida 2001; Koide et al. 2002). Blandford (1993) urgedobservers to search for Faraday rotation measure (RM) gradients transverse to the relativisticjet. Such a gradient is expected if a helical magnetic field wraps around the jet, and amongother effects, creates a Faraday screen. For a source with a jet axis in the plane of the skya helical field wrapping around a jet produces a maximum line-of-sight components alongthe jet boundary, no net line-of-sight magnetic field component at the center. Blandfordrealized that this would produce a gradient in the observed RM across the jet. Projectioneffects for inclined jets produce an offset in the RM (Asada et al. 2002) while preserving anRM gradient.Faraday rotation measure gradients in 3C273 (Asada et al. 2002) based on 5-8 GHzVLBA observations, present intriguing evidence for one source in which we may be able tosee the effects of helically wound fields, and measure the sense of rotation of an accretingblack hole. With the search for these RM gradients in mind, we have reanalyzed 12-22 GHzVLBA polarimetry observations for two epochs on 3C273 to obtain higher angular resolutiontransverse to the jet axis, while maintaining good sensitivity to the faint polarized emission.Throughout this discussion, we assume H0=71 km s−1 Mpc−1 (Spergel et al. 2003), ΩM= 0.27, and ΩΛ= 0.73. This gives a scale for 3C273 of 1 mas = 2.52 pc.2. Observations and ResultsThe observations were made on 2000 January 27 (2000.07) and 2000 August 11 (2000.61)with the ten element Very Long Baseline Array (VLBA)1. Data reduction and calibration forthe 2000.07 epoch were discussed in Zavala & Taylor (2001). We re-examined the data for2000.07 and made polarization angle maps using using five frequencies: 12.1, 12.6, 14.9, 15.4and 22.2 GHz. The 22.2 GHz maps were tapered to approximate the 12.1 GHz resolution,and all maps were convolved with a restoring beam matched to the 12.1 GHz beam. Theobservational setup for the 2000.07 observation was repeated for the 2000.61 epoch. Thesame data reduction and calibration procedures of 2000.07 (Zavala & Taylor 2001) were1The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooper-ative agreement with the National Science Foundation.– 3 –applied to the previously unpublished 2000.61 data. A datacube of polarization positionangle maps ordered in λ2 was used to create the rotation measure maps. Pixels in the RMmaps were blanked if the polarization position angle error was more than ± 10◦ at anyfrequency.Rotation measure images for the two epochs are presented in Figure 1a (2000.07) andFigure 1b (2000.61). Between the two epochs the region immediately southwest of the StokesI peak (i.e. in the projected direction of the jet) shows a change in RM. At 2000.07 the RMis ≈ 1800 rad m−2 and 6 months later shows a south to north gradient with an RM from−1000 to +1000 rad m−2.Beyond 3 mas from the Stokes I peak (hereafter this region is referred to as the “jet”)the RM distribution becomes smoother and does not change dramatically between the twoepochs. The mean RM for the jet is 642 rad m−2 for the 2000.07 epoch, and 652 rad m−2for 2000.61. An RM gradient transverse to the jet is apparent, and we discuss this in §3.1.In Figure 2 we show the RM corrected intrinsic B vectors for 3C273 under the assump-tion that the source is optically thin from 12-22 GHz. We simply took the Faraday correctedE vectors and rotated them by 90 degrees. This was done for the entire jet, and for the corewest of the Stokes I peak. This does not take relativistic aberration into account (Lyutikov,Pariev, & Blandford 2003; Lyutikov, Pariev & Gabuzda 2005) which could change the orien-tation of the B vectors by up to 20◦. The jet B vectors are initially oriented roughly parallelto the projected jet direction, then rotate to almost a N−S orientation 6 mas southwest ofthe Stokes I peak. This changing B vector orientation repeats itself as we move beyond 6mas from the Stokes I peak. For the 2000.61 epoch we detected polarization as far as 15 masdown the jet, and the B vectors there have returned to a direction parallel to the projectedjet axis.3. The Faraday screen in 3C 273In Fig. 1 we noted the change in the core RM at 3 mas southwest of the Stokes I peak.What causes the change in the observed RM? 1 mas corresponds to a projected distanceof 2.5 parsecs at the redshift of 3C273. A region of that size is unlikely to change over asix month time interval. We thus reject a change in the physical conditions of a foregroundFaraday screen as the cause of the changing rotation measure. A more likely explanationfor the RM changes in the core over the 6 month time interval is a relativistically movingpolarized jet component seen through a foreground Faraday screen. Changes in the observedrotation measure are caused by jet components sampling different sight lines as they move– 4 –out from the core (Zavala & Taylor 2004). The structure in the core is consistent with thesmall scale polarization structure reported by Attridge et al. (2003).The Faraday screen in front of the jet appears to be relatively constant in time as theRM does not significantly change over the 6 month interval presented here, or over threeyears (Zavala & Taylor 2001). The jet Faraday screen is also spatially uniform, as opposedto the sub-structure seen in the core (Fig. 1). The more uniform Faraday screen begins ata projected distance of approximately 10 parsecs from the core (Fig. 1). This may be thedistance from the central engine beyond which the magnetic field in the Faraday screen hasbecome more uniform and smooth.3.1. Evidence for a systematic RM GradientWe now examine the case for a transverse RM gradient as suggested by Blandford(1993). We wish to take a slice across the broadest RM distribution in the jet transverse tothe jet motion. We use Kellermann et al. (2004) to define a jet direction of −112.7◦ at thelocation of the broadest RM distribution in Figure 1. In Figure 1 we show the locations ofthe slices across the jet, with the orientation of the slice perpendicular to the line connectingthe Stokes I peak of the component at a relative RA= −3 mas, relative δ = −6.5 mas andthe map center. Figure 3 shows the resulting RM of the slices at the two epochs: 2000.07by the dashed line and 2000.61 by the solid line. A transverse gradient of approximately500 rad m−2 mas−1 is visible in Figure 3, with almost 4 beamwidths along the gradient.This is much larger than the gradient of 35 rad m−2 mas−1 across almost two beamwidthsobtained with the coarser resolution of Asada et al. (2002). This clearly demonstrates theadvantage of working at the highest frequencies the VLBA is capable of, consistent withenough sensitivity to polarized emission. The gradient expected by Blandford (1993) isclearly present, and is suggestive of a helical field wrapping around the jet.The RM is predominantly low and mostly negative along the southern edge of the jetin 3C273 (ignoring a few small and suspect patches of very high RM), even to a distance of15 mas (38 pc) in epoch 2000.61. Along the northern edge of the source the RMs are largerand predominantly positive. This overall RM structure is consistent with an ordered helicalfield, and suggests that this field is the dominant source of the Faraday screen on projectedlinear scales out to at least ∼40 pc.– 5 –3.2. An Alternative View – Evidence for localized RM enhancementsIn Figure 4 we present the 12 GHz fractional polarization along the slices shown in Figure1. Along with a gradient in the RM we also see a gradient in the fractional polarization.This indicates that the polarization is more ordered as we proceed from south to north alongthe transverse RM gradient. Consider the simplest case of a jet in the plane of the sky with ahelical magnetic field wrapped around the jet. The fractional polarization should be highestat the jet center, where the RM is zero. The fractional polarization then falls off as weapproach either edge of the jet, as the RM depolarizes the underlying emission (Gardner &Whiteoak 1966). Severe projection effects are surely present in 3C273, yet it is still surprisingto see that the highest RM in the jet corresponds to high fractional polarization. Could thelocalized RM and fractional polarization enhancements be the site of an interaction of thejet with the ambient thermal gas which forms the Faraday screen (Bicknell et al. 2003)? Inthis case the rotation of the B vectors in Figure 2 to approximately perpendicular to thejet motions seen by Kellermann et al. (2004) could be a compression and enhancement ofthe local magnetic field due to a collision with the Faraday rotating thermal gas. If suchan interaction is responsible for the observed RM and fractional polarization gradients, thenthe lack of free-free absorption of the jet of 3C273 may set interesting limits on the electrondensity of the thermal gas (Zavala & Taylor 2004).3.3. RM distributions in other sourcesSources with jets broad enough to be well resolved are rare (Pollack, Taylor, & Zavala2003), but a few examples do exist. The jet of M87 is resolved, but the RM data cover arelatively small area (Zavala & Taylor 2002). There is no obvious RM gradient, but thesign change of M87’s rotation measure is consistent with a helical magnetic field. Gabuzda,Murray, & Cronin (2004) report rotation measure gradients in 4 BL Lac sources with anangular resolution comparable to Asada et al. (2002). 3C147, a compact steep spectrum(CSS) quasar, has a relatively broad jet with an RM gradient observed at 8 GHz (Zhanget al. 2004). The quasars B1611+343 and B2251+158 are well resolved, but there is noevidence of a transverse RM gradient (Zavala & Taylor 2003). BL Lac is marginally resolvedand no gradient is apparent (Zavala & Taylor 2003). Another quasar with a resolved jet,B0736+017, fails to display an obvious transverse RM gradient. At present we can say thattransverse RM gradients do exist, but these gradients are not a universal feature.The very high fractional polarization we observe rules out internal Faraday rotation asa cause for the observed RM. Internal Faraday rotation is expected to cause severe depo-larization, and discontinuities in the observed position angle versus λ2 plots (Burn 1966;– 6 –Tribble 1991). If a helical field is responsible for the observed RM gradient, this implies asegregation of the synchrotron emitting electrons from the helical field region.4. SummaryWe confirm the presence of a transverse rotation measure gradient of 500 rad m−2mas−1, significantly larger than the gradient observed by Asada et al. (2002). This rotationmeasure gradient, as first suggested by Blandford (1993), is expected if a helical magneticfield wraps around the relativistic jet of 3C273. This field imposes an order on the Faradayscreen which we see as predominantly negative RMs along the south edge and positive RMsalong the north edge in the inner projected 40 parsecs of the jet. The increase in fractionalpolarization along the RM gradient, and turning of the B field vectors from parallel toperpendicular to the projected jet motion, suggest that an interaction may also explain thegradient, although in this case the overall order of the RM distribution has to be explainedas a coincidence, or some other mechanism that imposes an overall order to the surroundingmagnetic fields. The magnetic field responsible for the Faraday rotation appears to havestructure on sub-parsec scales within 3 milliarcseconds (8 pc) from the core of 3C273. Therelatively smooth RM structure of the jet implies a well ordered (on parsec scales) magneticfield in the Faraday screen.The larger RM gradient and the gradient in fractional polarization structure in 3C273revealed by these observations illustrate the advantages of observing broad jets at the highestfrequencies at which the VLBA operates. The superb resolution and polarization sensitivityof the VLBA from 12 – 22 GHz enables high spatial resolution tests of theoretical predictionswhich are key to understanding the physics of relativistic jets. A convincing test of thehelical magnetic field model awaits a clear predominance of RM gradients with the expectedhelicity in many sources, or the detection of a polarized counter-jet with gradients in opposeddirections (Asada et al. 2002). If a helical magnetic field is responsible for the observed RMgradients then because the high fractional polarization observed rules out internal Faradayrotation, the synchrotron emitting electrons must somehow be segregated from the helical Bfield region.We thank Phil Hardee for helpful discussions which improved the paper. RTZ thanksAlexandra Zavala for assistance with the submission. The National Science Foundation,through its support of NRAO, has funded this work. This research has made use of NASA’sADS Abstract Service and the NASA/IPAC Extragalactic Database (NED) which is operatedby the Jet Propulsion Laboratory, Caltech, under contract with NASA.– 7 –REFERENCESAsada, K., et al. 2002, PASJ, 54, L39Attridge, J. M., et al. 2003, American Astronomical Society Meeting, 203Bicknell, G. V., et al. 2003, New Astronomy Review, 47, 537Blandford, R. D. 1993, in Space Telescope Science Inst. Symp. Ser. 6,Astrophysical Jets, ed.D. Burgarella, M. Livio & C. P. 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Y., et al. 2004, A&A, 415, 477This preprint was prepared with the AAS LATEX macros v5.2.– 8 –Fig. 1.— Rotation Measure image for 3C273 on two epochs. Total intensity contours at12.1 GHz are overlaid with contour levels starting at 12 mJy/beam and 6.6 mJy/beam forepoch 2000.07 and 2000.61 respectively and increase by factors of 2. The synthesized beamis drawn in the bottom-left corner and has dimensions of 2.0 × 0.9 mas at 0◦ for both epochs.The line indicates the location of the slices shown in Fig. 3.– 9 –Fig. 2.— Rotation Measure corrected magnetic vectors for 3C273 for our two epochs overlaidon total intensity contours. Contour intervals are the same as for Figures 1a and b. Lengthsof magnetic vectors are proportional to the polarized intensity with scales of 1 mas = 100mJy beam−1 for both epochs.– 10 –Fig. 3.— Rotation Measure image slices perpendicular to the jet axis for both epoch 2000.07(dashed) and 2000.61 (solid). The region of the slice is indicated by the solid line in Fig. 1.– 11 –Fig. 4.— Fractional polarization slices perpendicular to the jet axis for both epoch 2000.07(dashed) and 2000.61 (solid). The region of the slice is indicated by the solid line in Fig. 1.

Faraday Rotation Measure Gradients from a Helical Magnetic Field in 3C273

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Faraday Rotation Measure Gradients from a Helical Magnetic Field in 3C273

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