Parsec-Scale Investigation of the Magnetic Field Structure of Several AGN Jets

1. Parsec-Scale Investigation of the Magnetic Field Structure of Several AGN Jets Abstract Blazar z Jet EVPA vs. Jet Direction Core RM (6cm-2cm) Core RM (7mm-2cm) 0954+658 0.368 ║ -41 ± 14 -2207 ± 386 2200+420 0.069 ║ 746 ± 76 -1144 ± 36 2007+777 0.342 ║ 808 ± 159 198 ± 200 1749+096 0.320 - -33 ± 24 -500 ± 347 1418+546 0.152 ┴ -65 ± 22 / -483 ± 55 No polarisation detected at 22 & 43 GHz 1156+295 0.729 ║ 136 ± 6 1647 ± 209 Conclusion Discussion References Shane O’Sullivan, Denise Gabuzda Department of Physics, University College Cork shaneosull@student.ucc.ie; gabuzda@physics.ucc.ie The fact that the Faraday corrected polarisation vectors for 2200+420 remain well aligned with the jet even as it goes through substantial bending can be understood if the implied transverse B-field represents the toroidal component of a helical magnetic field dominating in the observer’s rest frame (Lyutikov et al. 2005). 3. Multi-frequency (4.6, 5, 5.5, 8, 8.8, 13, 15, 22 & 43 GHz) polarisation observations of 6 blazars were obtained on the VLBA over a 24-hr period in July 2006. Observing at several different frequencies, separated by short and long intervals, enabled reliable information about the B-field structure to be obtained and the effect of Faraday Rotation to be determined and corrected. For 5 out of 6 sources the core Rotation Measure (RM) derived from the higher frequency data was higher than that derived from the lower frequency data. This is consistent with a higher e- density and/or B-field strength closer to the central engine. A transverse RM gradient was detected in the jet of 0954+658, providing evidence for the presence of a helical B-field surrounding the jet. The RM in the core region of 2200+420 displays sign changes in different frequency intervals (on different spatial scales). We suggest an explanation in terms of modest bends in a helical B-field surrounding the jet. 2. Figure 1 2200+420 6.5cm-2.3cm Rotation Measure & 0 maps Figure 4 1418+546 3.4cm 0 map A further 3 sources also have their jet EVPAs aligned with the jet direction. 1749+096 does not show appreciable jet polarisation. Fig.4 displays the only source in this sample with jet polarisation perpendicular to the jet direction. This behaviour of the jet EVPAs is natural if the jets have helical B-fields (eg. Lyutikov et al. 2005). Polarisation perpendicular to the jet direction occurs when the poloidal component of the helical B-field dominates, which is observed less often because the toroidal component is boosted due to the relativistic motion of the jet towards us. 2200+420 (BL Lac) has quite a complicated and variable structure. The inner jet has changed from a southwesterly direction in our Aug 2002 observations to directly southwards in our current observations (Fig. 2, 3), consistent with the “precessing nozzle” proposed by Stirling et al. 2003 and Mutel & Denn 2004. The RM map from 6.5cm-2.3cm (Fig. 1) shows a large positive value of +746 rad/m2 at the northern end, which we consider to be the true core, since our spectral index map (corrected for the frequency dependent core shift; see Lobanov 1998) shows this region to be most optically thick; a negative RM of -125 rad/m2 is observed just south of this region, while the rest of the jet has a slightly positive RM consistent with a much lower e- density in the optically thin jet. 1. The Faraday effect causes a rotation of the plane of polarisation, described by a (lambda)2 law:  = RM l2, where the rotation measure (RM) is determined by the integral of the e- density and the dot product of the B-field and the path length along the line of sight (LoS). A positive/negative RM indicates that the LoS B-field is pointing towards/away from the observer.  is the electric vector position angle (EVPA). Figure 2 2200+420 7mm-1.9cm Rotation Measure & 0 maps Previous results indicated the presence of different RM signs in the core regions of these 6 blazars in different frequency intervals (O’Sullivan & Gabuzda 2006) . This has two main possible origins: (1) the LoS B-field changes with distance from the centre of activity, or (2) since the previous observations were not simultaneous, it could be that there was an intrinsic change in the jet B field structure between observing epochs. Our new 8 frequency dataset is designed to test these possibilities. A summary of some of our results are displayed in the table below. Separate RMs were found for 1418+546 from (6.5cm–3.4cm) / (3.8cm–1.9cm) because no polarisation was detected at 22 & 43 GHz. The integrated (Galactic) RMs have been removed, to better isolate the RM distribution in the immediate vicinity of the AGNs (Pushkarev 2001 and references therein). Figure 5: 0954+658 6.5cm - 1.9cm RM map The RM map of 2200+420 from 7mm-1.9cm (Fig.2) displays a RM of -1144 rad/m2 in the core, which is larger in magnitude and different in sign than the observed northernmost RM for the lower frequency data; this may correspond to the region of negative RM in Fig.1. The inner jet RM has a smaller value of -732 rad/m2. An interesting feature is the high positive RM detected in the region where the jet bends. Fig.3 shows the RM map of 2200+420 from 7mm-2cm observed in Aug 2002 (Gabuzda et al. 2006). Note how the direction of the jet has changed from our current observations, while the intrinsic polarisation vectors remain aligned with the jet direction. A region of large positive RM is detected in the northern core region which may correspond to the northern region of positive RM in Fig.1 above. The colour scale is in kilo-rad/m2. Figure 3 2200+420 7mm-1.9cm RM & 0 maps (2002) The detection of a RM gradient across the jet of 0954+658 (Fig.5) is a strong signature of the presence of a helical magnetic field surrounding the jet (for more on this see poster by Mahmud & Gabuzda). The core region has a RM of -41 rad/m2 from 6.5cm-1.9cm, but a RM of -2207 rad/m2 from 7mm-1.9cm. As noted previously, the greater magnitude RM in the higher frequency data is thought to imply sampling of higher e- densities/B-field strengths. For 5 out of 6 of the sources the magnitude of the core RM increases in the higher frequency range. This is consistent with sampling an increased e- number density closer to the central engine. A larger magnetic field strength can also contribute to this increase. Why the higher frequency core RM of 2007+777 is lower is unclear at this time; we are investigating the possibility that the low frequency RM is over-estimated due to the presence of the optically thick-thin transition in this range. A side-on view of a right-handed (RH) helical B-field will produce polarisation that will be equally strong on both sides of the jet, hence, a zero net RM will be observed for an unresolved jet. For BL Lacs this would occur when the source is viewed at 1/ in the observer’s frame. (Black hole on left end) Side-on Regions with different RM signs in the jets of AGN can be explained within a helical B-field model as places where the jet is observed at angles greater than or less than 1/ due to bends in the jet. (A longitudinal jet B-field with a change in the angle to the line of sight could also cause a RM sign reversal, but this does not correspond to the observed B-field in most BL Lacs.) It’s important to note that VLBI resolution is usually not sufficient to completely resolve the true optically thick core, therefore, the VLBI “core” consists of emission from the true core and some of the optically thin inner jet. So if bends occur on scales smaller than the observed VLBI core, “core” RMs with different signs could be derived from observations at different frequencies (ie. probing different scales of the inner jet). In our future work, we will attempt to reconstruct the 3D path of the jet through space using the combined information from the observed distributions of the total intensity, linear polarisation, spectral index and rotation measure. Our results for 2200+420 confirm the presence of a RM sign-reversal in the core region. Since the dominant jet B-field is transverse to the jet and remains transverse when the jet bends, we will suppose a helical B-field surrounds the jet. The observed RM sign reversal can be explained by a slight bend of the jet, due to a collision with material in the parent galaxy or some instabilities inherent in the jet itself. Tail-on For a tail-on view of a RH helical B-field (ie.  > 1/), the dominant polarisation will be from the bottom half of the jet (Lyutikov et al. 2005) and a negative RM will be observed because the dominant LoS B-field will be pointing away from us. (Assuming jet not fully resolved in the transverse direction). Our view of the jet is affected by the relativistic motion of the jet towards us: Side-on:  = 1/ Line-of-Sight  Viewed at 1/ in observer frame For a head-on view of a RH helical B-field (ie.  < 1/), the dominant polarisation will be from the top half of the jet (Lyutikov et al. 2005) and a positive RM will be observed because the dominant LoS B-field will be pointing towards us. (Assuming jet not fully resolved in the transverse direction). Radiation emitted at 90o in jet rest frame Gabuzda, Rastorgueva, Smith & O'Sullivan 2006, MNRAS, 369, 1596 Lyutikov, Pariev & Gabuzda 2005, MNRAS, 360, 869 Lobanov 1998, A&A, 330, 79 Mutel & Denn 2004, arXiv:astro-ph/0412496 Nakamura, Uchida & Hirose 2001, New Aston., 6, 61 (background image) O'Sullivan & Gabuzda 2006, Proc. 8th European VLBI Network Symposium Pushkarev 2001, Astron. Rep., 45, 667 Stirling et al. 2003, MNRAS, 341, 405 Tail-on: Head-on:   1/   1/ Line-of-Sight Line-of-Sight   Viewed at < 1/ in observer frame Viewed at > 1/ in observer frame Head-on Radiation emitted at > 90o in jet rest frame Radiation emitted at < 90o in jet rest frame Funding was provided by the Irish Research Council for Science, Engineering and Technology.