Mapping Magnetic Field Profiles Along AGN Jets Using Multi-Wavelength VLBI Data

1. Mapping Magnetic Field Profiles Along AGN Jets Using Multi-Wavelength VLBI Data Mark McCann, Denise Gabuzda Department of Physics, University College Cork, Ireland Understanding Relativistic Jets, Krakow, Poland, 23rd-26th May, 2011 mark.mccann@umail.ucc.ie, d.gabuzda@ucc.ie 1. Abstract Lobanov (1998) developed an approach for deriving the local magnetic (B) fields in Active Galactic Nuclei (AGN) from the observed local frequency of the spectrum peak (νm) and corresponding peak flux density (Sm), based on a Blandford-Konigl jet model. Determining νm and Sm in individual regions of the source requires VLBI images encompassing, or nearly encompassing, the frequencies at which the spectral peaks lie, which have been accurately aligned relative to each other; we have used the 5-43GHz data of O'Sullivan & Gabuzda (2009) for the three AGN 1418+546, 2007+777, and 2200+420 (BL Lac), aligned using the cross-correlation algorithm of Croke & Gabuzda (2008). Our results suggest the presence of enhancements in the B field both in the core region and somewhat upstream of the observed core; we also find the B field to be enhanced in bright jet components well resolved from the core, consistent with the possibility that these are shocks. The jet B field of BL Lac falls off smoothly and monotonically with distance from the core, suggesting that the jet emission does not include a significant contribution from shocks. 2. Turnover Frequency We approximated the shape of the synchrotron spectrum at a given point in the VLBI images from a 4th order polynomial fit of the flux density against the frequency for that point, for the eight frequencies (4.6, 5.1, 7.9, 8.9, 12.9, 15.4, 22.2, and 43.1 GHz). We estimated lower and upper cutoff frequencies from fits in optically thick core regions, where a clear peak was observed. The spectral maximum, or turnover flux density Sm, was determined from the fit and the corresponding turnover frequency νm obtained. This process yielded distributions of Sm and νm throughout the source. 4. Results 1418+546 Frequency vs Flux Density in the VLBI Core of 2200+420 The derived Sm values are magnified by a factor of ten to show clear peaks near the core and a bright knot in the jet. The B field is strongest in the knot, and peaks slightly downstream and upstream of the core. Turnover Map of 2007+777 2007+777 Frequency vs Flux Density in the Jet of 2200+420 3. Magnetic Field Strength The turnover frequency can be sensitive to changes in the jet; namely velocity, particle density and magnetic field strength. For any point in the jet p, the magnetic field strength at that point Bp, is given by Lobanov (1998), where Bcore, νm,core and Sm,core are the magnetic field strength, turnover frequency, and spectral maximum of the VLBI core, and rp and rcore are the distances of the point and the VLBI core from the black hole. Since this cannot be determined, the distance of the points to the sonic point is used as a good approximation. Here νm is measured in GHz, Sm is in Jy, r in parsecs, and B in Gauss. We consider the case where m=1, since the core regions of these AGN are close to equipartition, O'Sullivan and Gabuzda (2009); the values of Bcore and rcore were taken from O'Sullivan and Gabuzda (2009). The profiles were constructed through the VLBI core region and along the jet, passing through any bright features in the jet; these show profiles of νm, Sm, and B along the jets. The derived Sm values are magnified again by a factor of ten to show clear peaks near the core and a bright knot in the jet. The B field has peaks slightly downstream and upstream of the core, and in the bright jet knot. 2200+420 5. Discussion The turnover frequency, turnover flux density and B field are smooth and physically plausible functions along the derived profiles. In all three AGN, the B field peaks slightly downstream from the core, possibly indicating strong B fields in emerging jet components; the B field also has a peak upstream from the observed “core”, possibly marking some region physically associated with the base of the jet. The jets 1418+546 and 2007+777 also have bright compact features with strong B fields, suggestive of shocks, whereas there is no evidence for shocks in the smooth B field distribution of the jet of 2200+420 (BL Lac). The B fields in the core-regions and bright knots of 1418+546 and 2007+777 are 5-10 G, whereas the B fields in BL Lac are lower, less than 1 G. The derived Sm shows only one peak near the core. The B field is magnified by a factor of a hundred; the B field is dominated by one peak slightly downstream from the core, and falls off smoothly in the jet. 6. Reference Lobanov, A.P. 1998A&AS, 132, 261L O'Sullivan, S.P. & Gabuzda, D.C. 2009MNRAS, 400, 26O Croke, S.M. & Gabuzda D.C. 2008MNRAS, 386, 619C