Rotation Measures in AGN jets seen by VLA at 21 cm to 6 mm
aa r X i v : . [ a s t r o - ph . H E ] D ec Extragalactic jets from every angleProceedings IAU Symposium No. xxx, 2014F. Massaro, C. C. Cheung, E. Lopez & A. Siemiginowska, eds. c (cid:13) Rotation Measures in AGN jetsseen by VLA at 21 cm to 6 mm
Evgeniya V. Kravchenko , William D. Cotton and Yuri Y. Kovalev Lebedev Physical Institute,Profsoyuznaya 84/32, 117997 Moscow, Russiaemail: [email protected] National Radio Astronomy Observatory,520 Edgemont Road, Charlottesville, 22903-2475 Virginia, USA
Abstract.
We present Faraday Rotation Measure (RM) properties of seven active galactic nuclei(AGN), observed with the NRAO VLA at three epochs in 2012-2014. Data was taken at 1.4,2.2, 5.0, 8.2, 15.4, 22.4, 33.5 and 43.1 GHz quasi simultaneously in full polarization mode. Forthe first time RMs were calculated in a range of wavelengths covering more than one order ofmagnitude: from 21 cm up to 6 mm. We measured RM for each source and showed a tendencyto increase its value toward high frequencies according to the law | RM | ∼ ν a with a =1.6 ± − ± · rad/m .Analysis of different epochs shows variations of the value and the sign of RM on short and longtime-scales. This may be caused by changing physical conditions in the compact regions of theAGN jets, e.g. strength of magnetic field, particle density and so on. Keywords. galaxies: active, nuclei, jets, magnetic fields.
1. Introduction
Due to presence of highly magnetized media in the close vicinity of, and in AGNjets themselves, polarized emission is the subject of Faraday effects (Faraday 1993).One of them is Faraday rotation, which causes rotation of the plane of polarization ofan electromagnetic wave. As a result, the intrinsic position angle (EVPA), χ , of thejet electric vector will be rotated on a factor, depending on the plasma properties andobserved wavelength. In the simplest case, when magnetized plasma isn’t intermixed withthe emission region and depolarization effects are small, the EVPA depends linearly on λ : χ observed = χ + RM · λ , where the constant RM is the Rotation Measure: RM = e π ǫ m c Z n e B k dl . Thus, RM is proportional to the magnetic field component, parallel to the line of sight, B k and particle volume density n e along the path d l . The study of Faraday Rotation canbe done only in multi-frequency observations.Study of the polarized emission variations in AGN jets on short time scales is ofparticularly interest because it provides information about the jet structure and pointsto the locations of the regions where the emission originates and how it propagates to anobserver.Variability of the electric vector position angle in blazars on weekly, monthly and year-time intervals was shown more than once (e.g. D’Arcangelo et al. 2007, Jorstad et al.2007, Agudo et al. 2014). So far, only few a works have been done (e.g. G´omez et al.1 Evgeniya V. Kravchenko, William D. Cotton & Yuri Y. KovalevFigure 1: (left) Example of reconstructed dependence of electric vector position angle withwavelength squared for 0710+439. (right) Derived Rotation Measure versus frequency for0710+439 at three epochs in logarithmic scale.2011) analyzing the region of the jet where this variability goes from and it remains tobe the subject of future studies.The goal of this work is tp probe the physical conditions in AGN jets and their structureby studying the Rotation Measures in different sources, through frequency and time.
2. Observations
We use data from the EVLA polarization calibration program (Taylor & Myers 2010),available from NRAO Archive † under project TPOL0003. Data consists of observationsmade at eight frequencies quasi simultaneously, with switching between frequencies madewithin 30 minutes sequentially. The bandwidth in each frequency band is 256 MHz.To avoid bandwidth smearing (Gardner & Whiteoak 1966) we split bands onto 2 to 4frequency channels, depending on the wavelength. The resulting frequency setup is givenin Table 1.Target sources are presented in Table 2. There are 13 epochs of observations availablewith the frequency setup given above, starting in 2011. The configuration of the VLAduring these observations goes through all possible setups. Here we present results fromlast 3 epochs only: 2 September 2012, 12 January 2014 and 12 February 2014.
3. Data reduction
Data calibration and imaging is done using Obit package ‡ (Cotton 2008).Since our observations cover wide frequency range, we probe regions in AGN jets withdifferent Faraday depths and different physical properties. Supposing the external to thejet nature of Faraday media, EVPA should depend from wavelength-squared linearly. Tosum up these two assumptions, EVPA is linear with λ within different intervals, butmay arbitrarily change through the whole range of λ . Thus we identify the RotationMeasure as a linear slope in the dependence EVPA- λ at individual λ intervals. We letEVPA wrap on 180 ◦ between different channels and pick the solutions with the minimal χ and minimal number of wraps. Example of the reconstructed curve is shown on Figure1. † https://archive.nrao.edu/archive/advquery.jsp ‡ D 11. Rotation Measures in AGN jets seen by VLA
Band Central frequency Channelof channels, GHz bandwidth, MHz
L 1.423, 1.485, 1.801, 1.863 64S 3.055, 3.124, 3.197, 3.253 64C 4.863, 4.925, 4.991, 5.053 64X 8.365, 8.431, 8.497, 8.559 64K u a Table 2: Target sources.
B1950 source name Alias Optical class Spectral type −
055 3C 279 Quasar double-peaked1308+326 OP 313 Quasar flat
Length and number of the individual intervals, where RM were obtained, varies fromsource to source. For instance for 0710+439, given on Figure 1, RM were identified atseven λ intervals, solutions for which with frequency are shown on Figure 1 with solidcurve.Target sources are roughly point-like even for the most extended VLA configuration.Thus, we carry out the analysis only on the central region of the map, which is composedof the optically thin and thick components of the parsec-scale jet. To conclude whatcomponent dominates in the map we analysed fractional polarization and spectral indexof the sources.Spectra have different types: flat or steep behavior, one or double peaked and are givenin Table 2. The distribution of the fractional polariation, given on Figure 2, also indicatesthat we observed a mix of regions with different optical depths.During the analysis, each epoch were considered independently. The typical value ofrotation from Galactic media (Taylor et al. 2009) is a few radians per meters squaredwhich is significant only at low frequencies. We didn’t correct our measurements for it.
4. Results
We have estimated RMs for all target sources through the whole λ -interval at threeepochs, the distribution of which is represented on Figure 2. Variations of a few orders ofmagnitude in the value of RM can be seen there, meaning strong variations of physicalconditions in AGN jets among sources. Evgeniya V. Kravchenko, William D. Cotton & Yuri Y. KovalevFigure 2: (left) Distribution of the fractional polarization for all sources at eight fre-quency bands and at three epochs of obsevations. Hatched region indicates 14 - 43 GHzfrequency range, white - 1.4 - 14 GHz range. (right) Distribution of the obtained Rota-tion Measures for all target sources at eight frequencies at three epochs. Shaded regionindicates measurements in 14 - 43 GHz range, while white region represents 1.4 - 14 GHzfrequency range.For the majority of the cases, wraps of 180 ◦ in EVPA at some frequency bands relativeto the other bands reduces χ considerably. Moreover, an analysis of different epochsshows good agreement between estimated values of RM and it’s trend with frequency. Itis important to note that sometimes we see non-trivial EVPA- λ behavior which can notbe fit by a linear slope. This means that other Faraday effects take place there, such asmixing of the rotating media with the emission region, multi component Faraday mediawith different optical depths and others.A tendency of RM to increase in value with frequency | RM | ∼ ν a , example of whichis given on Figure 1, is found for all sources. We have used RM values for all available λ intervals to determine power a for every target performing a linear regression of lg | RM | -lg( ν ) curves. An average over all targets and three epochs is found to be 1.6 ± n e B k d l by up to five orders of magnitude with distance from the centralblack hole. Theoretical estimations give a value of a = (Jorstad et al. 2007), asumingan outflowing sheath around a conically or spherically expanding jet with a helically-shaped magnetic field. Our low value of a may result from underestimated contributionof the emission from optically thin regions. We plan to conduct a detailed study of thiseffect in the future. Case of 0710 + . For this source we obtained the following values of RM (in rad/m )in the 33 - 43 GHz range: ( − ± · , ( − ± · and ( − ± · at threeconsecutive epochs given above. These values were measured independently and are ingood agreement. The behavior of RM over the full frequency range for 0710+439 is shownon Figure 1.So far, high values of Rotation Measure in AGNs were observed in only a few sources(e.g. Trippe et al. 2012, Jorstad et al. 2007, Attridge et al. 2005) with the highest valueof 5 . · rad/m for Sagittarius A ⋆ (Marrone et al. 2006). D 11. Rotation Measures in AGN jets seen by VLA
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