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Proceedings of the 7th European VLBI Network Symposium Bachiller, R. Colomer, F., Desmurs, J.F., de Vicente, P. (eds.) October 12th-15 2004, Toledo, Spain

Spectral properties of the core and the VLBI-jets of Cygnus A
U. Bach, T.P. Krichbaum, M. Kadler, E. Middelberg, W. Alef, A. Witzel and J.A. Zensus
Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, 53121 Bonn, Germany Ё Ё
Abstract. We present a detailed VLBI study of the spectral properties of the inner core region of the radio galaxy Cygnus A at frequencies between 5 GHz, 15 GHz, 22 GHz, 43 GHz and 86 GHz. The observations include a phase referencing epoch at 15 GHz and 22 GHz and the first 86 GHz VLBI observations of Cygnus A. We find a pronounced two-sided jet structure, with a steep spectrum along the jet and a highly inverted spectrum towards the counter-jet. The inverted spectrum and the frequency dependent jet to counter-jet ratio suggest that the inner counter-jet is covered by a circum-nuclear absorber as it is proposed by the unified scheme.

1. Introduction
Cygnus A is the closest (z = 0.057) strong FR II radio galaxy and therefore a key object for detailed studies of AGN. Its kiloparsec-scale structure in the radio bands is dominated by two huge radio lobes that contain bright hot-spots. The radio core lies in the centre of an elliptical galaxy (Hargrave & Ryle 1974) and is connected by two thin jets to the radio lobes (e.g., Perley et al. 1984). VLBI images from 1.6 GHz to 43 GHz obtained during the last 20 years (Carilli et al. 1991; Krichbaum et al. 1993; Carilli et al. 1994; Krichbaum et al. 1998; Bach et al. 2002, 2004) revealed a pronounced twosided core-jet structure also on parsec scales. According to the unified scheme, narrow line radio galaxies, like Cygnus A, should contain an obscuring torus around the central engine that blocks the emission from the broad line region (BLR) (e.g. Urry & Padovani 1995). Evidence for a hidden BLR in Cygnus A comes from UV spectroscopy (Antonucci et al. 1994) and optical spectro-polarimetry (Ogle et al. 1997). These results are consistent with VLBI observations of H I absorption (Conway & Blanco 1995) at the footpoint of the counter-jet suggesting that the absorption is caused by a ring or disk-like structure oriented perpendicular to the jet axis. This is also supported by multi-frequency VLBI studies, that show a highly inverted spectrum on the counter-jet side and a frequency dependent jet to counter-jet flux density ratio, which is likely due to free-free absorption by a foreground absorber (Krichbaum et al. 1998; Bach et al. 2002). In this study we intend to further constrain the existence of a circum-nuclear absorber. Therefore we prepared spectral index maps from VLBI images of Cygnus A obtained at frequencies from 5 GHz to 86 GHz.

Table 1. Observation log. Listed are the observing epoch, frequency , total flux density S tot (), beam size, beam position angle, peak flux density S peak , the rms noise of the map and polarization mode.
Epoch 1996.73 1996.73 1996.73 2002.03 2002.03 2002.51 2002.51 2003.04 2003.04 2003.24 2003.24 2003.27 2003.27 2003.27
a a a a a a a a a

[GHz] 15.4 22.2 43.2 4.9 15.4 4.9 15.4 15.4 22.2 4.9 15.4 14.4 43.1 86.2

S tot [Jy] 1.71 1.48 1.01 0.89 1.51 0.91 1.50 1.27 1.27 0.99 1.38 1.52 0.75 0.41

Beam, P.A. [mas в mas], [ ] 0.30 в 0.61, -18.3 0.24 в 0.47, -16.1 0.23 в 0.27, 3.6 0.92 в 1.54, -23.8 0.31 в 0.56, -21.3 0.87 в 1.56, -23.0 0.46 в 0.67, -14.4 0.46 в 0.73, -5.1 0.31 в 0.51, 10.4 1.10 в 1.72, -20.1 0.25 в 0.52, -23.4 0.45 в 0.68, 0.6 0.16 в 0.26, -11.4 0.32 в 0.36, 88.4

S peak Jy [ beam ] 0.40 0.36 0.40 0.12 0.28 0.15 0.39 0.32 0.34 0.21 0.23 0.35 0.23 0.33

[
mJy beam

Pol. ] dual dual dual dual dual dual dual dual dual dual dual LCP LCP LCP

0.36 0.82 1.90 0.17 0.12 0.16 0.17 0.26 0.45 0.14 0.12 0.26 0.64 1.89

Note: The array used was the VLBA, unless indicated by a footnote. Epochs in bold face denote own data. a : VLBA+Eb. : phase-referencing.

using NRAO's Astronomical Image Processing System (AIPS). The imaging of the source employing phase and amplitude selfcalibration was done using the CLEAN (Hogbom 1974) and Ё SELFCAL procedures in DIFMAP (Shepherd 1994). The selfcalibration was done in steps of several phase-calibrations followed by careful amplitude calibration. During the iteration process the solution interval of the amplitude self-calibration was shortened from intervals as long as the whole observational time down to minutes. Some of the observations (2003.04 & 2003.27) were performed using non standard techniques and a more detailed the description of their reduction can be found in Bach (2004).

3. Results and Discussion
To orient the reader, a collection of images at different frequencies from early 2003 is shown in Fig. 1. At 5 GHz one can see the jet and the counter-jet up to 50 mas from the core (1 mas 1.1 pc, using H0 = 71 km s-1 Mpc-1 , m = 0.3 and = 0.7 (Bennett et al. 2003)) separated by an emission gap of about 4 mas left of the maximum. The cross-identification was done using individual modelfit components along the jets. The identification was done using their relative separation from each other, their flux density and size. An upper limit of 0.2 mas

2. Observations and Data Reduction
We carried out six multi-frequency VLBI epochs of Cygnus A during the last years, including a phase-referencing observation at 15 GHz and 22 GHz in 2003 and for the first time VLBI observations at 86 GHz with the VLBA using the technique of fast frequency switching at 15 GHz, 43 GHz and 86 GHz developed by Middelberg et al. (2002). A detailed observing log is given in Tab. 1. The data were reduced in the standard manner


U. Bach et al.: Spectral properties of the core and the VLBI-jets of Cygnus A
3 2 1 0 -1 -2 3 2 1 0 -1 -2 3 2 1 0 -1 -2 3 2 1 0 -1 -2 -4
5/15 GHz



15/22 GHz





43/86 GHz



22/43 GHz

N

-3.5

-3

-2.5 -2

-1.5

-1 -0.5

0 0.5 r [mas]

1

1.5

2

2.5

3

3.5

4

Fig. 2. Spectral index profiles between frequency pairs at (5, 15, 22, 43, and 86) GHz. The profiles represent cuts through spectral index maps along the ridge-line of the jet. The dotted line marks the position of component N.

Fig. 1. VLBI images of Cygnus A at 5 GHz (2003.24), 15 GHz and 22 GHz (2003.04, phase-ref.), 43 GHz (2003.27) and 86 GHz (2003.27) (2003.24). The beam size, peak flux density and rms noise are given Tab. 1. The lowest contours start at 3 and increasing in steps of 2. The bottom panel also shows the overlaid modelfit components and their identification between different frequencies.

for the shift of the brightest component between 15 GHz and 22 GHz could be derived from the phase-referencing observation (2003.04) and was used to constrain the identification. Component N dealt as a reference component during the analysis, but does not represent the nucleus. It is most likely the first counter-jet component and the AGN is located somewhere between N and J14 (Bach 2004). Spectral index profiles of the inner region around the core are presented in Fig. 2 and show clearly the different behaviour of the spectral index at different frequency pairs. Most of the jet emission has a steep spectrum, whereas the counter-jet spectrum is flatter. The spectral properties in the core region are much more complex showing a highly inverted spectrum between 5 GHz and 15 GHz and also highly inverted regions at the higher frequency pairs, but always at different locations. Synchrotron self-absorption can produce spectral indices of up to 2.5, but between (-3 r 0) mas the spectral index between 5 GHz and 15 GHz exceeds this maximum value significantly. The most likely explanation is that in this region the lower frequencies become affected by free-free absorption. This is supported by recent simulation of radiative transfer models for obscuring tori in active galaxies that were applied to Cygnus A (van Bemmel & Dullemond 2003). The spectral energy distribution (SED) was best fitted by an inclined ( 50 ) torus of 10 pc to 30 pc, which would be in good agreement with our results. To estimate the amount of absorption in the radio bands we fitted a synchrotron spectrum that was modified by free-free absorption to jet-spectra that were calculated along the jet. At the positions where we think free-free absorption is most relevant the resulting optical depth at 5 GHz is on average 4.3 ± 1.0. Assuming a typical temperature of 104 K and a path length of about 5 pc for the absorber this correspond to an

absorbing column density of 7 в 1023 cm (see Bach (2004) for calculations), which is about two times higher than the column density inferred from X-ray absorption of 4 в 1023 cm-2 (Young et al. 2002). Since the X-ray measurement is made with a much lower resolution and therefore averages the absorption over a larger area, it is not surprising that we observe a higher column density. Thus, the emission gap between the jet and the counter-jet at 5 GHz seems to be the imprint of a circum nuclear absorber that might cover also a large fraction of the counter-jet up to 20 mas and becomes thinner further out, where the counter-jet shines through.
Acknowledgements. We thank the group of the VLBA 2cm Survey for providing their data. This work made use of the VLBA, which is an instrument of the National Radio Astronomy Observatory, a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. and of the 100 m telescope at Effelsberg, which is operated by the Max-Planck-Institut fur Ё Radioastronomie in Bonn. U.B. acknowledges partial support from the EC ICN RadioNET.

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