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Future Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA ASP Conference Series, Vol. ***, 2003 J. D. Romney & M. J. Reid (eds.)

Multi-frequency & Multi-epoch VLBI study of Cygnus A
U. Bach, M. Kadler, T.P. Krichbaum, E. Middelb erg, W. Alef, A. Witzel and J.A. Zensus Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, 53121 ¨ ¨ Bonn, Germany Abstract. We present the first multi-frequency phase-referenced observations of Cygnus A done with the VLBA at 15 and 22 GHz. We find a pronounced two-sided jet structure, with a steep sp ectrum along the jet and a highly inverted sp ectrum towards the counter-jet. The inverted sp ectrum and the frequency dep endent jet to counter-jet ratio suggest an obscuring torus in front of the counter-jet. From 14 ep ochs of 15 GHz VLBA data we accurately derive the jet and counter-jet kinematics. For the inner jet (r 5 mas) we measure motions of app 0.2 - 0.5 h-1 and on the counter-jet side we find app 0.03 ± 0.02 h-1 . We discuss the jet velocities within the unified jet model.

arXiv:astro-ph/0309403 v1 15 Sep 2003

1.

Introduction

Cygnus A is one of the most p owerful radio galaxies at a redshift of (z = 0.057). It is the archetypical FR I I radio galaxy. In the radio bands, Cygnus A is characterized by two strong lob es separated by 2 in the sky. Two highly collimated jets connect the lob es with the core (Perley et al. 1984; Carilli et al. 1991). On kiloparsec scales, the jet is oriented along P.A. 285 and the fainter counterjet along P.A. 107 . Due to the large inclination of the jet with resp ect to the observer, and the corresp ondingly reduced relativistic effects, Cygnus A is an ideal candidate for detailed studies of its jet physics, which is thought to b e similar to those of luminous quasars (e.g. Barthel 1989). We present and discuss results from multi-frequency VLBI observations at 15, 22 and 43 GHz and from the first multi-frequency phase referenced observations of Cygnus A done with the VLBA at 15 and 22 GHz. To complement our data at 15 GHz we used ten ep ochs from the VLBA 2 cm Survey (Kellermann et al. 1998; Zensus et al. 2002) to analyse the source kinematics with improved accuracy.

2.

Observations and Data Reduction

We observed Cygnus A in 1996 with the VLBA+Effelsb erg at 15, 22, and 43 GHz, in 2002 at 5 and 15 GHz and in 2003 with the VLBA only at 15 and 22 GHz in phase-referencing mode. All observations were done in dual circular p olarization and were correlated in Socorro. The data were reduced in the standard manner 1


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Bach et al.

using Aips. The imaging of the source implying phase and amplitude selfcalibration was done using Difmap. After imaging we fitted circular Gaussian comp onents to the self-calibrated data in order to parameterise the source structure. Conservatively, we assume errors of 10 % in the flux density arising from the uncertainties of the amplitude calibration and from the formal errors of the model fits. An estimate · for the p osition error is given by r = 2S (Fomalont 1989), where is the P residual noise of the map after the subtraction of the model, the width of the comp onent, and SP the p eak flux density. This formula tends to underestimate the error if the p eak flux density is very high or the width of the comp onent is small. In the case of a small FWHM we used the b eam size instead. 3. Results and Discussion

To investigate the sp ectral prop erties and the kinematics of Cygnus A on parsec scales we cross-identified individual model comp onents along the jet using their relative separation from each other, their flux density and size. Since the observations from 2003 were phase-referenced, we compared the alignment b etween the two frequencies from the modelfits with those from the phase-referencing and found similar results. The most likely p osition for the central engine is located b etween comp onent C3 and J10 (see Fig. 1). This p osition shows a slightly inverted sp ectrum and turned out to b e stable in our kinematical study. It seems to b e the same p osition, which Krichbaum et al. (1998) already used in their study.
4
C3 C2 C1 J10 J8 J7 J5 J3
15/22 GHz '96 22/43 GHz '96 15/22 GHz '03

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2

spectral index
J4 N J6 J9

1

0

-1

-2 -6

-4

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0 r [mas]

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Left: Phase referenced contour images of Cygnus A at 15 GHz (top) and 22 GHz (bottom). The dotted lines indicate the position of the modelfit components. Right: Profile of the spectral indices along the jet axis, 15/22 GHz from 1996 and 2003 and 22/43 GHz from 1996. r = 0 corresponds to the position of N in the left panel.

Figure 1.

3.1.

Sp ectral analysis

Figure 1 (left) shows the hybrid maps at 15 and 22 GHz with the p ositions of the modelfit comp onents denoted by dotted lines. The images are registered using phase-referencing to the quasar 2005+403 at a distance of 1.5 . On the right


Multi-frequency & multi-ep och VLBI study of Cygnus A

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hand side the sp ectral indices b etween 15, 22 and 43 GHz are plotted versus the core distance, assuming N is the core. Our analysis reveals that the core sp ectrum is slightly inverted with a sp ectral index of 15/22 22/43 0.5. The jet shows a steep sp ectrum with sp ectral indices of 15/22 -0.5 to 15/22 -1.2 and 22/43 -1.5. The counter-jet has a highly inverted sp ectrum with 15/22 up to 2.5 in the inner part (r 2 mas) and a flat sp ectrum b etween 15 and 22 GHz further out. Between 22 and 43 GHz the counter-jet sp ectrum is inverted with a sp ectral index of 22/43 1. The inverted sp ectrum on the counter-jet side is likely due to free-free absorption by a foreground absorb er. This is supp orted by the sp ectral b ehavior of the jet to counter-jet flux density ratio (Krichbaum et al. 1998; Bach et al. 2002). UV sp ectroscopy (Antonucci et al. 1994) and optical sp ectro-p olarimetry (Ogle et al. 1997) also show evidence for a hidden broad line region. According to the unified scheme, this is strong evidence for an obscuring torus around the central engine and is consistent with the results from 21 cm absorption line VLBI (Blanco & Conway 1996). 3.2. Kinematics and Geometry

Figure 2 shows the comp onent p ositions against time after a careful identification and linear fits to those, yielding apparent velocities app as follows. On the jet side the comp onents start with 0.1 mas yr-1 near the core and accelerate to 0.25 mas yr-1 at larger separations. In Cygnus A 1 mas corresp onds to 0.8 h-1 p c so that an apparent motion of 0.1 mas yr-1 corresp onds to app = 0.26 h-1 .
8 7 6 5 4
r [mas]

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J1 J2 J3 J4 J5 J6 J7 J8 J9 N C3 C2 C1 X lin fit

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Carilli 1994 (5 GHz) Bach 2002 (5 GHz) Krichbaum 1998 (22 GHz) Bach 2003 (15 GHz)

apparent velocity

0,6 0,4 0,2 0

3 2 1 0 -1 -2 -3 -4 1995
1996 1997 1998 1999 2000 epoch 2001 2002 2003

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Figure 2.

Left: Component distance at N versus time from 14 epochs of VLBI data at 15 GHz. Right: Apparent velocities versus separation from the co r e.

The situation on the counter-jet side is less clear than on the jet side. The comp onents are weaker and more extended than on the jet side. The innermost comp onent C3 shows a marginal motion of 0.010 ± 0.007 mas yr-1 and the next comp onent C2 seems to travel inwards with 0.02 ± 0.01 mas yr-1 . In the ep ochs with the highest quality data, comp onent C2 consists of two nearly equally bright comp onents. The apparent inward motion thus might b e due to blending effects of these sub-comp onents. A summary of the comp onent sp eeds is given in Table 2. From the right panel in Fig. 2 it seems that the jet comp onents accelerate as they travel down the jet and reach there maximum sp eed of 0.5 c


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Bach et al. Table 1. Prop er motions of the jet and cou Comp µ [mas/yr] app Comp C2 0.02 ± 0.01 0.05 ± 0.03 J5 J4 C3 -0.01 ± 0.01 -0.03 ± 0.02 J9 0.10 ± 0.02 0.25 ± 0.04 J3 J8 0.09 ± 0.01 0.24 ± 0.03 J2 J1 J7 0.09 ± 0.01 0.23 ± 0.02 J6 0.13 ± 0.02 0.32 ± 0.04 nter-jet com µ [mas/yr] 0.18 ± 0.01 0.18 ± 0.02 0.18 ± 0.03 0.22 ± 0.11 0.09 ± 0.01 p onents. app 0.46 ± 0.04 0.45 ± 0.04 0.46 ± 0.07 0.56 ± 0.28 0.23 ± 0.03

s n at a distance of r 2 mas. Using app = 1-ico , which is maximized at s cot = app and with app = 0.5 h-1 one can calculate the lower limit of the intrinsic velocity min of 0.45 h-1 c and the corresp onding angle to the line of sight of = 64 . Indeed, the analysis of the jet to counter-jet ratio also favors a large inclination of 80 ± 8 (Krichbaum et al. 1998; Bach et al. 2002). Due to the small relativistic effects at such conditions, the angle to the line of sight needs to change by more than 25 to explain the observed velocities by geometrical jet curvature. Since the jet app ears to b e very straight from parsec to kiloparsec scales, it is more likely that we observe a true acceleration, p ossibly caused by the collimation of the jet in the inner 2 parsec. Alternatively, we might observe a highly stratified jet with the different velocities b elonging to different layers in the jet. This idea can also explain the absence of detectable motions on the counter-jet side. Due to the fact that we see the counter-jet from its `back' the emission of the faster comp onents is b eamed away from us and we observe only the slower velocities of the outer sheath of the counter-jet. Bach et al. (2002) detected apparent motion of app = 0.15 ± 0.12 h-1 on the counter-jet side at a distance of r 8 mas from the core at 5 GHz, but this optically thin comp onent is no more visible at 15 GHz. From the assumption of a simple K¨nigl jet (K¨nigl 1981) and an inclination of 65 ­ 80 we would exp ect o o a prop er motion of app = 0.3 - 0.4 h-1 on the counter-jet side. That we do not see these `high' velocities on the counter-jet p oints to a more complicated jet structure.

4.

Conclusion

We carried out the first phase-referencing observations of Cygnus A with the VLBA at 15 and 22 GHz. The analysis of this data and a multi-frequency observation from 1996 at 15, 22 and 43 GHz revealed the sp ectral prop erties of the innermost jet structure in Cygnus A. We found a slightly inverted core and a steep jet sp ectrum. On the counter-jet side the inner part (r 2 mas) shows a highly inverted sp ectrum with an 15/22 of up to 2.5 and 22/43 1. In the outer regions the counter-jet shows a flat sp ectrum b etween 15 and 22 GHz and is still inverted b etween 22 and 43 GHz. Together with the frequency dep endence of the jet to counter-jet ratio (Krichbaum et al. 1998; Bach et al. 2002) there is strong evidence that this is due to free-free absorption by an obscuring torus. The apparent acceleration in the jet and the absence of detectable motions on the counter-jet side might reflect a more complicated jet structure than that


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of the simple K¨nigl jet with a well defined jet flow and questions the assumption o that the jet and counter-jet are intrinsically the same. The observations could b e explained by a stratification of the jet were we observe different velocities sheaths dep ending on the optical depth and the orientation of the jet. Acknowledgments. 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, op erated under coop erative agreement by Associated Universities, Inc. and of the 100 m telescop e at Effelsb erg, which is op erated by the MaxPlanck-Institut fur Radioastronomie in Bonn. ¨ References Antonucci, R., Hurt, T., & Kinney, A. 1994, Nature, 371, 313 Bach, U., et al. 2002, 6th EVN Symp., Bonn, ed. Ros E., (Bonn, MPIfR), 155 Barthel P.D. 1989, ApJ 336, 606 Blanco, P.R. & Conway, J. 1996, in Cygnus A ­ Study of a Radio Galaxy, ed. Carilli, C.L. & Harris, D.E., Cambridge University Press, 69 Carilli C.L., Bartel N., & Linfield R.P. 1991, AJ, 102, 1691 Fomalont, E.B. 1989 in Synthesis Imaging in Radio Astronomy, ASP Conf. Ser., Vol 6 Socorro, ed. Perley, R.A., Schwab, F.R. & Bridle, A.H., (San Francisco: ASP), 215 Kellermann, K.I., et al. 1998, AJ, 115, 1295 K¨nigl, A. 1981, ApJ, 243, 700 o Krichbaum, T.P., et al. 1998, A&A, 329, 873 Ogle, P.M., et al. 1997, ApJ, 482, L37 Perley R.A., Dreher J.W. & Cowan J.J. 1984, ApJ, 285, L35. Zensus, J.A., et al. 2002, AJ, 124, 662