Online material

Appendix A: The “tuning fork” and a possible obstacle

In Sect. 4.3 we discuss possible explanations of the jet widening (“tuning fork”), together with a decrease in surface brightness that appears to be stationary over the TANAMI monitoring period. The jet flow is likely to be disturbed by an obstacle. In the main text we conclude that a scenario including a red giant with a significant stellar wind is favored by our calculations and observations. In the following, we summarize the corresponding calculations when assuming the obstacle is a cloud.

The shock produced in the cloud by the interaction with the jet should propagate through it in a longer time than the penetration and crossing time if we expect the cloud to cross the whole jet diameter. Otherwise, the cloud would be disrupted by the shock when it has completely crossed it, and the bow-shock structure would have disappeared (Araudo et al. 2010; Bosch-Ramon et al. 2012). Knowing that the obstacle has been within the jet for at least ten years, hence a cloud shocking time of t_cs> 3.1 × 10⁸ s, we can therefore use Eq. (4) in Araudo et al. (2010) to give a lower limit of the density of the cloud: (A.1)where we have taken the distance to the core z = 1 pc = 3.1 × 10¹⁸ cm as a reference. Taking the aforementioned lower limit on time into account and taking as input data the obstacle radius R_c = 10^-3 pc, the jet power from the literature (L_j ≃ 10⁴⁴ erg s^-1, Abdo et al. 2010), and the location of the “tuning fork” along the jet z ≃ 0.4 pc, we can give a limit on the cloud density: (A.2)

Rivers et al. (2011) report an occultation of the central core for 170 days owing to the passage of a discrete clump of material through the line of sight in the context of a clumpy torus model. We suggest that the eclipse is short because the angular size of the eclipsed region must be small. Interestingly, for the hypothetical clump, which is located at a similar distance to the core to what is reported for the location of the “tuning fork” (0.1...0.3 pc, adopting measurements by Meisenheimer et al. 2007), they obtain a central density of n_clump = (1.8−3.0) × 10⁷ cm^-3 and a size of R_clump = (1.4−2.4) × 10¹⁵ cm. The size of the clump would agree with our result, but the density we obtain, as required for the cloud to survive the interaction for ten years, is three orders of magnitude higher. In addition, the cloud velocity estimated by Rivers et al. (2011) is one order of magnitude lower than the one we used for our calculations (v_clump ≃ 1000 km s^-1, Eq. (3)). Such a decrease in the velocity would have two effects: 1.) the crossing time would be increased by a factor of ten in Eq. (3), i.e., t_j ≃ 6.2 × 10⁹ s (approximately two hundred years); and 2.) owing to this increase in the crossing time, the lower limit in the cloud density obtained in Eq. (A.2) would be increased by a factor of 100, bringing it to n_c> 1.5 × 10¹² cm^-3. Following the result of Rivers et al. (2011), we could thus conclude that the cloud scenario should be ruled out for the “tuning fork” because: 1) it is difficult to expect clumps with higher densities at greater distances from the nucleus; and 2) the shock crossing time of the cloud, given by Eq. (A.1), would be reduced by a factor 20 with the numbers given in that paper for the cloud density, thus giving t_cs ≃ 3.4 × 10⁷ s (about one year). This means that clumps of the size and density, such as those obtained by Rivers et al. (2011) for the case of a clumpy torus, would survive for about one year before being disrupted and mixed with the jet if they come to collide with it; i.e., they would be destroyed close to the jet boundary, provided that their t_j is ~200 years. Therefore, this scenario cannot explain the steady situation that is observed at the “tuning fork”.

© ESO, 2014