Orientation of the Radio Source

The projected distance, d$_{\rm {w}}$, of the W radio hot spot of Pictor A from the nucleus is larger than that, d$_{\rm {e}}$, of the E radio hot spots. This observation is consistent with the W radio lobe being the nearer one, since the extra light travel time to the more distant lobe implies that it is seen at an earlier phase in its motion away from the nucleus and thus appears closer to the nucleus. Further, the east lobe is more depolarized than the west at long radio wavelengths (PRM), again suggesting that the W lobe is the nearer (Laing 1988). Both the radio and X-ray jets of Pictor A are seen on the W side of the nucleus. These considerations strongly suggest that the jet sidedness is a result of relativistic boosting.

The angle of the source axis to the line of sight is, of course, unknown, but can be estimated from d$_{\rm {w}}$ and d$_{\rm {e}}$ if the source is assumed to be intrinsically symmetric, since

$$\frac{d_{\rm {w}} - d_{\rm {e}}}{d_{\rm {w}} + d_{\rm {e}}} = \beta\cos\theta$$ (1)

where $\beta$ = V/c, V is the velocity with which the hot spots are receding from the nucleus and $\theta$ is the angle between the radio axis and our line of sight. A recent study (Arshakian & Longair 2000) has found the mean value of $\beta$ in a sample of FRII radio galaxies to be 0.11 $\pm$ 0.013. For Pictor A, we may then write

$$\cos \theta = 0.92\ \left (\rm {{V}\over{0.11 c}}\right)^{-1}$$ (2)

so $\theta$ = 23$^{\circ }$ if V = 0.11 c. In view of its sensitivity to V and the assumption of symmetry, this value of $\theta$ should not be taken too seriously.

The radio lobes are remarkably round and are quite well separated in the VLA images (PRM), although joined by a `waist' near the nucleus. This morphology suggests that $\theta$ is larger than 23$^{\circ }$. A larger value is also suggested by the fact that the hot spots project further away from the nucleus than the outer edges of the lobes themselves.