Theta is the angle to the line of sight
This animation shows how the twin relativistic jets in 3C31 would appear
when observed at different angles to the line of sight by
a radio telescope with a fixed (5000:1) dynamic range. The color
coding shows fixed fractions of the peak intensity, which
occurs in the central (nuclear) component in every image.
The jets appear identical when observed in the plane of the sky (90 degrees to the line of sight).
At angles closer to the line of sight, the apparent asymmetry between the jets grows, due to the different relativistic beaming of the approaching (right) and receding (left) jets. The unresolved nuclear component also appears to brighten, so the faintest jet emission becomes undetectable with an instrument that provides a finite dynamic range.
At the smallest angles to the line of sight, only the base of the brighter (approaching) jet can be detected, so the jet appears to be "one-sided" (and also much smaller than its true extent).
The relativistic jet model used for this display is the best fit by R.A.Laing and A.H.Bridle (2002) to 8 GHz VLA data for 3C31 at 0.25 arcsec resolution. The first 27.5 arcsec (about 9.4 kpc in projection) of each jet are shown.
The calculations end at 17 degrees to the line of sight as this is close to the limiting case that our code can compute, wherein the line of sight lies inside the widest cone angle subtended by the jet outflow at the galactic nucleus. The best fit to the VLA data is at 52 degrees to the line of sight.