The spectrum of extragalactic jets is a pure synchrotron continuum, which is essentially a power-law. This characteristic does not facilitate the determination of the material velocity within the jet. Brightness asymmetries between jets and their counterjets in radiosources are generally believed to be apparent and due to Doppler effects. But obviously, there are intrinsic structural differences in a lot of double-sided jets, so that this relativistic interpretation can be questioned. There are both theoretical and observational evidences that the jet asymmetries in radiosources are mainly due to intrinsic differences in radiation properties. To distinguish between these two interpretations, one should compare the physics of the jet and counterjet. This is however very difficult with a power-law continuum spectrum.
The cutoff frequency of the synchrotron spectrum is very sensitive to local physical conditions within the jet. It is unknown for nearly all extragalactic jets (being in the sub-mm or far-infrared domain) except for the very few cases where the synchrotron radiation is seen in the optical: M87, 3C273, 3C66B, PKS0521-36, 3C264, and 3C78. The precise shape of the cutoff is known for only the three brightest: M87 3C273 and 3C66B. All known optical jets are one-sided both in the optical and in the radio, with the notable exception of 3C66B which shows a counterjet in the radio. If the relativistic interpretation for asymmetry is correct, the counterjet should also radiate in the optical and should have the same intrinsic spectrum. By deriving the Doppler factor from the jet to counterjet intensity ratio in the radio, it is then possible to compute the Doppler shifted spectrum of the counterjet from the observed spectrum of the jet. 3C66B is the only source for which this is feasible. The deep exposure observations presented here aims at detecting the optical counterjet in order to discriminate between the two interpretations for brightness asymmetry and to help in understanding why so few jets are seen at optical wavelengths. Observations were made at the Canada-France-Hawaii Telescope (CFHT) on September 24-27 and November 23-29, 1995. The detector was MOCAM, but only one of the four 2048×2048 CCDs (chip U2) was used. The scale of 0.204 arcsec per pixel provides a 7 arcmin field of view. To avoid non-linearity at high exposure levels on the center of 3C66B (for galactic background modelling and photometric purposes), the exposure time was limited to 10 minutes, except for 3 exposures taken through clouds where the integration time was set to 20 minutes. 85 images through the I filter were obtained totalling nearly 15 hours of exposure time. The galaxy 3C66B and its companion (at 23 arcsec to the SE) were removed by elliptical modelling.
On the residual map (Fig. 2), several knots are visible on the counterjet side (SW). Radio contours (Hardcastle et al. 1996) show the optical knots that are coincident with radio structures. However, it is still not possible to determine precisely the origin of the optical emission, and one cannot rule out chance coincidence of totally unrelated objects. But from the radio-to-optical spectral index of these components, it is quite plausible that they are the optical counterparts of the radio counterjet. If this is the case, smaller spectral indices are found than for the jet, in contradiction with the prediction by the relativistic beaming interpretation of brightness asymmetry between the jet and the counterjet. This would indeed mean that the physics of the counterjet is intrinsically different from that of the jet: the magnetic field seems to be weaker in the counterjet, so that the radio intensity is lower and the cutoff frequency higher. Alternatively, if the detected knots have no relation with the counterjet, then our data indicate that its (undetected) optical emission is fainter than what is predicted by the relativistic beaming interpretation, favouring again an intrinsic brightness asymmetry. A complete discussion can be found in Fraix-Burnet (1996).
Hardcastle M.J., Alexander P., Pooley G.G., Riley J.M., 1996, MNRAS 278, 273