Annu. Rev. Astron. Astrophys. 1981. 19: 373-410 Copyright © 1981 by Annual Reviews. All rights reserved

4.3 Polarization Variations

A number of the compact radio sources show significant and variable linear polarization. Although the measured polarization is typically only a few percent, in some sources, particularly those identified with BL Lac objects, the linear polarization sometimes exceeds 10 percent. The absence of any detectable internal Faraday rotation in these sources has an important consequence for models of compact sources and theories of particle acceleration, since it places strict limits on the number of thermal electrons permitted. Wardle (1977) and Jones & O'Dell (1977) have shown, in this way, that the number of relativistic electrons must be considerably greater than the number of nonrelativistic ones, and furthermore, that the number of low energy relativistic electrons must be much smaller than that expected from the usual power-law distribution. Thus any type of stochastic acceleration process would be difficult to understand. Rather, it appears more probable that some very efficient mechanism must operate, and several authors (e.g. Wardle 1977, Jones & Hardee 1979, Spangler 1980) have pointed out that relativistic Maxwellian distributions are consistent with the restrictions imposed by the polarization and variability observations, and that such an energy distribution could be the natural consequence of acceleration by a relativistic blastwave.

Detailed observations of the variations of linear polarization are difficult, since the degree of polarization is typically only a few percent, and the time scale for significant changes appears to be much less than for variations in the total flux density. Observations by Altschuler & Wardle (1976), Andrew et al. (1978), and Aller et al. (1981) show no clear patterns. In some sources the polarization angle remains constant throughout several consecutive flux density outbursts. In other sources, the direction of polarization may change, either monotonically or quite erratically.

A particularly remarkable outburst occurred in the BL Lac object 0235 + 16 which reached a flux density maximum in late 1975 (MacLeod et al. 1976, Ledden et al. 1976). During a period of ~ 40 days during the decline of this outburst, the polarization direction rotated through an angle of ~ 130 degrees (Ledden & Aller 1979). As there was no significant difference between the position angles measured at 8 and 14 GHz, Faraday rotation is ruled out as a possible cause, and rotation or precession of the magnetic field appears the most straightforward interpretation. The 1975 outburst in 0235 + 16 is distinguished from other observed outbursts in that the flux density was several times greater than the quiescent value, and so the observed rotation probably more closely reflects the properties of the "central engine" than the more erratic variations that are observed in most other sources.