Abstract High brightness temperatures are a characteristic feature of IntraDay Variability (IDV) of extragalactic radio sources. Recent studies of the polarization properties of some IDV sources (e.g., 1150+812, PKS 0405–385 and 0716+714) have shown that these sources harbor several compact IDV components with angular sizes of ~10–30μas and very high polarizations (of up to ~50%–70%). These results indicate the possibility of the existence of uniform magnetic fields in the IDV components. We investigate the incoherent synchrotron and self- Compton radiation of an anisotropic distribution of relativistic electrons which spin around the magnetic field lines at small pitch angles. The brightness temperature limit caused by second-order Compton losses is discussed and compared to the brightness temperatures derived from energy equipartition arguments. It is found that anisotropic distributions of electrons moving in ordered magnetic fields can raise the equipartition and Compton brightness temperatures by a factor of up to ~3–5. This would remove some of the difficulties in the interpretation of extremely high intrinsic brightness temperatures of >1012 K (or apparent brightness temperatures of ~1014 K with a Doppler factor of ~30).
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