High Brightness Temperatures in IDV Sources

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 energyequipartitionarguments.It is found that anisotropic distributions of elec- trons 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 > 10 12 K (or apparent brightness temperatures of ∼ 10 14 K with a Doppler factor of ∼30).