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Abstract

We re-examined the brightness temperature (T_B)problem posed by intra-day variables (IDV's) such as S5 0716+714, which often display T_B>10^{12} K, far exceed the limit imposed by inverse Compton cooling. Several of such IDV's have also shown a high degree of circular polarisation (CP) of the order of ~1%, much higher that predicted by the conventional synchrotron theory. We adopted a synchrotron model that replaces the conventional power-law electron spectrum with a mono-energetic electron distribution, and found that a brightness temperature as high as T_B~10^{14} K can be reached with a moderate Doppler factor of D~10. The predicted intrinsic degree of CP is of the order of 1%, in agreement with observations. With a predicted spectrum of I_\nu\propto\nu^{1/3} between the radio and the infra-red frequencies, this model provides a good spectral fit to the IDV S5 0716+714 in the radio band.