The self absorption of gyro-synchrotron emission in a magnetic dipole field: Microwave impulsive burst and hard X-ray burst

Abstract

The gyro-synchrotron emission from a model source with a non-uniform magnetic field is computed taking into account the self absorption. This model seems adequate not only to interpret the radio spectrum and its time variation of microwave impulsive bursts but also to solve the discrepancy between the numbers of non-thermal electrons emitting radio burst and those emitting hard X-ray burst.

The decrease of flux of radio burst with decreasing frequency at low microwave frequencies is due to the self absorption and/or the thermal gyro-absorption. In this frequency range, the radio source is optically thick even at weak microwave bursts. The weakness of the bursts may be rather due to the small size of the radio source and/or the weakness of the magnetic field than the small number density of the non-thermal electrons.

The time variation of the flux of radio burst may be mainly attributed to the variation of source size in a horizontal direction (Φ direction) instead of the variation of the number density of non-thermal electrons itself, implying that the acceleration region progressively moves in the horizontal direction leaving the non-thermal electrons behind during the increasing phase of the radio burst.