Microwave plasma emission of a flare on AD Leo

Abstract

An intense radio flare on the dMe star AD Leo, observed with the Effelsberg radio telescope and spectrally resolved in a band of 480 MHz centred at 4.85 GHz is analysed. A lower limit of the brightness temperature of the totally right handed polarized emission is estimated as $T_{\rm b}\sim5\times10^{10}$ K (with values $T_{\rm b}\ga3\times10^{13}$ K considered to be more probable), which requires a coherent radio emission process. In the interpretation we favour fundamental plasma radiation by mildly relativistic electrons trapped in a hot and dense coronal loop above electron cyclotron maser emission. This leads to densities and magnetic field strengths in the radio source of $n\sim2\times10^{11}$ cm-3 and $B\sim800$ G. Quasi-periodic pulsations during the decay phase of the event suggest a loop radius of $r\sim7\times10^8$ cm. A filamentary corona is implied in which the dense radio source is embedded in hot thin plasma with temperature $T\ge2\times10^7$ K and density $n_{\rm ext}\le10^{-2}n$. Runaway acceleration by sub-Dreicer electric fields in a magnetic loop is found to supply a sufficient number of energetic electrons.