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 Tb ∼ 5 × 1010 K (with values Tb ≳ 3 × 1013 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 ∼ 2 × 1011 cm-3 and B ∼ 800 G. Quasi-periodic pulsations during the decay phase of the event suggest a loop radius of r ∼ 7 × 108 cm. A filamentary corona is implied in which the dense radio source is embedded in hot thin plasma with temperature T ≥ 2 × 107 K and density next ≤ 10-2n. Runaway acceleration by sub-Dreicer electric fields in a magnetic loop is found to supply a sufficient number of energetic electrons.