Millisecond radio spikes from the dwarf M flare star AD Leonis

Abstract

This paper presents Arecibo observations of highly lefthand circularly polarized (up to 100%) spikes from AD Leonis at 1415 MHz with rise times τ<SUB>R</SUB> ≤ 5 ms. These rise times provide an upper limit of L ≤ 1.5x10<SUP>8</SUP>cm, and a lower limit to T<SUB>B</SUB> ≥ 10<SUP>16</SUP>K for a symmetric emitter. Some of the spikes were part of a quasi-periodic train of spikes with a mean periodicity of τ<SUB>P</SUB> = 32±5 ms and a total duration of τ<SUB>D</SUB> = 150 ms. This spike train was itself one pulse in a quasi-periodic sequence of pulsations with a mean periodicity of τ<SUB>P</SUB> = 3.2±0.3 s and total duration of τ<SUB>D</SUB> = 25 s; the pulsations were superposed upon a longer lasting (50 s) burst. The high brightness temperatures and high circular polarization of the spikes are interpreted in terms of coherent maser emission processes. One possible explanation of the quasi-periodic pulsations and spikes is magnetoacoustic oscillations in a coronal loop that modulate the maser action.