DYNAMIC SPECTRA OF RADIO-BURSTS FROM FLARE STARS

Intense, highly circularly polarized radio bursts from dMe flare stars were recorded as dynamic spectra with 20 ms resolution using the correlator at the Arecibo 305 m telescope. We report observations of AD Leo at 1415 MHz with a bandwidth of 40 MHz and of YZ CMi at 430 MHz with a bandwidth of 10 MHz. The stellar bursts display a rich variety of structure in the frequency-time domain, including spikes with rise times less than 20 ms and durations less than 50 ms that attained flux densities up to 940 mJy at 1415 MHz and 1.6 Jy at 430 MHz; the corresponding brightness temperatures are near 1016 K in both cases. During a series of bursts at 1415 MHz, quasi-periodic pulsations were observed with an intensity modulation ΔI/I ≳ 50% and a period of about 0.7 s. In addition, "sudden reductions" of flux density were observed in some instances. Some of these properties are similar to those of certain solar bursts at decimetric wavelengths. We interpret the emission in terms of coherent emission processes driven by a loss-cone anisotropy. Two regimes are possible: (1) if the ratio of the electron plasma frequency ωp to the gyrofrequency Ωe is such that ωp/Ωe ≲ 3, then electromagnetic waves are directly amplified at low harmonics of the gyrofrequency, i.e., via the cyclotron maser mechanism; (2) if ωp/Ωe ≳3, then upper hybrid waves are amplified, and plasma radiation is produced near the fundamental and/or second harmonic of the plasma frequency. We suggest that the quasi-periodic pulsations observed during one event were caused by MHD oscillations in a magnetic loop. Radio-frequency interference, which can mimic stellar emissions, is of special concern with a single-dish telescope. We discuss four methods we have used to distinguish between them and give an example of an interference event that would have passed for a stellar burst had we not recorded its spectrum.