1995A&A...299..739S

Multifrequency observations of the flare of December 31, 1991 on UV Ceti are presented. The radio observations were carried out with the Effelsberg 100-m radio telescope at 4750MHz, whereas optical photometry was performed using the 0.8-m telescope of the Wendelstein Observatory in the five UBVRI colors. The radio burst started ∼5min after the maximum of an optical flare. X-ray emission observed with the ROSAT PSPC before and after the optical flare conclusively demonstrates that an X-ray flare has occured. Several narrow-bandwidth radio spikes with duration of about 0.1s, peak flux density of 250mJy, and ≥75% LH polarization were recorded. An interpretation of the spikes in terms of electron-cyclotron maser emission (ECME) and, alternatively, coherent plasma emission is proposed. The flare plasma parameters obtained from radio and soft X-ray data are as follows: n_e≃10¹¹/cm-3, T_e≃10⁷K, B≃(200-800)G. Using these values, the escape windows for ECME have been calculated at ν=4.75GHz. It has been shown that there is actually no "perpendicular" window for the ordinary mode at the second and third harmonics of the electron gyrofrequency. The optical flare's cross-section area and the temperature of the "cool" plasma were found to be 4x10¹⁶cm² and 16000K, respectively. Possible reasons for the time delay between the optical and radio flares as well as stellar flare models are discussed.