S. J. Avgoloupis

White-light oscillations during a flare on II Peg

We analyse the intensity oscillations observed in the gradual phase of a white-light flare on the RS CV n binary II Peg. Fast Fourier Transform power spectra and Wavelet analysis reveal a period of 220 s. The reliability of the oscillation is tested using several criteria. Oscillating coronal loop models are used to derive physical parameters such as temperature, electron density and magnetic field strength associated with the coronal loop. The derived parameters are consistent with the near-simultaneous X-ray observations of the flare. There is no evidence for oscillations in the quiescent state of the binary.

Fast colorimetry of the flare star EV lacertae from UBVRI observations in 2004

Aims. We report results of a quantitative colorimetric UBVRI analysis of two flare events on the red dwarf EV Lac. The photometric data were obtained in September 2004, during the multi-site synchronous monitoring from the four observatories in Ukraine, Russia, Greece, and Bulgaria. These observations confirmed the presence of small-scale high-frequency oscillations (HFO) initially detected by Rodono (1974, A&A, 32, 337) and recently reconfirmed by the authors. Here we discuss the color characteristics of flares and HFO. Methods. Colorimetric analysis had been performed with the help of the time tracks in the UBVRI color-color diagrams from the earliest phase of flare development. Digital filtering technique was used to evaluate the time-dependent color indices. Results. As can be clearly seen in the diagrams, color indices oscillate on a time scale of seconds, far exceeding instrumental errors. Regarding the HFO, we conclude that the bulk of a flare oscillates during a major part of its lifetime between the states of hydrogen plasma opaque and transparent in the Balmer continuum. We find that at the peaks of oscillations the color tracks drift into the regions of color-color diagrams corresponding to a blackbody radiation, which provides an estimate of color temperatures from 17 000 to 22 000 K. We also find that flares cover ∼1% of the stellar disc.

Photographic evidence of a short duration: Strong flash from the surface of the moon

Abstract We present photographic evidence of a very short duration, strong flash from the surface of the Moon (near an irregularly shaped crater in Palus Somni). The flash covered a region roughly 22 by 18 km wide with a total energy of the order of 10 17 erg. The event is established to be slightly above the surface of the Moon. An explanation is proposed involving outgassing and a subsequent electrical discharge caused by a piezoelectric effect.

The origin of the Moon flash of May 23, 1985

Abstract We present further evidence that the bright spot on the Moon observed on May 23, 1985, was a natural phenomenon that occurred slightly above the lunar surface. The interpretation that the spot could be attributed to an instantaneous reflection from a satellites solar panel fails to explain several important characteristics of the observation.

Coordinated Observations of the Red Dwarf Flare Star EV Lac in 1993

Results of cooperative observations of the flare star EV Lac in September 1993 are presented. One of the about 30 optical flares detected was powerful enough to permit a quantitative analysis of its intrinsic radiation with the colour-colour technique. Sinusoidal brightness variations due to spottedness of the stellar surface was found to have an amplitude Δ V = 0.m0.24. Behaviour of the K band stellar brightness during strong and weak U band flares are considered. The upper limits of very fast optical brightness variations were estimated during both a moderate flare and quiet state of the star. No decametric bursts were observed during the campaign that could be certainly attributed to flare activity.

Further investigation of the high frequency optical oscillations during the flare phase of the red dwarf EV Lac

In this paper we present the results of the analysis of the U‐light curve for a flare of magnitude 1.2, which was observed on September 2004, with the help of the 30‐inch Cassegrain telescope of the Stephanion Observatory. The results of both, DFT‐analysis and wavelet analysis, indicate that (a) Oscillations with period 1 to 1.5min, 11s, 7.5s and 5.5s appear during the pre‐flare state and persist during the whole flare state, (b) From the flare maximum phase on, a progressive increase of oscillations with periods 20s up to 4.0s is markedly indicated and (c) At the end of the flare only the oscillation of the pre‐flare state do remain. This result is in favour of (or does not contradict) the suggested explanation, i.e. the evolution of a fast mode magneto‐acoustic wave generated at the impulsive phase of the flare and traveling through the magnetic loop.

Imaging the Photosphere and Chromosphere of the RS CVn Star, II Peg

We present simultaneous starspot photometry and high resolution Hα spectroscopy of II Peg which gives evidence of differential motions in the chromosphere.