We have studied the initial phases of 65 optical flares on UV Cet type stars with a time resolution of 3x10–7 s. Ninety per cent of these flares have rapid brightenings shorter than 10 s and several are shorter than 1 s. We detected on UV Cet the shortest flare observed to date with a total duration of 1.7 s and a rise time of 0.3 s. The shortest flare rise times on the observed stars fit the predictions of the gas-dynamic flare model by Katsova et al. (1981) and Katsova and Livshits 1986).

High time resolution monitoring of the UV Cet type flare stars has enabled as to detect flares with total duration of a few seconds and with energy output comparable to solar subflares. We find that the time scales of decay for such flares correspond to recombination times of optically thin gas or to cooling times of optically thick gas heated in flares. Therefore, faster flares cannot be detected individually but their accumulated effect may be one source of brightness fluctuations of the “quiet” star. The upper limit for the total power of such microflares does not exceed the time-averaged power of individually detected flares.

Photoelectric observations of flares on two low-active red dwarfs (Gliese 171.2A and CM Dra) are reported.

Approximately 10 very bright flares in RS CVn systems have been observed with the IUE. In several cases, high-resolution spectra of the Mg II k line profiles were obtained before, during, and after the flare. Such data permit us to measure the position and size of the flaring region, the amount of line broadening, and any systematic flow velocity. In two cases, several spectra were obtained during the flare decay, permitting us to study the change in these quantities during the flare decay.

Ultraviolet spectroscopic observations of the RS CVn star II Peg in February 1983 show evidence for flare activity in greatly enhanced chromospheric and transition region emission lines. The total radiative losses from the chromosphere and transition region (i.e. the temperature interval 4.0 ≤ log Te ≤ 5.4) during the flare is 3.1 1035 erg. Over the whole atmosphere (i.e. the temperature interval 4.0 ≤ log Te ≤ 8.0), we estimate total radiative losses of 2.4 1036 erg, (excluding hydrogen line radiation). At flare peak, the flare radiated 1.5 1032 erg s–1. Adopting a two-ribbon flare model, where the filament is located between the two stars of the system, we can have 2039(l/R⊙) erg of magnetic energy available, where l is the filament length and we have taken a magnetic field strength of 1000G. Therefore, only a small fraction of this magnetic energy need be converted into heating of the flare plasma.

We have observed the white dwarf — K dwarf eclipsing binary system V471 Tauri with the VLA. We have detected in the radio flux an interesting dip centered near phase zero (the phase of white dwarf eclipse) and a 6 mJy flare shortly after mid—eclipse at phase 0.15. The best possible explanation for the dip is the self—eclipse of a large radio—emitting cloud anchored to a particular spot on the secondary, namely the sub—white dwarf point. The 6 cm flare observation and sudden variations seen in Hα suggest that this spot is an active flaring region.

We have re-analyzed the X-ray flare on Algol which was observed with EXOSAT (White et al. (1986)). The common practice of estimating loop volume and length from the decay time of the flare is discussed extensively. We show that during the decay phase of the flare both scaling laws for coronal loops are valid. This implies a unique determination of loop volume and length and allows a check whether additional heating occurs in the decay phase of a flare.

An X-ray flare from the star π UMA has been observed by EXOSAT on January 31, 1984. The data were collected using the-Low Energy Experiment and the Medium Energy Experiment, sensitive to the spectral bands 0.04 - 2 Kev and 1-20 Kev (Landini et al. 1986).

The temperature and emission measure variations during the flare are deduced under the isothermal assumption. Using a parametrized profile of the differential emission measure, wich is controlled by the conductive flux via the temperature gradient, the comparison with the observed data gives some insight on a “mean loop” model and its power supply.

We report the serendipitous discovery by EXOSAT of a flaring X-ray source in the field of the Seyfert type I galaxy III ZW 2. We identify this source with the visual binary HD 560 (B9V + G5Ve) and argue that virtually all of the observed X-ray flux, including the flare, came from its late-type component (HD 560 B). Optical studies have lead to the identification of HD 560 B as a post-T Tauri star. Since these stars are difficult to detect by optical methods, X-ray observations may prove to be the best way to identify them.

On the basis of 11 years of UBV photometry the light and color variability of the long-period RS CVn binary HK Lac (KOIII) is discussed. From the U-B data, which reflect the chromospheric activity, a quasi-periodic variation with the cycle length of about 5.5 years is suspected. Less evidently, a 2.7 years long overall spot coverage cycle is seen from the V(max) data.

No cyclic variation is found from the B-V data representing the photospheric temperature conditions, although there is an indication that the star was the reddest around 1985, when the chromospheric activity was in maximum.

At that time, in late 1985, we observed the star on 6 nights in UBV color in order to detect short-term variability. Fluctuations up to 0.03 magn. on time scales of 20-30 minutes were likely present. The mplitude is the highest in U colour.

A long term multifrequency campaign on the T Tauri star RU Lupi has been carried out in the X-Ray, UV optical and IR spectral regions with ASTRON and IUE satellites and ESO 1.5 m+IDS, 1.4 m CAT, 0.5 m UBVRI and 1 m IR telescopes, respectively.

We present two flare-like events occurred on April 17, 1984 and June 30, 1986. The first one was detected only in the UV, clue to lack the of simultaneous observations in other spectral regions. The second one was observed in UV, optical and IR regions showing a maximum roughly in the U band A comparison of the whole energy distribution of this event with that of a quiescent state observed on June 27 shows a flux enhancement of (89 ± 2)%. A detailed analysis of UV continuum and lines, namely N V, C I, CII, CIV, Si II and Si IV, shows that ihe surfaces fluxes of RU Lupi are always larger than those observed on typical flare stars and on the Sun by a factor of roughly 2 and 3 orders of magnitude, respectively. This fact allows us to conclude that RU Lupi activity cannot be explained even invoking a complete coverage by solar-like plages. On the contrary a patchy distribution of the emitting regions could explain the observed behaviour of this active star.

Photographic surveys completed during the past 30 years yielded data of about 600 flare stars in these fields. Although their average flare number is very low 17 of the stars produced 10… 120 flare ups. In order to investigate the possible long-term activity changes of these objects a new method - which is described below -was developed. Making use of it significant flare frequency variations were found at two out of three ‘active’ flare stars.

The results of the intensive observations of the Pleiades flare stars for more than 20 years (≈ 3000 h effective observational time) were compiled and published by G.Haro and collaborators in a catalogue (Haro et al.,1982) including the flare stars discovered in the Pleiades region up to 1981. The catalogue contains the data far 519 flare stars.

In the frame of the international programme for the study of flare stars in stellar aggregates the Department of Astronomy and the National Astronomical Observatory have carried out patrol observations in the Pleiades since 1979. Parallel to the observations, work on the creation of machine-readable versions of the published flare stars catalogues has begun. As a firatstep, a machine—readable version of the Tonantzintla catalogue (TC of the Pleiades flare stars has been prepared (Tsvetkov et al., 1987).

In the frames of the programme of nonstable star investigations in the Department, of Astronomy and National Astronomical Observatory the investigations of flare stars- stars of spectral type dK - dM whose nonstabi1ity appears first of all through the quick brightness increases take an essential place. The programme of systematic observations was initiated in 1979 with setting into operation of the telescopes of the Rozhen National Observatory.

We have been observing in the Sco-Oph dark cloud region for 12 years. We have only reduced a small part of the plates. Nevertheless, 12 new flare stars and more than 40 new variables stars have been found. Besides, we have discovered a new subgroup of RR stars in the-globular cluster M4.

The size, location, and flux of four solar soft X-ray sources, including a flare, were derived from the disk-integrating GOES X-Ray Sensors during a solar eclipse. The fluxes were used to derive the temperatures and emission measures of these sources. If an incorrect amount of flux is attributed to the flare source, then the evolution of these derived plasma properties seems unphysical.

We undertake a statistical analysis of the soft X-ray (3.5 – 5.5 keV) profiles of solar flares as observed with the Hard X-ray Imaging Spectrometer on the SMM. The durations, maximum intensities and intensity profiles of the flares are examined. The properties of the “typical” solar flare are discussed. The distributions of the measured parameters with respect to one another reveal some interesting results. In common with past studies, we conclude that there is no evidence to suggest that more than one type of event is being viewed, despite a desire evident in the literature to place events into distinct groups. We also conclude that commonly held views about the relatonships between flare duration and intensity, profile asymmetries and intensity etc.. are in error. For more details of the flare events, the selection of data and the method of analysis, the reader is referred to Pearce and Harrison (1988).

The eruption of a large flare on the east limb of the Sun was observed by the X-Ray Polychromator (XRP) on board the Solar Maximum Mission (SMM) on 19 May 1984. The XRP Flat Crystal Spectrometer (FCS) made polychromatic soft X-ray images during the preflare, flare and postflare phases. The XRP Bent Crystal Spectrometer (BCS) provided information on the temperature and dynamics of the hot (Te > 8 x 106K) coronal plasma from spectra integrated spatially over the whole region.

In two major solar flares, we have found evidence for hard X-ray footpoint emission using the X-ray imager on Hinotori. Possible factors are discussed as to why we only rarely detect footpoint emissions, which may not always take the form of double sources, depending on magnetic configurations in flares.

Neutrons are sometimes released from solar flares accompanied by generation of high energy protons and other nuclei and some of them occasionally reach the earth before they decay radioactively. Together with the satellite observations on gamma ray and neutron emissions associated with such flares, the ground-based observations of these neutrons give us a clue to understand the possible nuclear interactions of those protons and nuclei with gases ambient in the solar atmosphere. A review is given on the Japan-China collaborative program on the ground-based observations of these neutrons for the coming solar maximum, though there still remain obstacles to be resolved.