Flickering is a well-known, though poorly understood, signature of accretion in cataclysmic variables. HT Cas is an excellent system in which to study flickering because its well determined characteristics allow a calibration of the experimental system (see Horne et al. 1991).

Patterson (1981) argues that the flickering in HT Cas originates from regions very near the white dwarf. In U Gem, the flickering appears to be associated with the bright spot (Warner & Nather 1971). In RW Tri, the flickering occurs throughout the disk but is centrally concentrated (Horne & Stiening 1985); the same holds true for V2051 Oph (Warner & Cropper 1983).

HT Cas is a 16m.4 eclipsing dwarf nova with a binary period of 106 min and an inclination of ∼ 81°. The entire eclipse last about 15 min with the white dwarf totally eclipsed for ∼ 5 min. The mass ratio is 0.15 with the white dwarf having mass ∼ 0.6M⊙ (Horne et al. 1991).

We obtained 20 eclipses of HT Cas between 1982 and 1991 in U, B and V at Mt. Palomar and also McDonald Observatory with 0.5 – 2 sec integrations. These data were folded on the ephemeris given by Horne et al. (1991) and corrected for the 30 sec drift in the phasing as noted by Wood et al. (1995).

Decades of patrol observations of cataclysmic variables (CVs) show variations of the accretion rate also at a time scale of a few years which may be explained by a solar-type activity of the secondary and/or by orientation changes of the magnetic axis of the white dwarf in polars. Secondary photometric periods seen in TT Ari and possibly some other CVs need theoretical explanation. Irregular variability of some objects is characterized by power-law power spectra with a power index depending on the luminosity state of the system.

An X-ray burst is a sudden increase (rise time of order seconds) of the X-ray brightness of an X-ray source, which after reaching its peak decays, generally within a minute. The sky distribution of X-ray burst sources indicates that they are galactic objects (see Fig. 1); their concentration to the direction of the galactic center tells us that they lie at typical distances of ∼ 8 kpc, with corresponding peak luminosities of order 1038 erg s−1. The X-ray and optical properties of the persistent emission of X-ray burst sources show that they are low-mass X-ray binaries, in which mass is transferred from a rather normal low-mass (< 1 M⊙) star to a neutron star. The persistent emission is caused by the conversion of kinetic energy of the transferred matter into heat, at a rate of ∼ GM/R (∼ 0.1c2) per gram of accreted matter. The bursts are caused by unstable thermonuclear burning of material that has accumulated on the neutron star (‘thermonuclear flash’).

The global properties of X-ray bursts, in particular their dependence on the mass accretion rate, are fairly well understood. Different from the case of γ-ray bursts (see the contributions by Fishman, Hartmann and Kouveliotou to this Colloquium) the relevant question about X-ray bursts is not ‘What are they?’, but rather ‘What use are they?’. As we will argue here, X-ray bursts may provide us information on the mass and radius of a neutron star. This usefulness of X-ray bursts derives from the fact that the burst emission originates from the surface of the neutron star, unlike the persistent emission caused by mass accretion, of which we only know that it comes from the neutron star’s near vicinity.

Photospheric expansion has been observed during several powerful type I X-ray bursts, and it is associated with a supersonic outflow driven by the super-Eddington energy release in the thermonuclear He-burning at the base of the envelope (Lewin et al. 1993). Such expanded atmospheres represent perfect background for the propagation of sound, and we believe there is a number of arguments in favor of the reality of these phenomena.

We have suggested an acoustic origin for the second range photospheric oscillations detected in the X-ray bursters 1608-522 and 2127+119 (Lapidus et al. 1994). On the basis of our models of radiation driven winds from neutron stars during strongest bursts (Nobili et al. 1994), we have shown that purely radial standing sound waves in expanded spherical envelopes can have periods very close to the observed ones. Both the photosphere and the sonic surface may act as a reflecting boundary; this is the reason of quite different periods in the two sources (0.65 s and ∼ 7 s, respectively).

The X-ray spectrum and the rapid X-ray variability of accreting compact objects have a common origin, and their properties can therefore be expected to be coupled. Indeed, it turns out that the X-ray spectrum and the power spectrum show correlated variations, which occur as a function of variations in the mass fluxx Ṁ.

Stellar mass black holes and neutron stars have similar mass and size, and therefore their accretion phenomena may be expected to show similarities. Indeed, similarities exist that indicate that a unified description may be possible. If a particular phenomenon is seen in both neutron star and black-hole candidate systems this shows immediately that it cannot be due to any property that is unique to either neutron stars or black holes, such as the presence or absence of a surface, or of a strong non-aligned magnetic field. The quest for characteristics that are unique to black holes continues - I shall mention a few candidates below.

In order to study very rapid optical variability of astrophysical objects on time scales between 10−7 s and 102 s (Shvartsman 1977), at SAO the MANIA (Multichannel Analysis of Nanosecond Intensity Alterations) experiment is being used. A special photometric registration system and software has been developed (Beskin et al. 1982’ Plokhotnichenko 1983, Zhuravkov et al. 1994).

One of the applications of these tools is the detection of radiation from accreted or ejected plasma near compact objects. A choice between two models of accretion onto compact objects in binary systems - hydrodynamic flow or magnetc flaring (Shakura & Sunyaev 1973, Pustil’nik & Shvartsman 1974) has not yet been made. Recent optical and X-ray observations of X-ray binaries provide information on their (fast) variability, their nonthermal radio emission, generation of very high energy particles, and nonthermal processes in rapid optical flares (Bartolini et al. 1994, Beskin et al. 1994). However, the data are not fully described by classical hydrodynamical models.

X1755-338 is an X-ray binary source which displays X-ray dips with a 4.4 hour period (White et al. 1984). It was previously noted as an unusually soft X-ray source by Jones (1977) and was suggested later as a black-hole candidate (BHC) by White & Marshall (1984), and White et al. (1984), based on the similarity of its location in an X-ray colour-colour diagram to that of a group of BHCs.

The TTM is a coded-mask imaging spectrometer on board the KVANT module of the MIR space station. It is capable of producing images in the 2 – 30 keV band with an energy resolution of about 18% at 6 keV. The instrumental details are given in Brinkman et al. (1985).

We observed X1755-338 in 1989 March-September during the period of the TTM Galactic Centre Survey.

LMC X-3 is an X-ray binary in the Large Magellanic Cloud. It was discovered by UHURU and observations with various satellites showed that the X-ray source was variable by a factor of up to 100 (e.g. Traves et al. 1988). Using the GINGA and HEAO-1 observations, Cowley et al. (1991) found a strong ∼ 198 (or possibly ∼ 99) day modulation in the X-ray luminosities of LMC X-3. They suggested that this modulation may be due to an accretion disc precession, or periodic variations in the mass transfer rate, or a combination of both.

We observed LMC X-3 with the TTM in 1988-1990. The present paper gives some results from the analysis of the data obtained.

The TTM is a coded mask telescope on board the MIR space station. It is capable of producing images in the energy range 2-30 keV with a spectral resolution of about 18% at 6 keV (Brinkman et al. 1985).

The basic pioperties of γ-ray bursts are reviewed. Many of these properties have been known for over two decades, although new and more detailed observations have been made by Compton Observatory in the past three years. The new observation with the greatest impact has been the observed isotropic distribution of bursts along with a deficiency of weak bursts. This is not compatible with any known Galactic population of objects.

The observation of a γ-ray burst counterpart in another wavelength region has yet to be made, even though there have been substantial efforts in this pursuit from a wide variety of observations in many wavelength regions. Perhaps the greatest chance for a simultaneous or rapid observation of a γ-ray burst counterpart lies with the newly-developed BAtse Coordinates Distribution NEtwork (BACODINE) system. The objectives and characteristics of this system are described.

The Granat observatory was launched into a high apogee orbit on 1989 December 1 by a PROTON launcher. The initial Granat orbit had an apogee close to 200000 km and a perigee of 2000 km with an orbital period of 4 days. The satellite is entering the radiation belts for a few hours every orbit. Due to orbital evolution the perigee increased and the apogee decreased with time. After about 1.5 years of operation the perigee increased up to 20000 km. This has put the satellite completely outside of the proton radiation belts which makes the detector activation during the perigee passage negligible. Moreover, the satellite orbit is outside of the magnetosphere during the most parts of the mission. This makes such an orbit very attractive for high energy astrophysics missions which require low background level in order to achieve high sensitivity, and makes it essential for the investigation of the high energy transient phenomena such as cosmic γ-ray bursts and solar flares.

Gamma Ray Bursts (GRBs) have puzzled astronomers since their discovery more than 20 years ago. As no counterparts at wavelengths other than X- and γ-rays have yet been found the identification of the sources is still missing. Theoretical explanations range from colliding comets (1993) and merging neutron stars (1982) to more exotic objects, such as superconducting cosmic strings (1988). Data accumulated until now still do not discriminate between these models, although results from the BATSE (Burst and Transient Source Experiment) instrument aboard the Compton Gamma Ray Observatory (CGRO) strongly favor extragalactic models.

The Energetic Gamma Ray Experiment Telescope (EGRET) aboard CGRO has s ofar detected photons from 5 GRBs with its spark chamber. These are the highest energy γ-rays associated with GRBs to date. In this work we review previously published data and summarize the properties of these events. Elsewhere we present possible constraints from the data on the models proposed to explain GRBs.

We consider the hypothesis of the existence of a class of short (10−2-10−3s) and weak (10−6-10−7erg/s· cm2) γ-bursts of cosmological origin. These events arise from gravitational microlensing of bright, quiet γ-ray sources (with luminosity about 3·1038erg/s) located in distant (z ≥ 1) galaxies by stars inside foreground galaxies. If the size of the γ-ray sources is 104-106cm, motion with respect to the observer or the intermediate galaxy may bring them close to the neigborhood of cusps along critical curves generated by individual stars (corresponding to caustics in the observer’s plane). In these cases the observed fluxes increase by a factor 1011-1013, the temporal variation of intensity being due to the brightness distribution in the source and to the distance from the cusp (in the source plane). Using standard assumptions for the number and sizes of “quiet” γ-ray sources, the number and characteristics of galaxies, we estimate the observed burst rate to be between 1 and 10 bursts per year (see below). Their temporal and spectral properties are determined by the nature of sources (ejecting pulsar, accreting neutron star with a weak or strong magnetic field, black hole) and by their velocity relative to the cusp. Similar effects have been discussed in the context of flux variability and structure studies of quasars (e.g. Schneider & Weiss 1986, Grieger et al. 1988, Refsdal & Surdej 1994). However, in these cases the source sizes were larger than 1014cm3 and amplification did not exceed 102.

The mystery of the nature and origin of gamma-ray bursts (GRBs) has persisted over two decades of intensive research, observational and theoretical. The basic problem is the absence of quiescent or transient counterparts at other wavelengths, resulting in distance scale uncertainties of more than 10 orders of magnitude. Searches for counterparts are hampered by the lack of sufficiently accurate localizations and by a lack of criteria that uniquely identify the expected appearance of counterparts (although see Meszaros &: Rees 1993, Meszaros et al. 1994). The detection of archival optical transients (OT’s) inside γ-ray burst error boxes suggested burst recurrence and generated hope for a breakthrough in the γ-ray burst counterpart search (e.g., Schaefer 1990). However, counterparts of these OT’s have never been identified, and the reality of several archival images was questioned (Zytkow 1990, but see Schaefer 1990). The search for multi-wavelengths transients has recently been extended to neutrino- and gravitational wave emission. The status of multiwavelength surveys for quiescent or transient counterparts prior to 1988 was reviewed by Hartmann & Woosley (1988). Here we update that review and discuss “optical” emission in view of the current debate on the burster distance scale. Other recent reviews are by Schaefer (1994) and Greiner (1995).

We outline and discuss the recent results of optical searches for counterparts to gamma-ray burst sources as well as related problems.

This status report presents details on the project to search for serendipitous time-correlated optical photographic observations of γ-ray bursters. The ongoing photographic observations at nine observatories are used to look for plates which have been exposed simultaneously with a γ-ray burst detected by BATSE and contain the burst position. The results for the third year of BATSE operation are presented.

The Explosive Transient Camera (ETC) is a dedicated wide-field sky monitor for short-duration optical transients. The ETC operates completely automatically, searching for transients in the night sky without the need for a human observer on site. Data products from the ETC include data from any transients detected, as well as CCD images of the fields-of-view observed. We report here the imaging by the ETC of the 2.5σ error region of GRB 941014 90 seconds after the end of the burst.

Preliminary results are presented of the first (searching) stage of our observational program of the search and study of weak blue star-like objects in the two small γ-ray burst boxes GB 790418 and GB 790613. The basic search purposes are (1) the selection of star-like objects weaker than 23m inside these boxes, (2) the selection of the bluest ones in B — V among these objects. The first result is the fact that there are no blue (with B — V < 0) star-like objects brighter than 24m in both boxes. But in each of these GRB error boxes one blue object with smaller flux was selected for further study to elucidate their nature. In the smallest of the archival GRB error boxes (GB 790613) obtained by three satellite triangulation, the bluest star-like object with V = 24.56(±0.16) and B — V = −0.20(±0.19) is near the center (at the distance < 10") of the GRB box. In the GB 790418 error box, the bluest star-like object with B = 24.40(±0.20), B — V = −0.40(±0.40) is found at ≈50" from the center.

Two proposals for detection of optical flashes related to γ-ray bursts are briefly described. The aim of the first proposal is to develop a reliable monitoring service at optical wavelengths, and the second proposal deals with an automatically working telescope with full remote control via Internet network.

This review presents an overview of the large and important photographic plate collection at the Harvard College Observatory, including its history, uniqueness, contents, storage, uses and successes.

After outlining the achievements of photographic sky monitoring at Sonneberg Observatory we describe a concept of a future CCD-based sky patrol. We suggest a world-wide network of 6 identical observing stations which cover the entire sky down to a limiting magnitude of ∼ 18TO and with a time resolution of between 8 and 24 hours. A preliminary technical design is proposed, and the astrophysical and general impact of such a project is briefly discussed.