[en] We report the discovery of optical pulsations from the proposed candidate for the 7sup(s).7 x-ray pulsar 4U1626-67. High-sentivity, time-resolved, broad-band photometric observations of the faint blue star suggested by McClintock et al. were made on 3 May 1978 with the ESO 3.6m reflector. Analysis of 1h of data shows clear evidence of optical pulsations with a heliocentric period of 7sup(s).6809 +- 0sup(s).0035 (1sigma), which differs from the extra-polated X-ray period by only 0.5sigmaopt. We find no significant radial velocity changes over the observing interval. The optical light curve is broad, has a full amplitude of 4% of the mean net flux and a pulsed energy fraction of 2%. Our observations may be consistent with reprocessing of X-ray pulses in a highly compact binary system. (orig.)

[en] We present here the first results of fast optical photometry of the GX 339-4 counterpart obtained in late May 1981 when the object was found in an unusually bright state (V=15.4). The optical flux displays 20 second quasi-oscillations of 30-40% full amplitude together with very short time scale activity. We observe flares as short as 10-20 milliseconds during which the flux can be increased by a factor up to 5. The overall optical time behaviour is shown to be very similar to the X-ray time behaviour of Cyg X-1. Optical flates are likely to come from the hot inner part of the accretion disc. Possible interpretations of the 20 second quasi-oscillations are discussed. (orig.)

No abstract available

[en] During the periods 1979 June--July and 1980 June--July, a total of 15 and 26 optical bursts, respectively, were detected from the X-ray burst source 4U/MXB 1636--53. Four of these bursts were detected in more than one passband simultaneously. The maximum burst fluxes F/sub max/ above the persistent optical flux and integrated burst fluxes E/sub b/ range in size over a factor of approx.8. E/sub b/ and F/sub max/ are correlated, with an average burst duration, defined by E/sub b//F/sub max/, of 15 s with a standard deviation of 6 s. From a comparison with X-ray bursts simultaneously detected with Hakucho, it appears that the maximum optical and X-ray burst fluxes (including the persistent fluxes) are related according to a power law, consistent with the idea that the optical emission is the result of blackbody reprocessing of X-rays. The integrated optical and X-ray burst fluxes are approximately linearly related, which is not expected on the basis of this simple X-ray reprocessing picture. The average ratio of the observed integrated optical to X-ray burst flux equals 5 x 10^-5. Correcting for interstellar absorption, the intrinsic value of this flux ratio is between 2 x 10^-4 and 10^-3. The integrated optical burst fluxes are correlated with the waiting time since the previous burst: there is an indication that large optical bursts come after a long waiting time. Thus one expects that also the integrated X-ray burst flux on the average increases with the waiting time, in agreement with Ohashi's observation that X-ray bursts from 4U/MXB 1636--53 which occur within less than 100 minutes after the previous burst tend to be smaller (by a factor of approx.2) than the average burst size.is consumed in the thermonuclear flash which gives rise to the X-ray burst

[en] Simultaneous X-ray, optical and infrared observations of the absorption dips in the polar system EF Eri are presented. We use these observations to determine the properties of the dips, and develop a simple model for the dips in which they are caused by absorption in the accretion stream as it crosses the line-of-sight to the X-ray/optical emission region located in the vicinity of the magnetic pole of the white dwarf. Application of this model to the dips observed in EF Eri yields direct estimates of the density and structure of the accretion stream. (author)

[en] The discovery of periodic optical variations from the counterpart of the low-mass X-ray binary XB 1254-690 is reported. The period is 0.163890 + or - 0.000009 days, identical to the recurrence period of X-ray dips. The full amplitude of the light curve is 0.4 mag and the mean V-magnitude is 19.1. From spectrophotometric observations the authors derive a B-V color index of 0.31 + or - 0.05 and an interstellar reddening of E(B-V) = 0.4 + or - 0.1. The optical minimum occurs 0.15 in phase after X-ray dips. Most of the optical light curve is probably due to varying aspect of the X-ray-heated secondary atmosphere. The contribution of the X-ray-heated bulge does not dominate the optical variability. The X-ray-heated secondary explanation supports the interpretation that the X-ray dips are due to eclipses by a bulge located on the edge of the accretion disk and indicates that at least in the case of XB 1254-690 the accretion disk does not shadow completely the companion star. 37 references

[en] This paper presents the results of simultaneous optical and near-IR photometry, optical fast spectroscopy, and Exosat X-ray observations of the dwarf nova VW Hyi, obtained simultaneously during three consecutive orbital cycles, approximately two days after the 1983 November superoutburst terminated. The optical data show clear evidence for a late superhump, which is shifted + 0.7 in phase relative to the orbital modulation. An attempt is made to derive from the observed spectral distribution the contribution of the late superhump. The orbital hump and the late superhump apparently are not related to each other. This important effect excludes all models, in which the late superhump phenomenon is interpreted in terms of variations in the bright-spot brightness. 43 references

[en] We report the discovery with XMM-Newton of a hard-thermal (T ∼ 130 MK) and variable X-ray emission from the Be star HD 157832, a new member of the puzzling class of γ-Cas-like Be/X-ray systems. Recent optical spectroscopy reveals the presence of a large/dense circumstellar disk seen at intermediate/high inclination. With a B1.5V spectral type, HD 157832 is the coolest γ-Cas analog known. In addition, its non-detection in the ROSAT all-sky survey shows that its average soft X-ray luminosity varied by a factor larger than ∼3 over a time interval of 14 yr. These two remarkable features, 'low' effective temperature, and likely high X-ray variability turn HD 157832 into a promising object for understanding the origin of the unusually high-temperature X-ray emission in these systems.

[en] Holmberg IX X-1 is an archetypal ultraluminous X-ray source (ULX). Here we study the properties of the optical counterpart and of its stellar environment using optical data from SUBARU/Faint Object Camera and Spectrograph, GEMINI/GMOS-N and Hubble Space Telescope (HST)/Advanced Camera for Surveys, as well as simultaneous Chandra X-ray data. The V ∼ 22.6 spectroscopically identified optical counterpart is part of a loose cluster with an age ∼< 20 Myr. Consequently, the mass upper limit on individual stars in the association is about 20 M_sun. The counterpart is more luminous than the other stars of the association, suggesting a non-negligible optical contribution from the accretion disk. An observed UV excess also points to non-stellar light similar to X-ray active low-mass X-ray binaries. A broad He II λ4686 emission line identified in the optical spectrum of the ULX further suggests optical light from X-ray reprocessing in the accretion disk. Using stellar evolutionary tracks, we have constrained the mass of the counterpart to be ∼> 10 M_sun, even if the accretion disk contributes significantly to the optical luminosity. Comparison of the photometric properties of the counterpart with binary models show that the donor may be more massive, ∼> 25 M_sun, with the ULX system likely undergoing case AB mass transfer. Finally, the counterpart exhibits photometric variability of 0.14 mag between two HST observations separated by 50 days which could be due to ellipsoidal variations and/or disk reprocessing of variable X-ray emission.

[en] We report preliminary result of a deep optical search of the error box for the 1979 April 6 gamma-ray burst. A 90-min direct IIIa-J plate exposed at the prime focus of the ESO 3.6m telescope reveals several very faint objetcs in the box reported by Laros et al. (1981). Statistical arguments based on published faint object counts show that most of them are probably distant galaxies unrelated to the burst source. (orig.)