Search

We report 1-20 keV monitoring of the black hole candidate Cyg X-1 by the Ginga all-sky monitor (ASM), in 339 observations spanning from 1987 February to 1991 October. Cyg X-1 was in the X-ray low state in the entire 4.5 yr period. Flux modulation at the 5.6 days orbital period was observed clearly (if we assume no periodic variation, reduced kh2 value is 7.1) in the low-energy (1-2.1 keV) band. In the higher energy band, the modulation amplitude was small and less significant. The 1-2.1 keV folded light curve is a smooth function of the 5.6 days phase. These facts suggest that the modulation is caused by the orbital variation in the column density of photoelectrically absorbing circumstellar matter, as might be expected from the wind of the companion O9.7 Iab star, HDE 226868. The analysis of the spectral modulation suggests that the absorbing wind is not homogeneous. The ~300 days periodicity, reported previously by the Vela 5B investigators, was studied. Approximately 150 days periodicity, rather than ~300 days, was suggested in our data. Also, a broad peak around ~210-230 days period was found in both the power spectral density and the epoch-folding analysis. These periodicities seem to be unstable in their periods. The time-averaged 1-20 keV spectrum from the Ginga ASM alone can be characterized by a power law. However, the combined spectrum measured by Ginga in the 1-20 keV range and by the Solar Maximum Mission (SMM) hard X-ray and burst spectrometer (HXRBS), in the 25-400 keV range in 1987-1988 would require a more complex model, such as the unsaturated Comptonization model.

Future programs of x-ray astronomy in Japan are described. Following the Hakucho satellite, ASTRO-B and ASTRO-C are under preparation. ASTRO-B, to be launched in early 1983, is designed for the study of x-ray spectra and variability of x-ray sources. It is equipped with gas scintillation proportional counters of total area 1000 cm2, one dimensional x-ray focusing collectors, transient source monitors and a gamma-ray burst monitor. ASTRO-Cwill be launched in 1987 to study the time variability of cosmic x-ray sources with high statistical accuracies. The main emphasis is placed not only on the accurate timing analysis of the galactic sources but on the investigation of the time variability of active galactic nuclei. This satellite will be equipped with large area, low background proportional counters of total area of about 5000 cm2, an all sky monitor and a gamma-ray burst detector.

We observed Her X-1 using the Gingaobservatory in the spring of 1989 with the primary intention of studying the evolution of the pulse profile through the course of the 35 day X-ray HIGH-LOWcycle in that source. These observations cover 16 separate days in two MAINHIGHstates and in the intervening SHORTHIGHstate. We have augmented these data with four additional Gingaobservations of Her X-1 taken for other purposes but useful for our study. We present light curves in the 1-37 keV energy band for the seven high states covered by these data together with a representative sample of pulse profiles. The signal-to-noise ratio for these profiles is generally excellent, and collectively they provide a sound base for studying the evolution of the pulse profile. Of particular utility is the 1989 May observation, which for the first time provides extensive coverage of a short high state at high photon counting rate. By combining pulse phase and frequency information from all three high state observations in 1989, we are able to determine the phase alignment of main high and short high pulse profiles with high confidence. We identify components in the Her X-1 pulse profile by their distinctive spectral signatures, and we establish the existence of a definite, repetitive pattern of pulse shape variations tied to the 35 day high-low cycle. Comparing pre-Gingaobservations of pulse profiles superposed according to the 35 day phase indicates that this pattern has persisted over the past two decades. Moreover, the pulse phase alignment of the 1989 data allows the identification of components common to the main high and short high pulse profiles. One of the key elements of this pattern is the rapid change in pulse profile that occurs roughly 7 days into each main high state. In examining possible mechanisms for this interval of accelerated evolution, we are led to consider a class of models that involve dynamical changes in accretion flow geometry arising from neutron star obliquity. These models, however, suggest asymmetries in the X-ray illumination of the companion star that may conflict with extensive optical observations of the Her X-1 system. By contrast, kinematic changes in the geometrical aspect of obscuring matter flows near or within the magnetosphere that are tied to accretion disk precession provide a mechanism that may be consistent with observations.

Correlated variations in timing and spectral properties were observed near the peak of the outburst of Nova Muscae 1991 (GS 1124-683). Two regions with different spectral and timing characteristics can be distinguished in color-color and hardness-intensity diagrams. Two types of quasi-periodic oscillations (QPOs), each associated with regions in these diagrams, have been observed. Rapid flip-flop transitions sometimes occur between the regions. In one region the QPOs' centroid frequency is a strong, increasing function of X-ray intensity. Regions with similar characteristics and similar forms of QPOs were earlier found in GX 339-4 in its very high state. These two regions may turn out to be common and unique to black hole candidates in their very high state, even though they bear some similarities to the spectral branches in neutron star Z sources.