Sigenori Miyamoto

X-ray variability of GX 339-4 in its very high states

GX 339-4 was observed with the large area counters (LAC) onboard Ginga in its very high state, where the X-ray intensity was about a factor of 2-3 larger than its high state and it showed very rapid variations on time scales of less than several minutes, which had not been observed earlier in the high state of this source. The X-ray energy spectrum was very soft; it consisted of a low-energy component and a high-energy tail

Canonical time variations of X-rays from black hole candidates in the low-intensity state

X-rays from black hole candidates, Cyg X-1, GX 339-4, and GS 2023+338 in their low-intensity state, consist of various shots or burst events, and shapes of the shots are similar among these sources. In their low-intensity states, the normalized power spectrum density functions of the X-rays except for the Fourier frequencies below about 0.2 Hz are the same not only in their shape but also in their absolute values, even if the X-ray energy range is different.

CENTRAL BLACK HOLE MASSES IN ACTIVE GALACTIC NUCLEI INFERRED FROM X-RAY VARIABILITY

Central black hole masses in eight active galactic nuclei (AGNs) are evaluated by a method based on the X-ray flux variability of these AGNs and Cyg X-1 observed with Ginga. We introduce a new definition for the variability timescale by employing a normalized power spectral density function. Assuming a linear proportionality between the variability timescale and the mass of the central black hole, and using the relation between these two for Cyg X-1 as a reference point, we estimate the masses of eight AGNs (six Seyferts, one quasar, and one BL Lacertae type object). The masses evaluated in this way are lower than those in earlier estimates (or upper limits) using different methods by a factor of 1 or 2 orders of magnitude, for some sources. We thus obtain a logarithmically averaged Eddington ratio of 1.4 for eight sources, or 0.47 for six Seyferts excluding a quasar and a BL Lacertae-type object. We discuss significance of this high Eddington ratio for the emission mechanism and the cosmological evolution of AGNs.

Another canonical time variation of X-rays from black hole candidates in the very high flare state ?

An X-ray nova, Nova Muscae 1991 or GS 1124-68, was observed with the LAC on board Ginga. On 1991 January 22, 7 days after its X-ray maximum, the same time variations were discovered as those observed in GX 339-4 at its near-flare maximum. The phase lags between different energy X-rays showed a peculiar large and broad peak, which was the same as that observed in GX 339-4. Shapes of the power spectrum density functions were similar, and indices of the power-law component of the X-ray energy spectra were the same

Decrease in the orbital period of Hercules X-1

From a pulse-timing analysis of Ginga observations of the binary X-ray pulsar Her X-1 obtained during the interval 1989 April-June we have determined local orbital parameters for a Short High state. We have also determined an orbital epoch in the adjacent Main High state. By comparing these orbital solutions with previously published results, we have detected a decrease in the orbital period for Her X-1 at an average rate of dot-P/P = (- 1.32 +/- 0.16) x 10(exp -8) yr(exp -1) over the interval 1971-1989. This is substantially larger than the value predicted from current estimates of the mass transfer rate, and motivates consideration of other mechanisms of mass transfer and/or mass loss. A second result from these observations is a close agreement between orbital parameters determined separately in Main High and Short High states. This agreement places strong constraints on the obliquity of the stellar companion, HZ Her, if undergoing forced precession with a 35 day period. As a consequence further doubt is placed on the slaved-disk model as the underlying cause of the 35 day cycle in Her X-1.

Discovery of a prominent cyclotron absorption feature from the transient X-ray pulsar X0331+53

A remarkable absorption feature at 28.5 keV, attributable to electron cyclotron resonance, has been discovered in the 1.9-60-keV X-ray spectrum of the recurrent transient X-ray pulsar X0331 + 53. The observed resonance energy implies a neutron star surface magnetic field of 2.5(1 + z) x 10 to the 12th G, where z is the gravitational redshift. The detection was made with the Ginga observatory in October 1989, during an outburst of this transient with a flux level of about 0.3 Crab. The feature is very deep and has been resolved with excellent statistics. This is the fourth unambiguous detection of cyclotron resonant scattering features from X-ray pulsars, suggesting that these features are a common phenomenon among these objects. An empirical relation found between the cyclotron resonance energy and the spectral cutoff energy suggests that the magnetic field strengths of the known X-ray pulsars are clustered in a range (1-4) x 10 to the 12th G. 30 refs.

The 35 Day Evolution of the Hercules X-1 Pulse Profile: Ginga Observations and Their Implications

We observed Her X-1 using the Ginga observatory 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-LOW cycle in that source. These observations cover 16 separate days in two MAIN HIGH states and in the intervening SHORT HIGH state. We have augmented these data with four additional Ginga observations 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-Ginga observations 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.

Discovery and X-ray properties of GS 1124 - 683 (= Nova Muscae)

A bright X-ray nova: GS 1124-683 was discovered by the All Sky Monitor (ASM) on board Ginga on 1991 January 8. X-ray properties observed with the ASM from 1991 January 5 to the end of August are presented. The observed spectra were roughly described by a two-component model consisting of a power-law (a hard X-ray) and a disk-blackbody or a blackbody (a soft X-ray) component. In its rising phase, the hard X-ray component appeared first and the soft X-ray component followed with increase of the temperature and the emission area

The unstable long-term periodicity of Aquila X-1

We present the results of the periodicity analysis of 4.5 yr of data observed with the All Sky Monitor (ASM) on-board Ginga and of 10 yr of data accumulated by the Vela 5b satellite. ASM observed three outbursts from Aql X-1 which indicate a 309 day periodicity, but Vela 5b data instead show a 125 day periodicity. This result indicates that the Aql X-1 long-term periodicity is not stable and therefore, cannot be due to some regular mechanism such as orbital motion or precession

GINGAAll-Sky Monitor Observations of Cygnus X-1

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 χ2 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.