Shuang N. Zhang

Black Hole Spin in X-Ray Binaries: Observational Consequences

We discuss the observational consequences of black hole spin in X-ray binaries within the framework of the standard thin accretion disk model. When compared with theoretical flux distribution from the surface of a thin disk surrounding a Kerr black hole, the observed X-ray properties of the Galactic superluminal jet sources, GRO J1655-40 and GRS 1915+105, strongly suggest that each contains a black hole spinning rapidly in the same direction as the accretion disk. We show, however, that some other black hole binaries with an ultrasoft X-ray component probably harbor only non- or slowly spinning black holes, and we argue that those with no detectable ultrasoft component above 1-2 keV in their high luminosity state may contain a fast-spinning black hole but with a retrograde disk. Therefore, all classes of known black hole binaries are united within one scheme. Furthermore, we explore the possibility that spectral state transitions in Cyg X-1 are simply due to temporary disk reversal, which can occur in a wind accretion system.

Temporal Properties of Cygnus X-1 during the Spectral Transitions

We report the results from our timing analysis of 15 Rossi X-Ray Timing Explorer observations of Cygnus X-1 throughout its 1996 spectral transitions. The entire period can be divided into three distinct phases: (1) transition from the hard state to the soft state, (2) soft state, and (3) transition from the soft state back to the hard state. The observed X-ray properties (both temporal and spectral) in Phases 1 and 3 are remarkably similar, suggesting that the same physical processes are likely involved in triggering such transitions. The power density spectrum (PDS) during the transition can be characterized by a low-frequency red-noise (power-law) component, followed by a white-noise (flat) component that extends to roughly 1-3 Hz, where it is cut off, and a steeper power law (~1/f2) at higher frequencies. The X-ray flux also exhibits apparent quasi-periodic oscillations (QPOs), with the centroid frequency varying in the range of 4-12 Hz. The QPO shows no correlation with the source flux, but it becomes more prominent at higher energies. This type of PDS bears resemblance to that of other black hole candidates often observed in a so-called very high state, although the origin of the observed QPO may be very different. The low-frequency red noise has not been observed in the hard state, and thus seems to be correlated positively with the disk mass accretion rate, which is presumably low in the hard state and high in the soft state; in fact, it completely dominates the PDS in the soft state. In the framework of thermal Comptonization models, Cui et al. recently speculated that the difference in the observed spectral and timing properties between the hard and soft states is due to the presence of a fluctuating Comptonizing corona during the transition. Here we present the measured hard X-ray time lags and coherence functions between various energy bands, and we show that the results strongly support such a scenario.

The 1996 Soft State Transition of Cygnus X-1

We report continuous monitoring of Cygnus X-1 in the 1.3-200 keV band using All-Sky Monitor/Rossi X-Ray Timing Explorer and BATSE/Compton Gamma Ray Observatory for about 200 days from 1996 February 21 to early September. During this period, Cygnus X-1 experienced a hard-to-soft and then a soft-to-hard state transition. The low-energy X-ray (1.3-12 keV) and high-energy X-ray (20-200 keV) fluxes are strongly anticorrelated during this period. During the state transitions, flux variations of about a factor of 5 and 15 were seen in the 1.3-3.0 keV and 100-200 keV bands, respectively, while the average 4.8-12 keV flux remains almost unchanged. The net effect of this pivoting is that the total 1.3-200 keV luminosity remained unchanged to within ~15%. The bolometric luminosity in the soft state may be as high as 50%-70% above the hard state luminosity, after color corrections for the luminosity below 1.3 keV. The blackbody component flux and temperature increase in the soft state are probably caused by a combination of the optically thick disk mass accretion rate increase and a decrease of the inner disk radius.

Broadband High-Energy Observations of the Superluminal Jet Source GRO J1655–40 during an Outburst

The X-ray/radio transient superluminal jet source GRO J1655-40 was recently suggested to contain a black hole from optical observations. Because it is a relatively close-by system (d ~ 3.2 kpc), it can likely provide us with rich information about the physics operating in both Galactic and extragalactic jet sources. We present the first simultaneous broadband high-energy observations of GRO J1655-40 during the 1995 July-August outburst by three instruments: ASCA, WATCH/Granat, and BATSE/CGRO, in the energy band from 1 keV to 2 MeV. Our observations strengthen the interpretation that GRO J1655-40 contains a black hole. We detected a two-component energy spectrum, commonly seen from other Galactic black hole binaries, but never detected from a neutron star system. Combining our results with the mass limits derived from optical radial velocity and orbital period measurements, we further constrain the mass of the central object to be between 3.3 and 5.8 M☉, above the well-established mass upper limit of 3.2 M☉ for a neutron star (the optical mass function for GRO J1655-40 is 3.16 ± 0.2 M☉). This system is therefore the first Galactic superluminal jet source for which there is strong evidence that the system contains a stellar mass black hole. The inclination angle of the binary system is constrained to be between 76° and 87°, consistent with estimates obtained from optical light curves and radio jet kinematics.

Gamma-Ray Spectra and Variability of Cygnus X-1 Observed by BATSE

We present new BATSE Earth occultation observations of the 25 keV-1.8 MeV spectrum and variability of Cyg X-1 made between 1993 August and 1994 May. We observed that the normal soft γ-ray spectrum (γ2) of Cyg X-1 has two components: a Comptonized part seen below 300 keV and a high-energy tail in the 0.3-2 MeV range that was only hinted at in previous γ2 spectrum observed by HEAO 3. The source went through an extended sequence of changes between 1993 August and 1994 May; the 45-140 keV flux first decreased steadily from ~γ2 to below the γ1 flux level seen previously by HEAO 3 in 1979 to a new level, γ0, roughly one-quarter of its intensity over a period of ~140 days. The flux remained at this low level for about 40 days before returning swiftly (~20 days) to approximately the initial γ2 level. The γ2 spectrum may be interpreted in terms of an interacting two-region model, consisting of a high-temperature (~210-250 keV) core embedded in an ~50 keV corona. In this scenario, the observed 25-300 keV photons were produced by Compton scattering of soft photons (~0.5 keV) by the hot electrons in the outer corona. These same hard X-rays were further upscattered by a population of energetic electrons in the inner core, producing the spectral tail above 300 keV. During the excursion of the 45-140 keV flux from the γ2 to the γ0 level, the spectrum evolved to a form consistent with either a power law with a photon index of ~2.6 or a single-temperature Compton model with an electron temperature, kT, of 108 ± 11 keV and an optical depth, τ, of 0.40 ± 0.06 and then returned essentially to the original γ2 spectrum at the end of the active period. The overall cooling of the system during the low-flux period may be due to an increase in the soft photon population that effectively quenched the hot electrons in these regions through Compton scattering.

Energy Spectra and High Frequency Oscillations in 4U 0614+091

We investigate the behavior of the high-frequency quasi-periodic oscillations (QPOs) in 4U 0614+091, combining timing and spectral analyses of RXTE observations. The energy spectra of the source can be described by a power law (α ~ 2.8) and a blackbody (kT ~ 1.5 keV), with the blackbody accounting for 10%-20% of the total energy flux. We find a robust correlation of the frequency, ν, of the higher frequency QPO near 1 kHz with the flux of the blackbody, FBB. The slope of this correlation, d log ν/d log FBB, is 0.27-0.37. The source follows the same relation even in observations separated by several months. The QPO frequency does not have a similarly unique correlation with the total flux or the flux of the power-law component. The rms fraction of the higher frequency QPO rises with energy from 6.8% ± 1.5% (3-5 keV) to 21.3% ± 4.0% (10-12 keV). For the lower frequency QPO, however, it is consistent with a constant value of 5.4% ± 0.9%. The results may be interpreted in terms of a beat-frequency model for the production of the high-frequency QPOs.

The transient hard X-ray tail of GX 17+2: New BeppoSAX results

We report on results of two BeppoSAX observations of the Z source GX 17+2. In both cases the source is in the horizontal branch of the colour-intensity diagram. The persistent continuum can be fit by two-component models consisting of a blackbody plus a Comptonization spectrum. With one of these models, two solutions for the blackbody temperature of both the observed and seed photons for Comptonization are equally accepted by the data. In the first observation, when the source is on the left part of the horizontal branch, we observe a hard tail extending up to 120 keV, while in the second observation, when the source moves towards right in the same branch, the tail is no longer detected. The hard (>30 keV) X-ray emission can be modeled either by a simple power-law with photon index Γ ∼ 2.8, or assuming Comptonization of ∼1 keV soft photons off a hybrid thermal plus non-thermal electron plasma. The spectral index of the non-thermal injected electrons is p ∼ 1.7. The observation of hard X-ray emission only in the left part of the horizontal branch could be indicative of the presence of a threshold in the accretion rate above which the hard tail disappears. An emission line at 6.7 keV with equivalent width ∼ 30 eV is also found in both observations. We discuss these results and their physical implications.

KiloHertz Quasi-Periodic oscillations in Island State of 4U 1608-52 as observed with RXTE/PCA

We report RXTE/PCA observations of 4U 1608-52 on March 15, 18, and 22 immediately after the outburst in early 1996. The persistent count rates ranged from 190 to 450 counts s-1 (1-60 keV). During this period of time, 4U 1608-52 was in the island state. We detected quasi-periodic oscillation (QPO) features in the power density spectra (PDS) at 567-800 Hz on March 15 and 22, with source fractional root mean square (rms) amplitude of 13%-17% and widths of 78-180 Hz. The average rms amplitude of these QPO features is positively correlated with the energy. Our results imply that the neutron star spin frequency is possibly between 300 and 365 Hz.

Hard X-Ray Variability of the Black Hole Candidate GRO J0422+32 during Its 1992 Outburst

We have studied the hard X-ray variability of the soft X-ray transient GRO J0422+32 with BATSE in the 20-100 keV energy band. Our analysis covers 180 days following the first X-ray detection of the source on 1992 August 5, fully covering its primary and secondary X-ray outbursts. We compute power density spectra (PDSs) in the 20-50, 50-100, and 20-100 keV energy bands, in the frequency interval 0.002-0.488 Hz. The PDSs of GRO J0422+32 are approximately flat up to a break frequency, and decay as a power law above it, with index ~1. During the first 70 days of the X-ray outburst, the PDSs of GRO J0422+32 show a significant quasi-periodic oscillation (QPO) peak near ~0.2 Hz, superposed on the power-law tail. The break frequency of the PDSs obtained during the primary X-ray outburst of GRO J0422+32 occurs at 0.041±0.006 Hz; during the secondary outburst, the break is at 0.081±0.015 Hz. The power density at the break ranged between 44% and 89% Hz-1/2 (20-100 keV). The canonical anticorrelation between the break frequency and the power density at the break, observed in Cyg X-1 and other BHCs in the low state, is not observed in the PDSs of GRO J0422+32. We compare our results with those of similar variability studies of Cyg X-1. The relation between the spectral slope and the amplitude of the X-ray variations of GRO J0422+32 is similar to that of Cyg X-1; however, the relation between the hard X-ray flux and the amplitude of its variation is opposite to what has been found in Cyg X-1. Phase lags between the X-ray flux variations of GRO J0422+32 at high and low photon energies could only be derived during the first 30 days of its outburst. During this period, the variations in the 50-100 keV band lag those in the 20-50 keV energy band by an approximately constant phase difference of 0.039(3) rad in the frequency interval 0.02-0.20 Hz. The time lags of GRO J0422+32 during the first 30 days of the outburst decrease with frequency as a power law with index 0.9 for ν>0.01 Hz.

Anticorrelated Hard/Soft X-Ray Emission from the X-Ray Burster 4U 0614+091

We have detected transient X-ray activity from the X-ray burster 4U 0614+091 simultaneously with BATSE on board the Compton Gamma Ray Observatory (20-100 keV) and the all-sky monitor (ASM) on board the Rossi X-Ray Timing Explorer (1-12 keV). The peak fluxes reach approximately 40 mcrab in both instruments over a period of about 20 days. The variable emission shows a clear anticorrelation of the hard X-ray flux with the soft X-ray count rate. The observed anticorrelation is another clear counterexample to the notion that only black hole binaries exhibit such correlations. The individual spectra during this period can be fitted by power laws with photon indices 2.2 ± 0.3 (ASM) and 2.7 ± 0.4 (BATSE), while the combined spectra can be described by a single power law with index 2.09 ± 0.08. BATSE and the Rossi X-Ray Timing Explorer ASM are a good combination for monitoring X-ray sources over a wide energy band.