arXiv subjects

Wei Cui

Publications and source records attributed to Wei Cui.

At least 217 records · Page 12Linked to original sources

arXiv1998

Strong Aperiodic X-ray Variability and Quasi-Periodic Oscillation in X-ray Nova XTE J1550-564

We report the discovery of strong aperiodic X-ray variability and quasi-periodic oscillation (QPO) in the X-ray light curves of a new X-ray Nova, XTE J1550-564, and the evolution of the observed temporal properties during the rise of the recent X-ray outburst. The power spectral analysis of the first observation reveals strong aperiodic X-ray variability of the source (~28%), as well as the presence of a QPO at ~82 mHz with fractional rms amplitude ~14% over the 2-60 keV energy range. Also apparent is the first harmonic of the QPO with the amplitude ~9%. As the X-ray flux increases, the source tends to become less variable, and the QPO frequency increases rapidly, from 82 mHz to 4 Hz, over the flux (2-50 keV) range of 1.73-5.75 x 10^{-8} ergs cm^{-2} s^{-1}. The amplitude of the fundamental component of the QPO varies little, while that of the harmonic follows a decreasing trend. The fundamental component strengthens toward high energies, while its harmonic weakens. Initially, the power spectrum is roughly flat at low frequencies and turns into a power law at high frequencies, with the QPO harmonic sitting roughly at the break. In later observations, however, the high-frequency portion of the continuum can actually be better described by a broken power law (as opposed to a simple power law). This effect becomes more apparent at higher energies. The overall amplitude of the continuum shows a similar energy dependence to that of the fundamental component of the QPO. Strong rapid X-ray variability, as well as hard energy spectrum, makes XTE J1550-564 a good black hole candidate. We compare its temporal properties with those of other black hole candidates.

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arXiv1998

On Gravitomagnetic Precession and Qpo in Black Hole Candidates

The X-ray observation of black hole candidates provides a valuable tool to probe regions very close to the central black hole, where strong-field relativistic effects become important. Recent studies have shown that these effects seem to manifest themselves in the observed X-ray spectrum, in terms of the shape of X-ray continuum and the profile of emission lines, and in the X-ray light curves, in terms of certain quasi-periodic oscillations (QPOs). The latter will be the focus of this paper. We will review the proposed observational evidence for gravitomagnetic precession in black hole candidates, in light of new observational data and alternative models. Quantitative comparison will be made between the data and the models. We will comment on recent theoretical and observational efforts to address whether the gravitomagnetic precession of accreted, orbiting matter around a rotating black hole is a viable process for producing the observed QPOs. The results are inconclusive. We will mention a few areas where further progress can be made to possibly shed more light on the issue.

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arXiv1998

Probing the Structure of Accreting Compact Sources Through X-Ray Time Lags and Spectra

We exhibit, by compiling all data sets we can acquire, that the Fourier frequency dependent hard X-ray lags, first observed in the analysis of aperiodic variability of the light curves of the black hole candidate Cygnus X-1, appear to be a property shared by several other accreting black hole candidate sources and also by the different spectral states of this source. We then present both analytic and numerical models of these time lags resulting by the process of Comptonization in a variety of hot electron configurations. We argue that under the assumption that the observed spectra are due to Comptonization, the dependence of the lags on the Fourier period provides a means for mapping the spatial density profile of the hot electron plasma, while the period at which the lags eventually level--off provides an estimate of the size of the scattering cloud. We further examine the influence of the location and spatial extent of the soft photon source on the form of the resulting lags for a variety of configurations; we conclude that the study of the X-ray hard lags can provide clues about these parameters of the Comptonization process too. Fits of the existing data with our models indicate that the size of the Comptonizing clouds are quite large in extent ($\sim$ 1 light second) with inferred radial density profiles which are in many instances inconsistent with those of the standard dynamical models, while the extent of the source of soft photons appears to be much smaller than those of the hot electrons by roughly two orders of magnitude and its location consistent with the center of the hot electron corona.

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arXiv1998

Temporal X-ray Properties of Galactic Black Hole Candidates: Observational Data and Theoretical Models

I review the observed temporal X-ray properties of Galactic black hole candidates and the status of theoretical modeling of these properties. Like spectral X-ray properties, which has so far attracted most of the attention, the temporal properties, such as power spectral density, hard X-ray time lags, and coherence functions, contain information on some of the same physical properties of inverse Comptonization processes, which are generally believed to be responsible for producing hard X-ray emission in black hole candidates. In addition, the temporal properties add new information about the size of the Comptonizing region and perhaps its density structure. Therefore, the spectral and temporal properties taken together may help break the degeneracy, intrinsic to most spectral studies, in determining the temperature and optical depth of hot electrons. Moreover, significant insights might be gained into the dynamics of mass accretion in black hole candidates from the inferred density structure of accreted gas (from the temporal properties). Unfortunately, until very recently little has been done to {\it quantitatively} model the observed temporal properties of black hole candidates. I will discuss the recently proposed models, which have been formulated in the context of thermal Comptonization, and compare them to the data. Emphasis will be made on simultaneous studies of the observed spectral and temporal properties, the ultimate approach to studying X-ray production mechanisms in Galactic black holes.

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arXiv1998

Evidence for Frame-Dragging Around Spinning Black Holes in X-Ray Binaries

In the context of black hole spin in X-ray binaries, we propose that certain type of quasi-period oscillations (QPOs) observed in the light curves of black hole binaries (BHBs) are produced by X-ray modulation at the precession frequency of accretion disks, due to relativistic dragging of inertial frames around spinning black holes. These QPOs tend to be relatively stable in their centroid frequencies. They have been observed in the frequency range of a few to a few hundred Hz for several black holes with dynamically determined masses. By comparing the computed disk precession frequency with that of the observed QPO, we can derive the black hole angular momentum, given its mass. When applying this model to GRO J1655-40, GRS 1915+105, Cyg X-1, and GS 1124-68, we found that the black holes in GRO J1655-40 and GRS 1915+105, the only known BHBs that occasionally produce superluminal radio jets, spin at a rate close to the maximum limit, while Cyg X-1 and GS 1124-68, typical (persistent and transient) BHBs, contain only moderately rotating ones. Extending the model to the general population of black hole candidates, the fact that only low-frequency QPOs have been detected is consistent with the presence of only slowly spinning black holes in these systems. Our results are in good agreement with those derived from spectral data, thus strongly support the classification scheme that we proposed previously for BHBs.

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arXiv1998

Soft Phase Lags of Pulsed Emission from the Millisecond X-ray Pulsar SAX J1808.4-3658

We report the discovery of phase shifts between X-ray pulses at different energies in the newly discovered millisecond (ms) X-ray pulsar SAX J1808.4-3658. The results show that low-energy pulses lag high-energy pulses by as much as $\sim$0.2 ms (or $\sim$8% of the pulse period). The measurements were made in two different ways: (1) computing cross power spectra between different energy bands, and (2) cross-correlating the folded pulse profiles in different energy bands; consistent results were obtained. We speculate that the observed soft lags might be related to the lateral expansion and subsequent cooling of a ``hot spot'' on the neutron star surface in which the pulsed X-ray emission originates. Also presented is the possibility of producing soft lags via Compton down scattering of hard X-ray photons from the hot spot in the cool surrounding atmosphere. We will discuss possible X-ray production mechanisms for SAX J1808.4-3658 and constraints on the emission environment, based on the observed soft lags, pulse profiles, and energy spectrum.

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arXiv1998

Evolution of AQL X-1 During the Rising Phase of its 1998 Outburst

We present results from 16 snapshots of Aql X-1 with RXTE during the rising phase of its recent outburst. The observations were carried out at a typical rate of once or twice per day. The source shows interesting spectral evolution during this period. Phenomenologically, it bears remarkable similarities to ``atoll'' sources. Shortly after the onset of the outburst, the source is seen to be in an ``island'' state, but with little X-ray variability. It then appears to have made a rapid spectral transition (on a time scale less than half a day) to another ``island'' state, where it evolves slightly and stays for 4 days. In this state, the observed X-ray flux becomes increasingly variable as the source brightens. Quasi-period oscillation (QPO) in the X-ray intensity is detected in the frequency range 670--870 Hz. The QPO frequency increases with the X-ray flux while its fractional rms decreases. The QPO becomes undetectable following a transition to a ``banana'' state, where the source continues its evolution by moving up and down the ``banana'' branch in the color-color diagram as the flux (presumably, the mass accretion rate) fluctuates around the peak of the outburst. Throughout the entire period, the power density spectrum is dominated by very-low frequency noises. Little power can be seen above ~1 Hz, which is different from typical ``atoll'' sources. In the ``banana'' state, the overall X-ray variability remains low (with fractional rms ~3--4%) but roughly constant. The observed X-ray spectrum is soft with few photons from above $\sim$25 keV, implying the thermal origin of the emission. The evolution of both spectral and temporal X-ray properties is discussed in the context of disk-instability models.

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arXiv1997

Frequency Dependent Lags - A Common Phenomenon of Accreting Sources

The Fourier frequency dependent hard X-ray lag, first discovered from the analysis of aperiodic variability of the light curves of the black hole candidate Cygnus X-1, turns out to be a property shared by several other accreting compact sources. We show that the lag can be explained in terms of Comptonization process in coronae of hot electrons with inhomogeneous density distributions. The density profile of a corona, like the optical depth and electron temperature, can significantly affect the Comptonization energy spectrum from it. This means, by fitting the energy spectrum alone, it is not possible to uniquely determine the optical depth and temperature of the Comptonization cloud if its density profile is unknown. The hard X-ray time lag is sensitive to the density distribution of the scattering corona. Thus simultaneous analysis of the spectral and temporal X-ray data will allow us to probe the density structure of the Comptonizing corona and thereby the dynamics of mass accretion onto the compact object.

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arXiv1997

Evidence for Centrifugal Barrier in X-ray Pulsar GRO J1744-28

We present further observational evidence of the effects of a centrifugal barrier in GRO J1744-28, based on continued monitoring of the source with RXTE. For X-ray pulsars, the centrifugal barrier manifests itself in the cessation of pulsed emission when the source becomes faint. We show that such phenomenon occurred repeatedly for GRO J1744-28, following the decay of two X-ray outbursts. This has allowed a direct measurement of the dipole field strength for this pulsar. Here we argue that some of the other peculiar properties observed of this source may also be related to an active centrifugal barrier.

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arXiv1997

Cygnus X-1: X-ray Emission Mechanism and Geometry

We summarize recent observations of Cyg X-1 during its 1996 state transition. The results favor an emission geometry with a central optically thin corona surrounded by an optically thick, geometrically thin accretion disk. As the soft state is approached, the inner edge of the disk moves closer to the black hole, and the corona shrinks, perhaps due to more efficient cooling. Soft X-rays are from the innermost disk, and inverse-Comptionization by the corona appears to be responsible for the hard X-ray emission. Both Compton reflection and an iron emission line have been observed in both states. They are likely to originate in the inner region of the disk. Also briefly discussed is the theoretical effort to simultaneously model the observed spectral and temporal X-ray properties of Cyg X-1.

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arXiv1997

Cygnus X-1: A Spinning Black Hole?

It has been a long-standing puzzle what triggers the spectral transition between the hard and soft states in Cyg X-1. Also, the origin of the frequently observed X-ray flares in the source still remains a mystery. In this paper, we will make an attempt to address these issues in light of our recent work on black hole spin and its observational consequences.

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arXiv1997

Spectral Transitions in Cyg X-1 and Other Black Hole X-Ray Binaries

We show that the model proposed by Esin, McClintock & Narayan (1997) for the low state, intermediate state and high state of the black hole soft X-ray transient, Nova Muscae 1991, is consistent with the spectral evolution of the black hole X-ray binary, Cyg X-1, during the hard-to-soft state transition observed in 1996. We also apply the model to the outbursts of two other black hole X-ray transients, GRO J0422+32 and GRO J1719-24.

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arXiv1997

Simultaneous ASCA And RXTE Observations of Cygnus X--1 During Its 1996 State Transition

We report results from simultaneous ASCA and RXTE observations of Cyg X-1 when the source made a rare transition from the hard (= low) state to the soft (= high) state in 1996. These observations together cover a broad energy range ~0.7--50 keV with a moderate energy resolution at the iron K-band, thus make it possible to disentangle various spectral components. The low-energy spectrum is dominated by an ultra-soft component, which is likely to be the emission from the hottest inner portion of the accretion disk around the black hole. At high energies, the X-ray spectrum can be described by a Comptonized spectrum with a reflection component. The Compton corona, which upscatters soft ``seed photons'' to produce the hard X-ray emission, is found to have a y-parameter ~0.28. The hard X-ray emission illuminates the accretion disk and the re-emitted photons produce the observed ``reflection bump''. We show that the reflecting medium subtends only a small solid angle ($\sim 0.15\times 2π$), but has a large ionization parameter such that iron is ionized up to \ion{Fe}{24}-\ion{Fe}{26}. The presence of a broad iron line at $6.58\pm0.04$ keV is also consistent with a highly ionized disk, if we take into account the gravitational and Doppler shift of the line energy. These results imply a geometry with a central corona surrounding the black hole and the reflection occurring in the innermost region of the disk where matter is highly ionized.

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arXiv1997

Modeling the Hard X-Ray Lag and Energy Spectrum of Cyg X-1

In an effort to model the observed energy spectrum of Cygnus X-1 as well as its hard X-ray lag by Comptonization in inhomogeneous clouds of hot electrons with spherical geometry and various radial density profiles we discovered that: 1) Plasma clouds with different density profiles will lead to different Comptonization energy spectra even though they have the same optical depth and temperature. On the other hand, clouds with different optical depths can produce the same energy spectra as long as their radial density distributions are properly chosen. Thus by fitting the energy spectrum alone, it is not possible to uniquely determine the optical depth of the Comptonization cloud, let alone its density structure. 2) The phase or time difference as a function of Fourier frequency or period for the X-rays in two energy bands is sensitive to the radial density distribution of the scattering cloud. Comptonization in plasma clouds with non-uniform density profiles can account for the long standing puzzle of the frequency-dependent hard X-ray lags of Cygnus X-1 and other sources. Thus simultaneously fitting the observed spectral and temporal X-ray properties will allow us to probe the density structure of the Comptonizing atmosphere and thereby the dynamics of mass accretion onto the compact object.

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arXiv1997

Rxte Observations of a New X-Ray Transient: GRS 1737-31

We present the results from RXTE observations of a new X-ray transient, GRS 1737-31, near the Galactic center during an outburst. Over a span of two weeks, the source was seen to vary significantly on time scales down to a few seconds. The rapid flares in the light curves bear resemblance to those observed for Cygnus X-1, a well-known black hole candidate (BHC). The power-density spectrum is also very typical of BHCs in the HARD (or LOW) state: it is flat below a characteristic break frequency of ~0.03 Hz, and becomes roughly a 1/f power law above. The observed X-ray spectrum can be characterized by a simple power law with a photon index of ~1.7 over a broad energy range 2-200 keV, which is remarkably similar to that of Cyg X-1 in the HARD state. The similarities to Cyg X-1 make GRS 1737-31 a likely BHC. However, unlike most transient BHCs, GRS 1737-31 does not seem to reach a ``soft state'' even during an outburst. We discuss this phenomenon in light of the comparison to a class of hard X--ray BHCs including transients GRO J0422+32, V404Cyg and GRO J1719-24, and persistent sources 1E1740.7-2942 and GRS 1758-258.

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arXiv1997

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 to 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 ultra-soft X-ray component probably harbor only non- or slowly-spinning black holes, and we argue that those with no detectable ultra-soft 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.

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arXiv1997

Evidence for "Propeller" Effects In X-ray Pulsars GX 1+4 And GROJ1744-28

We present observational evidence for "propeller" effects in two X-ray pulsars, GX 1+4 and GROJ1744-28. Both sources were monitored regularly by the Rossi X-ray Timing Explorer (RXTE) throughout a decaying period in the X-ray brightness. Quite remarkably, strong X-ray pulsation became unmeasurable when total X-ray flux had dropped below a certain threshold. Such a phenomenon is a clear indication of the propeller effects which take place when pulsar magnetosphere grows beyond the co-rotation radius as a result of the decrease in mass accretion rate and centrifugal force prevents accreting matter from reaching the magnetic poles. The entire process should simply reverse as the accretion rate increases. Indeed, steady X-ray pulsation was reestablished as the sources emerged from the non-pulsating faint state. These data allow us to directly derive the surface polar magnetic field strength for both pulsars: 3.1E+13 G for GX 1+4 and 2.4E+11 G for GROJ1744-28. The results are likely to be accurate to within a factor of 2, with the total uncertainty dominated by the uncertainty in estimating the distances to the sources. Possible mechanisms for the persistent emission observed in the faint state are discussed in light of the extreme magnetic properties of the sources.

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arXiv1997

Temporal Properties of Cygnus X-1 During the Spectral Transitions

We report the results from our timing analysis of 15 RXTE observations of Cygnus X-1 throughout its 1996 spectral transitions. The entire period can be divided into 3 distinct phases: (1) transition from the hard to soft state, (2) soft state, and (3) transition from the soft state back to the hard state. The observed X-ray properties 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 red noise component, followed by a white noise component which extends to roughly 1-3 Hz where it is cut off, and a steeper power law at higher frequencies. The X-ray flux also exhibits apparent quasi-periodic oscillation (QPO) with the centroid frequency varying in the range of 4-12 Hz. The QPO shows no correlation with the source flux, but 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, thus seems to be positively correlated 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 thermalComptonization models, Cui et al. (see astro-ph/9610071 and astro-ph/9610072) 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 show that the results strongly support such a scenario.

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