Guo-Qiang Ding

THE ENERGY DEPENDENCE OF THE CENTROID FREQUENCY AND PHASE LAG OF THE QUASI-PERIODIC OSCILLATIONS IN GRS 1915+105

We present a study of the centroid frequencies and phase lags of quasi-periodic oscillations (QPOs) as functions of photon energy for GRS 1915+105. It is found that the centroid frequencies of the 0.5-10 Hz QPOs and their phase lags are both energy dependent, and there exists an anticorrelation between the QPO frequency and phase lag. These new results challenge the popular QPO models, because none of them can fully explain the observed properties. We suggest that the observed QPO phase lags are partially due to the variation of the QPO frequency with energy, especially for those with frequency higher than 3.5 Hz.

Timescale Analysis of Spectral Lags

A technique for timescale analysis of spectral lags performed directly in the time domain is developed. Simulation studies are made to compare the time domain technique with the Fourier frequency analysis for spectral time lags. The time domain technique is applied to studying rapid variabilities of X-ray binaries and γ-ray bursts. The results indicate that in comparison with the Fourier analysis the timescale analysis technique is more powerful for the study of spectral lags in rapid variabilities on short time scales and short duration flaring phenomena.

A statistical study on the low-frequency quasi-periodic oscillation amplitude spectrum and amplitude in GRS 1915+105

A statistical study was made on both the energy dependence of the low-frequency quasi-periodic oscillation (QPO) amplitude (LFQPO amplitude spectrum) and the LFQPO amplitude from all the RXTE observations of GRS 1915+105. Based on the two-branch correlation of the LFQPO frequency and the hardness ratio, the observations that were suitable for evaluating the LFQPO amplitude spectrum were divided into two groups. According to a comparison between the radio and X-ray emissions, we deduced that the jets during the two groups of observations are very different. A negative correlation between the LFQPO frequency and the radio flux was found for one group. The LFQPO amplitude spectrum was fitted by a power law with an exponential cutoff in order to describe it quantitatively. It reveals that as the LFQPO frequency increases, the power law hardens. Furthermore, the cutoff energy first decreases, and then smoothly levels off. The fit also shows that the LFQPO amplitude spectra of the two groups are essentially the same, suggesting that the LFQPO does not originate from the jet. The LFQPO amplitude spectra are hard, indicating a possible origin of the LFQPO in the corona. As the LFQPO frequency increases, the LFQPO amplitude first increases and then decreases. The effects of the low-pass filter and the jet on the LFQPO amplitude are discussed.

Evolution of Hard X-Ray Spectra along the Branches in Circinus X-1

Using the data from the Proportional Counter Array and the High-Energy X-Ray Timing Experiment on board the Rossi X-Ray Timing Explorer satellite, we investigate the evolution of the 3-200 keV spectra of the peculiar low-mass X-ray binary Circinus X-1 along the branches on its hardness-intensity diagram (HID) from the vertical horizontal branch, through the horizontal horizontal branch and normal branch, to the flaring branch. We detect a power-law hard component in the spectra. It is found that the derived photon indices (Γ) of the power-law hard component are correlated with the position on the HID. The power-law component dominates the X-ray emission of Cir X-1 in the energy band higher than ~20 keV. The fluxes of the power-law component are compared with those of the bremsstrahlung component in the spectra. A possible origin of the power-law hard component is discussed.

A systematic study on energy dependence of quasi-periodic oscillation frequency in GRS 1915+105

Systematically studying all the RXTE/PCA observations for GRS 1915+105 before November 2010, we have discovered three additional patterns in the relation between Quasi-Periodic Oscillation (QPO) frequency and photon energy, extending earlier outcomes reported by Qu et al. (Astrophys. J. 710:836, 2010). We have confirmed that as QPO frequency increases, the relation evolves from the negative correlation to positive one. The newly discovered patterns provide new constraints on the QPO models.

Cross-correlations between soft and hard light curves depending on luminosity in the transient neutron star XTE J1701-462

Using all the observations from Rossi X-ray Timing Explorer for the accreting neutron star XTE J1701-462, we carry out a systematic study of the cross-correlation function between its soft and hard light curves. Over the entire outburst, XTE J1701-462 evolves from super-Eddington luminosities to quiescence and shows both Z and atoll behaviours. Following previous work, we divide the outburst into five intervals: one Cyg-like interval, three Sco-like intervals and one atoll interval, according to their different behaviours in the corresponding colour-colour diagrams (CCDs). With cross-correlation analyses, anti-correlation, positive and ambiguous correlations are found in the different intervals in this source. Both anti-correlated soft and hard time lags are detected, where hard lags mean that the hard photons lag behind the soft ones and soft lags mean the reverse. In the Cyg-like interval, anti-correlations are presented in the hard vertex and upper normal branch (NB) and positive correlations dominate the horizontal branch (HB) and lower NB. In the first two Sco-like intervals, anti-correlations are detected first and most of them are found in the HB and/or upper NB, while positive correlations are mostly detected in the lower NB and flaring branch (FB). In the following interval, i.e. the third Sco-like interval, anti-correlations occur on the upper FB and positive correlations are mainly distributed in the lower FB. The different intervals correspond to various luminosities; therefore, the position of anti-correlations in the CCD might depend on the luminosity. It is noted that, in the Cyg-like interval, positive correlations dominate the HB, which is not consistent with the behaviour of the Cyg-like Z sources GX 5-1 and Cyg X-2, the HBs of which host ambiguous correlations and anti-correlations. Hence, for comparison with GX 5-1, we analyse the spectra of the HB and the hard vertex of the Cyg-like interval. The fitting results show that, in contrast to GX 5-1, the ratio of hard emission to soft emission basically remains unvaried from the HB to the hard vertex, which might result in positive correlation. Additionally, we compare the spectra of the third Sco-like interval with atoll source 4U 1735-44 and find that their spectral evolution along the tracks in the CCD is similar, indicating that in this interval XTE J1701-462 could approach an atoll source. The truncated disc model might be responsible for the detected time lags in XTE J1701-462; the possible origins of anti-correlated hard and soft time lags are also discussed.

A timing view of the heartbeat state of GRS 1915+105

We present a timing analysis of two Rossi X-ray Timing Explorer observations of the micro-quasar GRS 1915+ 105 during the heartbeat state. The phase-frequency-power maps show that the intermediate-frequency aperiodic X-ray variability weakens as the source softens in the slow rise phase, and when the quasi-periodic oscillation disappears in the rise phase of the pulse of the double-peaked class, its sub-harmonic is still present with a hard phase lag. In the slow rise phase, the energy-frequency-power maps show that most of the aperiodic variability is produced in the corona, and may also induce the aperiodic variability observed at low energies from an accretion disc, which is further supported by the soft phase lag especially in the intermediate-frequency range (with a time delay up to 20 ms). In the rise phase of the pulse, the low-frequency aperiodic variability is enhanced significantly and there is a prominent hard lag (with a time delay up to 50 ms), indicating that the variability is induced by extension of the disc towards small radii as implied by the increase in flux and propagates into the corona. However, during the hard pulse of the double-peaked class, the variability shows no significant lag, which may be attributed to an optically thick corona. These timing results are generally consistent with the spectral results presented by Neilsen, Remillard & Lee which indicated that the slow rise phase corresponds to a local Eddington limit and the rise phase of the pulse corresponds to a radiation pressure instability in the disc.

The 2009 outburst from the new X-ray transient and black-hole candidate XTE J1652-453

We analyse the Rossi X-ray Timing Explorer (RXTE) and Swift observations of the 2009 outburst from a newly discovered transient and black hole candidate XTE J1652-453. The behaviour of the source was observed by RXTE to be a sequence of spectral states that are typical of black-hole X-ray binaries. During the first seven observations, it is determined that the source remains in a high/soft state. This is determined from the spectrum that is dominated by a soft thermal component, which contributes an average of similar to 85 per cent to the X-ray flux at 2-20 keV, and from the hardness similar to 0.1 showing up in the hardness-intensity diagram. For the last 20 observations, the spectral state is classified as low/hard, according to an average hardness of similar to 0.8 and a balance between the thermal and the non-thermal components; a power-law component takes >= 80 per cent of the total 2-20 keV flux. Located in between is an intermediate state that the source might have experienced. The usual relationship between the rms and hardness is also present in the hardness-rms diagram. Throughout the outburst, no quasi-periodic oscillations are found in XTE J1652-453.

The cross-correlation analysis in Z source GX 349+2

Using all the observations from Rossi X-ray Timing Explorer for Z source GX 349+2, we systematically carry out cross-correlation analysis between its soft and hard X-ray light curves. During the observations from 1998 January 9 to January 29, GX 349+2 traced out the most extensive Z track on its hardness-intensity diagram, making a comprehensive study of cross-correlation on the track. The positive correlations and positively correlated time lags are detected throughout the Z track. Outside the Z track, anti-correlations and anti-correlated time lags are found, but the anti-correlated time lags are much longer than the positively correlated time lags, which might indicate different mechanisms for producing the two types of time lags. We argue that neither the short-term time lag models nor the truncated accretion disc model can account for the long-term time lags in neutron star low-mass X-ray binaries (NS-LMXBs). We suggest that the extended accretion disc corona model could be an alternative model to explain the long-term time lags detected in NS-LMXBs.

Disc–corona interaction in the heartbeat state of GRS 1915+105

Timing analysis provides information about the dynamics of matter accreting on to neutron stars and black holes, and hence is crucial for studying the physics of the accretion flow around these objects. It is difficult, however, to associate the different variability components with each of the spectral components of the accretion flow. We apply several new methods to two Rossi X-ray Timing Explorer observations of the black hole binary GRS 1915+105 during its heartbeat state to explore the origin of the X-ray variability and the interactions of the accretion-flow components. We offer a promising window into the disc-corona interaction through analysing the formation regions of the disc aperiodic variabilities with different time-scales via comparing the corresponding transition energies of the amplitude-ratio spectra. In a previous paper, we analysed the Fourier power density as a function of energy and frequency to study the origin of the aperiodic variability, and combined that analysis with the phase lag as a function of frequency to derive a picture of the disc-corona interaction in this source. We here, for the first time, investigate the phase-lag as a function of energy and frequency, and display some interesting details of the disc-corona interaction. Besides, the results from the shape of amplitude-ratio spectrum and from several other aspects suggest that the quasi-periodic oscillation originates from the corona.