J. L. Qu

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.

Distribution of Spectral Lags in Gamma-Ray Bursts

Using the data acquired in the time-to-spill (TTS) mode for long gamma-ray bursts (GRBs) by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma Ray Observatory (CGRO), we have carefully measured spectral lags in time between the low (25-55 keV) and high (110-320 keV) energy bands of individual pulses contained in 64 multipeak GRBs. We find that a temporal lead by higher energy gamma-ray photons (i.e., positive lags) is the norm in this selected sample set of long GRBs. While relatively few in number, some pulses of several long GRBs do show negative lags. This distribution of spectral lags in long GRBs is in contrast to that in short GRBs. This apparent difference poses challenges to and places constraints on the physical mechanism(s) for producing long and short GRBs. The relation between the pulse peak count rates and the spectral lags is also examined. Observationally, there seems to be no clear evidence for a systematic spectral lag-luminosity connection for pulses within a given long GRB.

The correlations between the spin frequencies and kHz QPOs of neutron stars in LMXBs

Aims. We studied the correlations between spin frequencies and kilohertz quasi-periodic oscillations ( kHz QPOs) in neutron star low-mass X-ray binaries. Methods. The updated data on kHz QPOs and spin frequencies are statistically analyzed. Results. We find that when two simultaneous kHz QPOs are present in the power spectrum, the minimum frequency of upper kHz QPO is at least 1.3 times higher than the spin frequency, i. e. nu(s) = -( 0.19 +/- 0.05)nu(s) + ( 389.40 +/- 21.67) Hz. If we shift this correlation in the direction of the peak separation by a factor of 1.5, this correlation matches the data points of the two accretionpowered millisecond X- ray pulsars, SAX J1808.4- 3658 and XTE J1807-294.

X-ray spectral and timing properties of the black hole X-ray transient SWIFT J1753.5-0127

We have carried out a detailed analysis of the black hole candidate (BHC) and X-ray transient SWIFT J1753.5-0127 as observed during its outburst in 2005-2006 by the Rossi X-Ray Timing Explorer. The spectral analysis shows that the emission is dominated by hard X-rays, thus revealing the low/hard state of the source during the outburst. The peak luminosity is found to be lower than the typical value expected from balance between the mass flow and evaporation of the inner edge of the disk. As a result, the disk is unable to extend inward to produce strong soft X-rays, which would correspond to the so-called high/soft state. These are typical characteristics of a small subset of BHCs, those soft X-ray transients that stay in the low/hard state during outburst. For most of our observation time, the frequency of the quasi-periodic oscillation (QPO) in the spectrum of this source is found to vary roughly linearly with the flux and spectral index, while a deviation from this relationship at peak luminosity may provide the first observational evidence of a partially evaporated inner edge to the accretion disk. An anticorrelation between QPO frequency and spectral color suggests that the global disk oscillation model proposed by Titarchuk & Osherovich is not likely at work in this case.

Evolution of Cross-Correlation and Time Lag of Cyg X-2 Along the Branches

We report the detections of the anticorrelated soft and hard X-rays, and the time lags of ~hectosecond from the neutron star low-mass X-ray binary Cyg X-2, a well-known Z-type luminous source. Both anticorrelation and the positive correlation were detected during the low-intensity states, while only the latter showed up during high-intensity states. Comparing with the lower part of normal branch and flaring branch, we find that more observations located on the horizontal and the upper normal branches are accompanied with anticorrelation, implying the occurrence of the anticorrelation when there is a low mass accretion rate. So far anticorrelated hard lags of 1000 s timescale are only reported from Galactic black hole candidates in their hard states. Here we provide the first evidence that a similar feature can also be established in neutron star systems such as Cyg X-2. Finally, the possible origins of the observed time lags are discussed under current LMXB models.

The 2009 outburst of H 1743-322 as observed by RXTE

Context. Six outbursts have been observed by RXTE from the source since 2003. Following “a failed outburst” with a missing high/soft state in 2008, the most recent one occurred in 2009, exhibiting all spectral transitions. Aims. An outburst was observed by RXTE from the black hole candidate H 1743-322 in 2009. We investigate this particular outburst in the context of the source outburst history. Methods. We analyze the RXTE observations of the 2009 outburst of H 1743-322, as well as the observations of the previous five outbursts for comparison. All available RXTE observations taken since 2003 sum up to a total exposure of ∼1200 ks. Results. The hardness-intensity diagram (HID) shows a complete counter-clockwise q-track for the 2009 outburst. It leaves the low/hard state but does not reach the leftmost edge of the overall HID. Near maximum, the X-ray spectra show a power law with photon index <2.4; the peak luminosity during the whole outburst reached only 20% of the Eddington luminosity. The highest disk fractional luminosity is about 0.75 for the 2009 outburst, which is close to the lower limit observed from the black hole XRB sample in their state transitions from the intermediate to the high/soft state. While the lowest hardness (6–19 keV/3–6 keV) values in the HID is about 0.3–0.4 for the 2009 outburst, the timing analysis shows that a transition to the high soft state occurred. During the low/hard state of the 2009 outburst, the inner radius of the accretion disk is found to be closer to the central black hole and anti-correlated with the disk temperature implying reprocessing of the hot corona on the disk’s soft X-rays. In the luminosity diagram of the corona versus the disk, the tracks of the outbursts in 2003 and 2009 cross the line which represents a roughly equal contribution to the emission from the thermal and the non-thermal components; the track of the 2008 outburst has its turn-over falling on this line. This may indicate an emission balance between the corona and the disk that prevents the state transition from going further than the low/hard state.

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.

AN ULTRA-LOW-MASS AND SMALL-RADIUS COMPACT OBJECT IN 4U 1746-37?

Photospheric radius expansion (PRE) bursts have already been used to constrain the masses and radii of neutron stars. RXTE observed three PRE bursts in 4U 1746-37, all with low touchdown fluxes. We discuss here the possibility of a low-mass neutron star in 4U 1746-37 because the Eddington luminosity depends on stellar mass. With typical values of hydrogen mass fraction and color correction factor, a Monte Carlo simulation was applied to constrain the mass and radius of a neutron star in 4U 1746-37. 4U 1746-37 has a high inclination angle. Two geometric effects, the reflection of the far-side accretion disk and the obscuration of the near-side accretion disk, have also been included in the mass and radius constraints of 4U 1746-37. If the reflection of the far-side accretion disk is accounted for, a low-mass compact object (mass of 0.41 +/- 0.14 M-circle dot and radius of 8.73 +/- 1.54 km at 68% confidence) exists in 4U 1746-37. If another effect operated, 4U 1746-37 may contain an ultra-low-mass and small-radius object (M = 0.21 +/- 0.06 M-circle dot, R = 6.26 +/- 0.99 km at 68% confidence). Combining all possibilities, the mass of 4U 1746-37 is 0.41(-0.30)(+0.70) M-circle dot at 99.7% confidence. For such low-mass neutron stars, it could be reproduced by a self-bound compact star, i.e., a quark star or quark-cluster star.

The last three outbursts of H1743−322 observed by RXTE in its latest service phase

After 2010, three outbursts of H1743-322 were detected by RXTE. We have carried out timing and spectral analysis of the data, emphasizing on the two with relatively complete evolution history presented in the RXTE/Proportional Counter Array observations. We then constitute an enlarged outburst sample for H1743-322 which allows us to investigate the spectral transitions in more details. We find that the spectral transitions to high-soft state constrain a region for four outbursts in hardness-intensity diagram. An extension of the region locates in the vicinity of the failed outburst in 2008, and excludes it from a successful group. We therefore suggest that the failed outburst in 2008 may present the first almost successful outburst sample, which is important for modelling the outburst, especially upon the conditions required for transition to high-soft state.