ALBERT

Description: We infer the emission positions of twin kilohertz quasi-periodic oscillations (kHz QPOs) in neutron star low-mass X-ray binaries (NS-LMXBs) based on the Alfvén wave oscillation model (AWOM). For most sources, the emission radii of kHz QPOs cluster around a region of 16–19 km with the assumed NS radii of 15 km. Cir X-1 has the larger emission radii of 23 ~ 38 km than those of the other sources, which may be ascribed to its large magnetosphere-disc radius or strong NS surface magnetic field. SAX J1808.4–3658 is also a particular source with the relative large emission radii of kHz QPOs of 20 ~ 23 km, which may be due to its large inferred NS radius of 18 ~ 19 km. The emission radii of kHz QPOs for all the sources are larger than the NS radii, and the possible explanations of which are presented. The similarity of the emission radii of kHz QPOs (~16–19 km) for both the low/high luminosity Atoll/Z sources is found, which indicates that both sources share the similar magnetosphere-disc radii.

Description: 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.

Description: We investigated the spectral and timing properties of the type B quasi-periodic oscillations (QPOs) showing up in the transient black hole binary GX 339-4 during its four outbursts observed by Rossi X-ray Timing Explorer (RXTE)/PCA and HEXTE in 2002, 2004, 2007 and 2010. We find that, the dependence on variability of the accretion flow turns out to be similar for the type B QPOs occurring in these four outbursts. We therefore take the results from the 2010 outburst for presentation. Our spectral results obtained from both the energy and time domains show that, the occurrence of the type B QPO is accompanied with sudden increase of hard component flux, relatively smaller inner disk radius, stable disk but variable corona. The latter may be understood in a scenario of variable input of seed photons for Comptonization in the corona. Further clues to probing the possible origination of the type B QPO come from our analysis of time lag and its energy dependence. The energy dependence of type B QPO amplitude suggests that the hard component dominate the variability and the time lag spectral analysis results suggests the type B QPO could be related to inverse Compton scattering. The time lag between hard and soft energy band is about 10 ms and found to depend on frequency in a form of –0.7 for type B QPOs in the rising phase. Finally we put these results in a context of a blob mechanism at work.

Description: We test the relativistic precession model (RPM) and the magnetohydrodynamics Alfvén wave oscillation model (AWOM) for the kilohertz (kHz) quasi-periodic oscillations (QPOs) from sources with measured neutron star (NS) masses and twin kHz QPO frequencies. For the RPM, the derived NS masses of Cyg X-2, SAX J1808.4–3658 and 4U 1820–30 are 1.96 ± 0.10, 2.83 ± 0.04 and 1.85 ± 0.02 M , respectively. These are, respectively, ~30, 100 and 40 per cent higher than the measured results 1.5 ± 0.3, 〈1.4 and $1.29^{+0.19}_{-0.07}$ M . For the AWOM, where the free parameter of the model is the density of the star, we infer the NS radii to be around 10–20 km for the above three sources. Based on this, we can infer the matter compositions inside the NSs with the help of the equations of state. In particular, for SAX J1808.4–3658, the AWOM shows a lower mass density for its NS than those of the other known kHz QPO sources, with a radius range of 17–20 km, which excludes the strange quark matter inside its star.

Description: Based on Rossi X-ray Timing Explorer ( RXTE ) observational data, we study the timing and spectral properties of some peculiar low-frequency (LF) quasi-periodic oscillations (QPOs), which have been found at the peak luminosity of the outburst of some transient black hole (BH) binaries: the 2005 outburst of GRO J1655–40, the 2003 outburst of H1743–322 and the 1998 outburst of XTE J1550–564. Appearing in the ultraluminous state, these QPOs from different sources show some common properties. The amplitude is very weak (less than 1 per cent) and the quality factor is larger than 6. Moreover, these QPOs (about several Hz) sometimes show up simultaneously with another QPO (about 10 Hz), but their frequencies are not harmonically related. We also find that the frequencies of these QPOs are inversely correlated with the mass of the BH, which implies that these QPOs might be correlated with the innermost stable circular orbit. The QPO frequency is also negative correlated with the inner disc radius among BHs. However, its frequency is too low to ascribe it to the Keperlian orbit frequency. Moreover, we discuss the physical origin of these QPOs and we suggest that they are not produced by the viscous variability of the inner disc either.

Description: 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.

Description: 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.

Description: We present a timing analysis of two Rossi X-ray Timing Explorer observations of the microquasar 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.

Description: We investigate the properties of the centroid frequency of low-frequency quasi-periodic oscillation (LF QPOs, 0.1–30 Hz) during the 2003 outburst of H1743–322, by using the observational data of the Rossi X-ray Timing Explorer . We find that the frequency shows different energy dependences for each of the sub-classes of LF QPOs: the QPO frequency is proportional to photon energy for the type C QPOs while it is ambiguous for the type A and B QPOs. For type C QPOs, the slope of the frequency–energy relation versus frequency plot can be well described by a power law with frequency till ~7.5 Hz. Beyond ~7.5 Hz the slope goes down. The LF QPO amplitude decreases monotonically with the frequency for the type C but increases for the other two types. These properties provide a joint diagnostic for discriminating the different types of LF QPO.

Description: 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.