Wenfei Yu

Kilohertz Quasi-periodic Oscillation Frequency Anticorrelated with Millihertz Quasi-periodic Oscillation Flux in 4U 1608-52

We analyzed Rossi X-Ray Timing Explorer data of the low-mass X-ray binary and atoll source 4U 1608-52 obtained on 1996 March 3 in which the source simultaneously showed a strong single kilohertz quasi-periodic oscillation (kHz QPO) around 840 Hz and a 7.5 mHz QPO detected at energies below 5 keV. We find that the frequency of the kHz QPO is approximately anticorrelated with the 2-5 keV X-ray count rate associated with the mHz QPO. The average kHz QPO frequency varies by about 0.6 Hz (0.07%) during a mHz QPO cycle over which the average 2-5 keV count rate varies by about 60 counts s(-1) (4%). This is the opposite of the frequency-count rate correlation observed in the same data on longer timescales and, hence, constitutes the first example of a sign reversal in the frequency-flux correlation related to the origin of the flux. Such a sign reversal is predicted by the radiative disk truncation model for the case where the flux variations originate on the neutron star but are not due to disk accretion rate fluctuations. The results support the nuclear burning interpretation of the mHz QPO and the interpretation of the kHz QPO frequency as an indicator of the orbital frequency at the inner edge of the accretion disk. The varying radiative stresses exerted on the inner disk by the flux due to the quasi-periodic nuclear burning lead to changes in the inner disk radius and, hence, to the observed anticorrelation between kHz QPO frequency and X-ray count rate. We found a time lag of about 10 s of the X-ray count rate relative to the kHz QPO frequency in terms of the anticorrelation between these two quantities, which could be caused by the propagation of the nuclear burning front on the neutron star away from the equatorial region.

The Kilohertz QPO Frequency and Flux Decrease in Aquila X-1 and the Effect of Soft X-Ray Spectral Components

We report on a Rossi X-Ray Timing Explorer observation of Aquila X-l during its outburst in 1997 March in which, immediately following a type I burst, the broadband 2-10 keV flux decreased by about 10% and the kilohertz quasi-periodic oscillation (kHz QPO) frequency decreased from 813 +/- 3 to 776 +/- 4 Hz. This change in kHz QPO frequency is much larger than expected from a simple extrapolation of a frequency-flux correlation that was established in data before the burst. Meanwhile, a very low frequency noise component in the broadband fast Fourier transform power spectra, with a fractional rms amplitude of 1.2% before the burst, ceased to exist after the burst. All these changes were accompanied by a change in the energy spectral shape. If we characterize the energy spectra with a model composed of two blackbody (BB) components and a power-law component, almost all the decrease in flux was in the two BE components. We attribute the two BE components to the contributions from a region very near the neutron star, or even from the neutron star itself, and from the accretion disk, respectively.

X-ray outbursts of ESO 243-49 HLX-1: comparison with galactic low-mass x-ray binary transients

We studied the outburst properties of the hyper-luminous X-ray source ESO 243-49 HLX-1, using the full set of Swift monitoring observations. We quantified the increase in the waiting time, recurrence time, and e-folding rise timescale along the outburst sequence, and the corresponding decrease in outburst duration, total radiated energy, and e-folding decay timescale, which confirms previous findings. HLX-1 spends less and less time in outburst and more and more time in quiescence, but its peak luminosity remains approximately constant. We compared the HLX-1 outburst properties with those of bright Galactic low-mass X-ray binary transients (LMXBTs). Our spectral analysis strengthens the similarity between state transitions in HLX-1 and those in Galactic LMXBTs. We also found that HLX-1 follows the nearly linear correlations between the hard-to-soft state transition luminosity and the peak luminosity, and between the rate of change of X-ray luminosity during the rise phase and the peak luminosity, which indicates that the occurrence of the hard-to-soft state transition of HLX-1 is similar to those of Galactic LMXBTs during outbursts. We found that HLX-1 does not follow the correlations between total radiated energy and peak luminosity, and between total radiated energy and e-folding rise/decay timescales we had previously identified in Galactic LMXBTs. HLX-1 would follow those correlations if the distance were several hundreds of kiloparsecs. However, invoking a much closer distance for HLX-1 is not a viable solution to this problem, as it introduces other, more serious inconsistencies with the observations.

ENERGY-DEPENDENT POWER SPECTRAL STATES AND ORIGIN OF APERIODIC VARIABILITY IN BLACK HOLE BINARIES

We found that the black hole candidate MAXI J1659-152 showed distinct power spectra, i.e., power-law noise (PLN) versus band-limited noise (BLN) plus quasi-periodic oscillations (QPOs) below and above about 2 keV, respectively, in observations with Swift and the Rossi X-ray Timing Explorer during the 2010 outburst, indicating a high energy cutoff of the PLN and a low energy cutoff of the BLN and QPOs around 2 keV. The emergence of the PLN and the fading of the BLN and QPOs initially took place below 2 keV when the source entered the hard intermediate state and settled in the soft state three weeks later. The evolution was accompanied by the emergence of the disk spectral component and decreases in the amplitudes of variability in the soft and hard X-ray bands. Our results indicate that the PLN is associated with an optically thick disk in both hard and intermediate states, and the power spectral state is independent of the X-ray energy spectral state in a broadband view. We suggest that in the hard or intermediate state, the BLN and QPOs emerge from the innermost hot flow subjected to Comptonization, while the PLN originates from the optically thick disk farther out. The energy cutoffs of the PLN and the BLN or QPOs then follow the temperature of the seed photons from the inner edge of the optically thick disk, while the high frequency cutoff of the PLN follows the orbital frequency of the inner edge of the optically thick disk as well.

Early Phase Detection and Coverage of Extragalactic and Galactic Black Hole X-ray Transients with SKA

SKAs large field of view and high sensitivity at low frequencies will provide almost a complete coverage of the very early rising phase of extragalactic and Galactic transients which undergo a flare or outburst due to an abrupt accretion onto either supermassive (such as tidal disruption events, TDEs) or stellar mass black hole transients (such as black hole LMXB) , when their broadband emission is supposed to be jet-dominated at low luminosities, allowing SKA to be the first facility to make source discoveries and to send out alerts for follow-up ground or space observations as compared with the sensitivity of future X-ray wide-field-view monitoring. On the other hand, due to extremely large rate-of-change in the mass accretion rate during the rising phase of TDE flares or transient outbursts, SKA will be able to cover an extremely large range of the mass accretion rate as well as its rate-of-change not accessible with observations in persistent black hole systems, which will shape our understanding of disk-jet coupling in accreting black holes in the non-stationary accretion regimes.

3XMM J181923.7–170616: An X-Ray Binary with a 408 s Pulsar

We carry out a dedicated study of 3XMM J181923.7-170616 with an approximate pulsation period of 400 s using the XMM-Newton and Swift observations spanning across nine years. We have refined the period of the source to 407.904(7) s (at epoch MJD 57142) and constrained the 1σ upper limit on the period derivative \dot{P}≤slant 1.1× {10}-8 {{s}} {{{s}}}-1. The source radiates hard, persistent X-ray emission during the observation epochs, which is best described by an absorbed power-law model (Γ ˜ 0.2-0.8) plus faint Fe lines at 6.4 and 6.7 keV. The X-ray flux revealed a variation within a factor of 2, along with a spectral hardening as the flux increased. The pulse shape is sinusoid-like and the spectral properties of different phases do not present significant variation. The absorption {N}{{H}} (˜ 1.3× {10}22 {{cm}}-2) is similar to the total Galactic hydrogen column density along the direction, indicating that it is a distant source. A search for the counterpart in optical and near-infrared surveys reveals a low-mass K-type giant, while the existence of a Galactic OB supergiant is excluded. A symbiotic X-ray binary (SyXB) is the favored nature of 3XMM J181923.7-170616 and can essentially explain the low luminosity of 2.78× {10}34{d}102 {erg} {{{s}}}-1, slow pulsation, hard X-ray spectrum, and possible K3 III companion. An alternative explanation of the source is a persistent Be X-ray binary (BeXB) with a companion star no earlier than B3-type.