oalib

Publish in OALib Journal

ISSN: 2333-9721

APC: Only $99

Submit

Any time

2020 ( 16 )

2019 ( 114 )

2018 ( 767 )

2017 ( 790 )

Custom range...

Search Results: 1 - 10 of 61098 matches for " Wenfei Yu "
All listed articles are free for downloading (OA Articles)
Page 1 /61098
Display every page Item
Probing the X-ray Variability of X-ray Binaries
Wenfei Yu
Physics , 2003,
Abstract: Kilohertz quasi-periodic oscillations (kHz QPOs) has been regarded as representing the Keplerian frequency at the inner disk edge in the neutron star X-ray binaries. The so-called ``parallel tracks'' on the plot of the kHz QPO frequency vs. X-ray flux in neutron star X-ray binaries, on the other hand, show the correlation between the kHz QPO frequency and the X-ray flux on time scales from hours to days. This is suspected as caused by the variations of the mass accretion rate through the accretion disk surrounding the neutron star. We show here that by comparing the correlation between the kHz QPO frequency and the X-ray count rate on a certain QPO time scale observed approximately simultaneous in the Fourier power spectra of the X-ray light curve, we have found evidences that the X-ray flux of millihertz QPOs in neutron star X-ray binaries is generated inside the inner disk edge if adopting that the kilohertz QPO frequency is an orbital frequency at the inner disk edge. This approach could be applied to other variability components in X-ray binaries.
State Transitions in Bright Galactic X-ray Binaries: Luminosities Span by Two Orders of Magnitude
Wenfei Yu,Zhen Yan
Physics , 2009, DOI: 10.1088/0004-637X/701/2/1940
Abstract: Using X-ray monitoring observations with the ASM on board the RXTE and the BAT on board the Swift, we are able to study the spectral state transitions occurred in about 20 bright persistent and transient black hole and neutron star binaries. We have confirmed that there is a correlation between the X-ray luminosity corresponding to the hard-to-soft transition and the X-ray luminosity of the following soft state. This correlation holds over a luminosity range spanning by two orders of magnitude, with no indication of a flux saturation or cut-off. We have also found that the transition luminosity correlates with the rate of increase in the X-ray luminosity during the rising phase of an outburst or flare, implying that the origin of the variation of the transition luminosity is associated with non-stationary accretion in both transient sources and persistent sources. The correlation between the luminosity corresponding to the end of the soft-to-hard transition and the peak luminosity of the preceding soft state is found insignificant. The results suggest that the hysteresis effect of spectral state transitions is primarily driven by non-stationary accretion when the mass accretion rate increases rather than the mass accretion rate decreases. Our results also imply that Galactic X-ray binaries can reach more luminous hard states during outbursts of higher luminosities and of similar rise time scales as those observed. Based on the correlations, we speculate that bright hard state beyond the Eddington luminosity will be observed in Galactic binaries in the next century. We also suggest that some ultra-luminous X-ray sources in nearby galaxies, which stay in the hard states during bright, short flares, harbor stellar-mass compact stars.
Low frequency QPOs and possible change in the accretion geometry during the outbursts of Aquila X$-$1
Wenda Zhang,Wenfei Yu
Physics , 2014, DOI: 10.1088/0004-637X/805/2/139
Abstract: We have studied the evolution of the Low Frequency Quasi-Periodic Oscillations (LFQPOs) during the rising phase of seven outbursts of the neutron star Soft X-ray Transient (SXT) Aql X$-$1 observed with the \textit{Rossi X-ray Timing Explorer (RXTE)}. The frequency correlation between the low frequency break and the LFQPO sampled on the time scale of $\sim$2 days was seen. Except for the peculiar 2001 outburst, the frequency of the LFQPOs increased with time before the hard-to-soft state transition up to a maximum $\nu_{max}$ at $\sim$31 Hz, a factor of $\sim$5 higher than those seen in black hole transients such as GX 339$-$4, making the maximum QPO frequency a likely indicator of the mass of the central compact object. The characteristic frequencies increased by around ten percent per day in the early rising phase and accelerated to nearly one hundred percent per day since $\sim$2 days before the hard-to-soft state transition. We examined the dependence of the frequency $\nu_{LF}$ on the source flux $f$ and found an anti-correlation between the maximum frequency of the LFQPOs and the corresponding X-ray luminosity of the hard-to-soft transition (or outburst peak luminosity) among the outbursts. We suggest that X-ray evaporation process can not be the only mechanism that drives the variation of the inner disk radius if either of the twin kHz QPO corresponds to the Keplerian frequency at the truncation radius.
Discovery of the correlation between peak episodic jet power and X-ray peak luminosity of the soft state in black hole transients
Hui Zhang,Wenfei Yu
Physics , 2015,
Abstract: Episodic jets are usually observed in the intermediate state of black hole transients during their X-ray outbursts. Here we report the discovery of a strong positive correlation between the peak radio power of the episodic jet $P_{\rm jet}$ and the corresponding peak X-ray luminosity $L_{\rm x}$ of the soft state (in Eddington units) in a complete sample of the outbursts of black hole transients observed during the RXTE era of which data are available, which follows the relation $\log P_{\rm jet}=(2.2\pm{0.3})+(1.6\pm0.2)\times \log {L_{\rm x}}$. The transient ultra-luminous X-ray source in M31 and HLX-1 in EXO 243-49 fall on the relation if they contain stellar mass black hole and either stellar mass black hole or intermediate mass black hole, respectively. Besides, a significant correlation between the peak power of the episodic jet and the rate-of-increase of the X-ray luminosity $\rm dL_{x}/dt$ during the rising phase of those outbursts is also found, following $\log P_{\rm jet}=(2.0\pm{0.4})+(0.7\pm0.2)\times \log {\rm d}L_{\rm x}/{\rm d}t$. In GX 339$-$4 and H 1743$-$322 in which data for two outbursts are available, measurements of the peak radio power of the episodic jet and the X-ray peak luminosity (and its rate-of-change) shows similar positive correlations between outbursts, which demonstrate the dominant role of accretion over black hole spin in generating episodic jet power. On the other hand, no significant difference is seen among the systems with different measured black hole spin in current sample. This implies that the power of the episodic jet is strongly affected by non-stationary accretion instead of black hole spin characterized primarily by the rate-of-change of the mass accretion rate.
Phase Drifts of Sub-pulses during the 2004 Giant Flare of SGR 1806-20 and Settling of the Magnetic Fields
Yi Xing,Wenfei Yu
Physics , 2010, DOI: 10.1088/0004-637X/729/1/1
Abstract: We analyzed the observations of SGR 1806-20 performed with the \textit{Rossi X-ray Timing Explorer} (RXTE) during its 2004 giant flare. We studied the phase evolution of the sub-pulses identified in the X-ray waveform and found that the sub-pulses varied in phase with time and then gradually settled, which might indicate drifts of the emission regions in relative to the neutron star surface, or changes in the local emission geometry before the magnetic field became stable. The characteristic e-folding timescale of the phase drifts measured starting about 15 s following the initial flux spike are in the range between 37 s and 84 s. This leads to the first measurements of the characteristic timescale for the magnetic field of the neutron star to settle after a field reconfiguration during the giant flare.
X-ray outbursts of low-mass X-ray binary transients observed in the RXTE era
Zhen Yan,Wenfei Yu
Physics , 2014, DOI: 10.1088/0004-637X/805/2/87
Abstract: We have performed a statistical study of the properties of 110 bright X-ray outbursts in 36 low-mass X-ray binary transients (LMXBTs) seen with the All-Sky Monitor (2--12 keV) on board the {\it Rossi X-ray Timing Explorer} ({\it RXTE}) in 1996--2011. We have measured a number of outburst properties, including peak X-ray luminosity, rate of change of luminosity on a daily timescale, $e$-folding rise and decay timescales, outburst duration, and total radiated energy. We found that the average properties such as peak X-ray luminosity, rise and decay timescales, outburst duration, and total radiated energy of black hole LMXBTs, are at least two times larger than those of neutron star LMXBTs, implying that the measurements of these properties may provide preliminary clues as to the nature of the compact object of a newly discovered LMXBT. We also found that the outburst peak X-ray luminosity is correlated with the rate of change of X-ray luminosity in both the rise and the decay phases, which is consistent with our previous studies. Positive correlations between total radiated energy and peak X-ray luminosity, and between total radiated energy and the $e$-folding rise or decay timescale, are also found in the outbursts. These correlations suggest that the mass stored in the disk before an outburst is the primary initial condition that sets up the outburst properties seen later. We also found that the outbursts of two transient stellar-mass ULXs in M31 also roughly follow the correlations, which indicate that the same outburst mechanism works for the brighter outbursts of these two sources in M31 that reached the Eddington luminosity.
Similar Shot Noise in Cyg X-1, GRO J0422+32 and 1E 1724-3045
Wenfei Yu,Tipei Li
Physics , 1999,
Abstract: We study the rapid variability in Cyg X-1 observed with the Rossi Timing Explorer (RXTE) on Jan 17 and 20 of 1997. The power spectra below 200 Hz can be characterized by two ``shot noise'' components and a peaked-noise centered at 0.2 Hz. This is similar to those ``shot noise'' observed in another black hole candidate GRO J0422+32 and a neutron star X-ray binary 1E 1724-3045. The similarity suggests that the generation mechanism of ``shot noise'' and peaked-noise is probably similar in both black hole systems and neutron star systems, and in both high mass X-ray binaries (HMXBS) and low mass X-ray binaries (LMXBs). We have also analyzed ~ 500 brightest shots selected from the Cyg X-1 light curves. The time scale of the spectral variation around the shot can be as long as ~ a few seconds, consistent with Ginga results. The difference between the superposed shot profile in different energy bands can be attributed to the time lag and the different shot width between different energy bands. The shot width defined from the auto-correlation function of shot profile does not show a bimodal distribution.
A UV flux drop preceding the X-ray hard-to-soft state transition during the 2010 outburst of GX 339-4
Zhen Yan,Wenfei Yu
Physics , 2011, DOI: 10.1111/j.1745-3933.2012.01333.x
Abstract: The black hole X-ray transient GX 339$-$4 was observed with the {\it Swift} satellite across the hard-to-soft state transition during its 2010 outburst. The ultraviolet (UV) flux measured with the filter UVW2 of the {\it Swift}/UVOT started to decrease nearly 10 days before the drop in the hard X-ray flux when the hard-to-soft state transition started. The UV flux $F_\mathrm{UV}$ correlated with the X-ray flux $F_\mathrm{X}$ as $F_\mathrm{UV}\propto F_\mathrm{X}^{0.50\pm0.04}$ before the drop in the UV flux. During the UV drop lasting about 16 days, the X-ray flux in 0.4--10 keV was increasing. The drop in the UV flux indicates that the jet started to quench 10 days before the hard-to-soft state transition seen in X-rays, which is unexpected.
Energy dependent power spectral states and origin of aperiodic variability in black hole binaries
Wenfei Yu,Wenda Zhang
Physics , 2013, DOI: 10.1088/0004-637X/770/2/135
Abstract: We found the black hole candidate MAXI J1659-152 showed distinct power spectra, i.e., a power-law noise (PLN) vs. band-limited noise (BLN) plus quasi-periodic oscillations (QPOs), below and above about 2 keV respectively, in the observations with the Swift and the RXTE during the 2010 outburst, indicating a high energy cut-off of the PLN and a low energy cut-off of the BLN and the QPOs around 2 keV. The emergence of the PLN and the fading of the BLN and the QPOs initially took place from below 2 keV when the source entered the hard intermediate state and finally 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 X-ray and the hard X-ray bands. Our results indicate that the PLN is associated with the optically thick disk in both hard and intermediate states, and power spectral state is independent of the X-ray energy spectral state in a broadband view. We suggest that in the hard and the intermediate state, the BLN and the QPOs emerge from the innermost hot flow subjected to Comptonization, while the PLN originates from the optically thick disk further out. The energy cut-offs 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 cut-off of the PLN follows the orbital frequency at the inner edge of the optically thick disk as well.
Kilohertz QPO Frequency Anti-Correlated with mHz QPO Flux in 4U 1608-52
Wenfei Yu,Michiel van der Klis
Physics , 2002,
Abstract: We analysed {\it Rossi X-ray Timing Explorer} (RXTE) data of the low-mass X-ray binary and atoll source 4U 1608-52 obtained on March 3, 1996 in which the source simultaneously showed a strong single kilohertz quasi-periodic oscillation (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 anti-correlated 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 c/s (4%). This is opposite to the frequency-count rate correlation observed in the same data on longer time scales 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 on the inner disk exerted by the flux due to the quasi-periodic nuclear burning lead to changes in the inner disk radius and hence to the observed anti-correlation between kHz QPO frequency and X-ray count rate. There is a time lag of about ten seconds of the X-ray count rate relative to the kHz QPO frequency in terms of the anti-correlation we found 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.
Page 1 /61098
Display every page Item


Home
Copyright © 2008-2017 Open Access Library. All rights reserved.