259 research outputs found
Probing the X-ray Variability of X-ray Binaries
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.Comment: 4 pages including 1 figure, To appear in "High Energy Processes and
Phenomena in Astrophysics, IAU Symposium 214", X. Li, Z. Wang, V. Trimble
(eds
X-ray outbursts of low-mass X-ray binary transients observed in the RXTE era
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,
-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 -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.Comment: Accepted to Ap
Low frequency QPOs and possible change in the accretion geometry during the outbursts of Aquila X1
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 X1 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 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 at
31 Hz, a factor of 5 higher than those seen in black hole
transients such as GX 3394, 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 2 days before
the hard-to-soft state transition. We examined the dependence of the frequency
on the source flux 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.Comment: 8 pages, 8 figures, 1 table; accepted for publication in Ap
Energy dependent power spectral states and origin of aperiodic variability in black hole binaries
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.Comment: 5 pages, 6 figures, accepted for publication in Ap
State Transitions in Ultracompact Neutron Star LMXBs: towards the Low Luminosity Limit
Luminosity of X-ray spectral state transitions in black hole and neutron star
X-ray binaries can put constraint on the critical mass accretion rate between
accretion regimes. Previous studies indicate that the hard-to-soft spectral
state transitions in some ultracompact neutron star LMXBs have the lowest
luminosity. With X-ray monitoring observations in the past decade, we were able
to identify state transitions towards the lowest luminosity limit in 4U
0614+091, 2S 0918-549 and 4U 1246-588. By analysing corresponding X-ray pointed
observations with the Swift/XRT and the RXTE/PCA, we found no hysteresis of
state transitions in these sources, and determined the critical mass accretion
rate in the range of 0.002 - 0.04 and 0.003 - 0.05
for the hard-to-soft and the soft-to-hard transition,
respectively, by assuming a neutron star mass of 1.4 solar masses. This range
is comparable to the lowest transition luminosity measured in black hole X-ray
binaries, indicating the critical mass accretion rate is not affected by the
nature of the surface of the compact stars. Our result does not support the
Advection-Dominated Accretion Flow (ADAF) model which predicts that the
critical mass accretion rate in neutron star systems is an order of magnitude
lower if same viscosity parameters are taken. The low transition luminosity and
insignificant hysteresis in these ultracompact X-ray binaries provide further
evidence that the transition luminosity is likely related to the mass in the
disc.Comment: 12 pages, 4 figures, to appear in MNRA
Coupling between the 45 Hz Horizontal-Branch Oscillation and the Normal Branch Oscillation in Scorpius X-1
The observations of the bright persistent neutron star low-mass X-ray binary
(LMXB) Sco X-1 performed with the {\it Rossi X-ray Timing Explorer} (RXTE) show
a 6 Hz normal-branch oscillation (NBO), a 45 Hz horizontal-branch
oscillation (HBO), and twin kHz quasi-periodic oscillations (QPOs) on its
normal branch simultaneously. We have found that the fractional amplitude of
the HBO corresponding to the NBO phase of high flux is 1.1%, while that of the
NBO phase of low flux is undetectable, with a 3 upper limit of 0.4%,
implying that the HBO strength varies with the NBO phase in an opposite way to
that of the lower kHz QPO previously found, and suggests that the condition for
the generation of the HBO is met when the NBO flux is high. The 6 Hz NBO in Sco
X-1 connects the 45 Hz HBO and the twin kHz QPO together, showing a unique
picture indicating a coupling between the QPOs, which has never been observed
in other neutron star LMXBs. We discuss the implications for current models of
the 45 Hz HBO, the 6 Hz NBO, and the twin kHz QPOs.Comment: 10 pages, 2 figures, accepted for publication in ApJ
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