3,464 research outputs found
The X-ray fast-time variability of Sco X-2 (GX 349+2) with RXTE
Sco X-2 (GX 349+2) is a low-mass X-ray binary and Z source. We have analysed
156 ks of Rossi X-ray Timing Explorer data, obtained in 1998 January, on this
source. We investigated the fast-time variability as a function of position on
the Z track. During these observations, Sco X-2 traced out the most extensive Z
track ever reported from this object, making this the most comprehensive study
thus far. We found the broad peaked flaring branch noise that is typical of Sco
X-2, with a centroid frequency in the range 3.3--5.8 Hz. We also discovered low
frequency noise, and a new peaked noise feature, with centroid frequencies in
the range 5.4--7.6 Hz and 11--54 Hz, respectively. We discuss the phenomenology
of these features, their relationship with the power spectral components found
in other low-mass X-ray binaries, and the implications for current models. In
particular, the low frequency noise we observed was strongest at intermediate
energies, in contrast to the low frequency noise seen in other Z sources. We
also detected very low frequency noise, and have calculated complex cross
spectra between intensity and hardness. We found that the very low frequency
noise is not entirely due to motion along the Z track.Comment: 17 pages, 9 figures, minor improvements, accepted for publication in
MNRA
The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U 1728-34, 4U 1608-52, and Aql X-1, as a Function of X-ray Intensity
We study the kilohertz quasi-periodic oscillations (kHz QPOs) in the low-mass
X-ray binaries 4U 1728-34, 4U 1608-52, and Aql X-1. Each source traces out a
set of nearly parallel lines in a frequency vs. X-ray count rate diagram. We
find that between two of these tracks, for similar QPO frequency, the source
count rate can differ by up to a factor of ~ 4, whereas at the same time the
rms amplitude of the kHz QPOs is only a factor of ~ 1.1 different. We also find
that, for 4U 1608-52 and Aql X-1, the rms spectrum of the kHz QPOs does not
depend upon which track the source occupies in the frequency vs. X-ray count
rate diagram. Our results for 4U 1728-34, 4U 1608-52, and Aql X-1 are
inconsistent with scenarios in which the properties of the kHz QPOs are only
determined by the mass accretion rate through the disk, whereas X-ray count
rate also depends upon other sources of energy that do not affect the QPOs.Comment: Submitted to ApJ.(7 pages; 9 figures
Are There Three Peaks in the Power Spectra of GX 339-4 and Cyg X-1?
Among the variability behaviour exhibited by neutron star systems are the
so-called ``horizontal branch oscillations'' (HBO, with frequencies ~50 Hz),
the ``lower-frequency kHz quasi-periodic oscillation'' (QPO) and the
``upper-frequency kHz QPO'', with the latter two features being separated in
frequency by an amount comparable to, but varying slightly from, the suspected
spin-frequency of the neutron star. Recently, Psaltis, Belloni, & van der Klis
(1999) have suggested that there exists a correlation between these three
frequencies that, when certain identifications of variability features are
made, even encompasses black hole sources. We consider this hypothesis by
reanalyzing a set of GX 339-4 observations. The power spectral density (PSD)
constructed from a composite of 7 separate, but very similar, observations
shows evidence for three broad peaks in the PSD. If the peak frequencies of
these features are identified with ``QPO'', then their frequencies
approximately fit the correlations suggested by Psaltis, Belloni, & van der
Klis (1999). We also reanalyze a Cyg X-1 observation and show that the
suggested QPO correlation may also hold, but that complications arise when the
``QPOs'' (which, in reality, are fairly broad features) are considered as a
function of energy band. These fits suggest the existence of at least three
separate, independent physical processes in the accretion flow, a hypothesis
that is also supported by consideration of the Fourier frequency-dependent time
lags and coherence function between variability in different energy bands.Comment: 8 pages, 7 figures, to appear in MNRA
The discovery of a 7-14 Hz Quasi-Periodic Oscillation in the X-ray Transient XTE J1806-246
We have studied the correlated X-ray spectral and X-ray timing behavior of
the X-ray transient XTE J1806-246 using data obtained with the proportional
counter array onboard the Rossi X-ray Timing Explorer. In the X-ray color-color
diagram two distinct patterns are traced out. The first pattern is a curved
branch, which is observed during the rise and the decay of the outburst. This
pattern resembles the so-called banana branch of those low-luminosity neutron
star low-mass X-ray binaries (LMXBs) which are referred to as atoll sources.
The power spectrum of XTE J1806-246 on this curved branch consisted of a power
law and a cutoff power law component. The presence of these components and
their dependence on position of the source on the branch is also identical to
the behavior of atoll sources on the banana branch. Near the end of it outburst
XTE J1806-246 formed patches in the color-color diagram, the spectrum was
harder, and the power spectrum showed strong band limited noise, characteristic
of the atoll sources in the island state. A second pattern was traced out
during the only observation at the peak of the outburst. It consists of a
structure which we interpret as formed by two distinct branches. This pattern
resembles the normal-flaring branches of the high-luminosity neutron star LMXBs
(the Z sources). The discovery of a 7-14 Hz QPO during this observation
strengthens this similarity. We conclude that if XTE J1806-246 is a neutron
star, it is most likely an atoll source that only at the peak of its outburst
reached a luminosity level sufficiently high to show the type of QPO that Z
sources who on their normal and flaring branches.Comment: Accepted for publication in ApJ Main Journal (14 April 1999). Only
minor changes to the text and to some of the figure
A comparison of the fast timing behaviour of 4U 1705-44 to that of 4U 1608-52 and Cyg X-1
We studied the fast timing behaviour of the atoll source 4U 1705-44 using the
entire EXOSAT dataset, four observations covering a total of 230,000 seconds of
1-20 keV spectral and timing data. In one of the observations, 4U 1705-44 was
in a low intensity "island" state and had an unusually hard spectrum. The fast
timing analysis of this hard island state shows a power spectrum very similar
to that of black hole candidates in the "low state", with a flat-topped
band-limited noise component that gradually steepens towards higher frequency.
We perform for the first time a quantitative comparison of the timing behaviour
of an atoll source in the hard island state (4U 1705-44) with that of a black
hole candidate in the low state (Cygnus X-1). We also compare the power
spectrum of 4U 1705-44 in the hard island state with those of the atoll source
4U 1608-52 in a similar state as reported by Yoshida et al. (1993). Our results
confirm that there are similarities between the fast timing behaviour of the
hard island states of these atoll sources and the low state of black hole
candidates, yet we also find significant differences in power spectral
parameters; the power spectra of the neutron star systems have a lower rms
amplitude and are less steep. We find a trend among the neutron star power
spectral properties, in the sense that the lower the centroid frequency of the
fitted Lorentzian is, the higher its fractional rms amplitude, and the steeper
the continuum underneath it. We propose a new method to fit the power spectra
of Cyg X-1 and other black hole candidates in the low state, that provides a
significantly better fit than previous models.Comment: Accepted for publication in A&A 7 pages 4 figure
The first radius-expansion X-ray burst from GX 3+1
During several observations in 1999 August with RXTE of the low-mass X-ray
binary GX 3+1, we found a single short and strong X-ray burst. This is the
first burst from GX 3+1 which clearly shows evidence for radius expansion of
the neutron-star photosphere during the thermo-nuclear runaway. We show that
the cooling phase of the neutron star photosphere starts already just before
the end of the contraction phase. Considering the fact that the radius
expansion is due to the burst luminosity being at the Eddington luminosity,
assuming standard burst parameters and accounting for gravitational redshift
effects we derive a distance to the source of ~4.5 kpc, although relaxing these
assumptions may lead to uncertainties up to ~30%. By comparing the persistent
flux with that observed at the peak of the burst we infer that near the time of
the X-ray burst the persistent luminosity of GX 3+1 is ~0.17*L_edd, confirming
predictions from theoretical modeling of X-ray spectra of bright sources like
GX 3+1.Comment: 5 pages, 4 figures, accepted for publication in A&A Letter
The statistics of Sco X-1 kHZ QPOs
Recently an additional technique was applied to investigate the properties of
kHz QPOS, i.e. the analysis of the distribution of frequency ratios or
frequencies themselves. I review the results of such work on the data from
ScoX-1: Abramowicz et al. (2003), which was later criticized by Belloni et
al.(2005). I conclude that the findings of the latter paper are consistent with
results presented earlier: kHz QPOs cluster around the value corresponding to
the frequency ratio of 2/3. I also discuss the random walk model of kHz QPOs
and possible future observations needed to verify it.Comment: Astronomische Nachrichten, in pres
Pulse amplitude depends on kHz QPO frequency in the accreting millisecond pulsar SAX J1808.4-3658
We study the relation between the 300-700 Hz upper kHz quasi-periodic
oscillation (QPO) and the 401 Hz coherent pulsations across all outbursts of
the accreting millisecond X-ray pulsar SAX J1808.4-3658 observed with the Rossi
X-ray Timing Explorer. We find that the pulse amplitude systematically changes
by a factor of ~2 when the upper kHz QPO frequency passes through 401 Hz: it
halves when the QPO moves to above the spin frequency and doubles again on the
way back. This establishes for the first time the existence of a direct effect
of kHz QPOs on the millisecond pulsations and provides a new clue to the origin
of the upper kHz QPO. We discuss several scenarios and conclude that while more
complex explanations can not formally be excluded, our result strongly suggests
that the QPO is produced by azimuthal motion at the inner edge of the accretion
disk, most likely orbital motion. Depending on whether this azimuthal motion is
faster or slower than the spin, the plasma then interacts differently with the
neutron-star magnetic field. The most straightforward interpretation involves
magnetospheric centrifugal inhibition of the accretion flow that sets in when
the upper kHz QPO becomes slower than the spin.Comment: 5 pages, 4 figures, Accepted for publication in ApJ
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