2,408 research outputs found
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
Identification of black hole power spectral components across all canonical states
From a uniform analysis of a large (8.5 Ms) Rossi X-ray Timing Explorer data
set of Low Mass X-ray Binaries, we present a complete identification of all the
variability components in the power spectra of black holes in their canonical
states. It is based on gradual frequency shifts of the components observed
between states, and uses a previous identification in the black hole low hard
state as a starting point. It is supported by correlations between the
frequencies in agreement with those previously found to hold for black hole and
neutron stars. Similar variability components are observed in neutron stars and
black holes (only the component observed at the highest frequencies is
different) which therefore cannot depend on source-specific characteristics
such as the magnetic field or surface of the neutron star or spin of the black
hole. As the same variability components are also observed across the jet-line
the X-ray variability cannot originate from the outer-jet but is most likely
produced in either the disk or the corona. We use the identification to
directly compare the difference in strength of the black hole and neutron star
variability and find these can be attributed to differences in frequency and
strength of high frequency features, and do not require the absence of any
components. Black holes attain their highest frequencies (in the
hard-intermediate and very-high states) at a level a factor ~6 below the
highest frequencies attained by the corresponding neutron star components,
which can be related to the mass difference between the compact objects in
these systems.Comment: 17 pages, 16 figures, accepted for publication in Ap
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 aperiodic timing behaviour of the accretion-driven millisecond pulsar SAX J1808.4-3658
We studied the aperiodic X-ray timing behaviour of the accreting millisecond
pulsar SAX J1808.4-3658. The source was recently found to be the first
accreting millisecond pulsar that shows the kilohertz quasi-periodic
oscillations (kilohertz QPOs) that are found in many other X-ray binaries with
accreting neutron stars. The high frequency of these signals reflects the short
dynamical time scales in the region near the compact object where they
originate. We find that in addition to the kilohertz QPOs SAX J1808.4-3658
shows several low frequency timing features, based on which the source can be
classified as a so-called atoll source. The frequencies of the variability
components of the atoll sources follow a universal scheme of correlations. The
correlations in SAX J1808.4-3658 are similar but show a shift in upper
kilohertz QPO frequency. This discrepancy is perhaps related to a stronger or
differently configured magnetic field.Comment: 4 pages, 3 figures. To appear in the proceedings of the "The Restless
High-Energy Universe" (Amsterdam, The Netherlands), 2003, eds. E.P.J. van den
Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijer
Cross-spectral modelling of the black hole X-ray binary XTEJ1550-564: challenges to the propagating fluctuations paradigm
Timing properties of black hole X-ray binaries in outburst can be modeled
with mass accretion rate fluctuations propagating towards the black hole. Such
models predict time lags between energy bands due to propagation delays. First
application of a propagating fluctuations model to black hole power spectra
showed good agreement with the data. Indeed, hard lags observed from these
systems appear to be in agreement with this generic prediction. Our PROPFLUC
code allows to simultaneously predict power spectra, time lags, and coherence
of the variability as a function of energy. This was successfully applied to
Swift data on the black hole MAXIJ1659-152, fitting jointly the power spectra
in two energy bands and the cross-spectrum between these two bands. In the
current work, we attempt to to model two high signal to noise Rossi X-ray
Timing Explorer (RXTE) observations of the black hole XTE J1550-564. We find
that neither observation can be adequately explained by the model even when
considering, additionally to previous PROPFLUC versions, different propagation
speeds of the fluctuations. After extensive exploration of model extensions, we
tentatively conclude that the quantitative and qualitative discrepancy between
model predictions and data is generic to the propagating fluctuations paradigm.
This result encourages further investigation of the fundamental hypotheses of
the propagating fluctuations model. We discuss some of these hypotheses with an
eye to future works.Comment: 14 pages, 12 figures, accepted for publication in MNRA
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