722 research outputs found
Reversal of the amplitude difference of kHz QPOs in six atoll sources
AIMS: For six neutron-star atoll sources (4U 1608-52, 4U 1636-53, 4U 0614+09,
4U 1728-34, 4U 1820-30 and 4U 1735-44) we investigate the relationship between
the observed fractional rms amplitudes of the twin kHz QPOs. We discuss whether
this displays features that could have a physical meaning in terms of the
proposed QPO models. METHOD: We consider the difference in rms amplitude
between the upper and lower kHz QPOs, as a function of the frequency ratio R.
We compare two data sets. Set I is a collection taken from published data. Set
II has rms amplitude values obtained by automatic fitting of continuous
segments of RXTE-PCA observations. RESULTS: For each of the six sources, we
find that there is a point in the R domain around which the amplitudes of the
two twin kHz QPOs are the same. We find such a point located inside a narrow
interval R=1.5 +-3%. Further investigation is needed in the case of two sources
to explore this finding, since we have not determined this point in Set II.
There is evidence of a similar point close to R = 1.33 or R = 1.25 in the four
sources. We suggest that some of these points may correspond to the documented
clustering of the twin kHz QPO frequency ratios. CONCLUSIONS: For the sources
studied, the rms amplitudes of the two kHz peaks become equal when the
frequencies of the oscillations pass through a certain ratio R, which is
roughly the same for each of the sources. In terms of the orbital QPO models,
with some assumptions concerning the QPO modulation, this finding implies the
existence of a specific orbit at a common value of the dimensionless radius, at
which the oscillations corresponding to the two peaks come into balance. In a
more general context, the amplitude difference behaviour suggests the possible
existence of an energy interchange between the upper and lower QPO modes.Comment: 5 pages, 2 figures, proofreaded versio
The atoll source states of 4U 1608-52
We have studied the atoll source 4U 1608-52 using a large data set obtained
with the Rossi X-ray Timing Explorer. We find that the timing properties of 4U
1608-52 are almost exactly identical to those of the atoll sources 4U 0614+09
and 4U 1728-34 despite the fact that contrary to these sources 4U 1608-52 is a
transient covering two orders of magnitude in luminosity. The frequencies of
the variability components of these three sources follow a universal scheme
when plotted versus the frequency of the upper kilohertz QPO, suggesting a very
similar accretion flow configuration. If we plot the Z sources on this scheme
only the lower kilohertz QPO and HBO follow identical relations. Using the
mutual relations between the frequencies of the variability components we
tested several models; the transition layer model, the sonic point beat
frequency model, and the relativistic precession model. None of these models
described the data satisfactory. Recently, it has been suggested that the atoll
sources (among them 4U 1608-52) trace out similar three-branch patterns as the
Z sources in the color-color diagram. We have studied the relation between the
power spectral properties and the position of 4U 1608-52 in the color-color
diagram and conclude that the timing behavior is not consistent with the idea
that 4U 1608-52 traces out a three-branched Z shape in the color-color diagram
along which the timing properties vary gradually, as Z sources do.Comment: 43 pages, 16 figures, ApJ accepte
Polar decompositions of quaternion matrices in indefinite inner product spaces
Polar decompositions of quaternion matrices with respect to a given
indefinite inner product are studied. Necessary and sufficient conditions for
the existence of an -polar decomposition are found. In the process an
equivalent to Witt's theorem on extending -isometries to -unitary
matrices is given for quaternion matrices
Natuurrapport 2003: toestand van de natuur in Vlaanderen: cijfers voor het beleid: samenvatting / English summary
Timing properties and spectral states in Aquila X-1
We have analyzed five X-ray outbursts of the neutron-star soft X-ray
transient Aql X-1 and investigated the timing properties of the source in
correlation with its spectral states as defined by different positions in the
color-color and hardness-intensity diagrams. The hard color and the source
count rate serve as the distinguishing parameters giving rise to three spectral
states: a low-intensity hard state, an intermediate state and a high-intensity
soft state. These states are respectively identified with the extreme island,
island and banana states that characterize the atoll sources. The large amount
of data analyzed allowed us to perform for the first time a detailed timing
analysis of the extreme island state. Differences in the aperiodic variability
between the rise and the decay of the X-ray outbursts are found in this state:
at the same place in the color-color diagram, during the rise the source
exhibits more power at low frequencies (< 1 Hz), whereas during the decay the
source is more variable at high frequencies (> 100 Hz). The very-low frequency
noise that characterizes the banana-state power spectra below 1 Hz cannot be
described in terms of a single power law but a two-component model is required.
In two outbursts a new 6-10 Hz QPO has been discovered and tentatively
identified with the normal/flaring branch-like oscillation observed only at the
highest inferred mass accretion rates. We have compared the spectral and timing
properties of Aql X-1 with those of other atoll and Z sources. Our results
argue against a unification scheme for these two types of neutron-star X-ray
binaries.Comment: 24 pages, 4 tables, 9 figures, accepted for publication in Ap
XTE J1701-462 and its Implications for the Nature of Subclasses in Low-Magnetic-Field Neutron Star Low-Mass X-Ray Binaries
We report on an analysis of RXTE data of the transient neutron star low-mass
X-ray binary (NS-LMXB) XTE J1701-462, obtained during its 2006-2007 outburst.
The X-ray properties of the source changed between those of various types of
NS-LMXB subclasses. At high luminosities the source switched between two types
of Z source behavior and at low luminosities we observed a transition from Z
source to atoll source behavior. These transitions between subclasses primarily
manifest themselves as changes in the shapes of the tracks in X-ray color-color
and hardness-intensity diagrams, but they are accompanied by changes in the kHz
quasi-periodic oscillations, broad-band variability, burst behavior, and/or
X-ray spectra. We find that the low-energy X-ray flux is a good parameter to
track the gradual evolution of the tracks in color-color and hardness-intensity
diagrams, allowing us to resolve the evolution of the source in greater detail
than before and relate the observed properties to other NS-LMXBs. We further
find that during the transition from Z to atoll, characteristic behavior known
as the atoll upper banana can equivalently be described as the final stage of a
weakening Z source flaring branch, thereby blurring the line between the two
subclasses. Our findings strongly suggest that the wide variety in behavior
observed in NS-LXMBs with different luminosities can be linked through changes
in a single variable parameter, namely the mass accretion rate, without the
need for additional differences in the neutron star parameters or viewing
angle. We briefly discuss the implications of our findings for the spectral
changes observed in NS LMXBs and suggest that, contrary to what is often
assumed, the position along the color-color tracks of Z sources is not
determined by the instantaneous mass accretion rate.Comment: Submitted to ApJ, comments are welcome. 13 pages, 8 figure
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