572 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
Strong Correlation Between Noise Features at Low Frequency and the Kilohertz QPOs in the X-Ray Binary 4U 1728-34
We study the timing properties of the low mass X-ray binary 4U 1728-34 using
recently released data from the Rossi X-Ray Timing Explorer. This binary, like
many others with accreting neutron stars, is known to exhibit strong
quasi-periodic oscillations (QPOs) of its X-ray flux near 1 kHz. In addition to
the kilohertz QPOs, the Fourier power spectra show a broken power law noise
component, with a break frequency between 1 and 50 Hz, and a Lorentzian between
10 and 50 Hz. We find that the frequencies of the break and the low-frequency
Lorentzian are well correlated with the frequencies of the kilohertz QPOs. The
slope of the correlation is similar to that expected if the oscillations are
due to relativistic frame dragging (Lense-Thirring precession) in the inner
accretion disk (Stella & Vietri 1998). The correlation is also nearly identical
to the one found in Z-sources between the the well known QPOs on the horizontal
branch and the kilohertz QPOs, suggesting that the low frequency oscillations
are a similar phenomenon in these sources. The frequency of the break in the
power spectra is also correlated with the frequencies of the kilohertz QPOs. As
previously noted for the similar binaries 4U 1608-50 and 4U 1705-44, this
broken power law component closely resembles that of black hole candidates in
the low state, where the break frequency is taken as an indicator of mass
accretion rate. The relation between break frequency and kilohertz QPO
frequency thus provides additional proof that the frequency of the kilohertz
QPOs increases with mass accretion rate.Comment: ApJL in press, see the 'QPO page' at
http://www.astro.uva.nl/ecford/qpos.htm
The Complex Phase Lag Behavior of the 3-12 Hz Quasi-Periodic Oscillations during the Very High State of XTE J1550-564
We present a study of the complex phase lag behavior of the low-frequency
(<20 Hz) quasi-periodic oscillations (QPOs) in the X-ray transient and
black-hole candidate XTE J1550-564 during its very high state. We distinguish
two different types of low-frequency QPOs, based on their coherence and
harmonic content. The first type is characterized by a 6 Hz QPO with a Q (the
QPO frequency divided by the QPO width) of <3 and with a harmonic at 12 Hz. The
second type of QPO is characterized by a 6 Hz QPO with a Q value of >6 and with
harmonics at 3, 12, 18, and possibly at 9 Hz. Not only the Q values and the
harmonic content of the two types are different, but also their phase lag
behavior. For the first type of QPO, the low energy photons (<5 keV) of both
the 6 Hz QPO and its harmonic at 12 Hz lag the hard energy photons (>5 keV) by
as much as 1.3 radian. The phase lags of the second type of QPO are more
complex. The soft photons (<5 keV) of the 3 and the 12 Hz QPOs lag the hard
photons (>5 keV) by as much as 1.0 radian. However, the soft photons of the 6
Hz QPO precede the hard ones by as much as 0.6 radian. This means that
different harmonics of this type of QPO have different signs for their phase
lags. This unusual behavior is hard to explain when the lags are due to
light-travel-time differences between the photons at different energies, e.g.,
in a Comptonizing region surrounding the area where the QPOs are formed.Comment: Accepted for publication in ApJ Letters, 29 September 199
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
Detection of anti-correlated hard X-ray time lag in Cygnus X-3
The wide-band X-ray spectra of the high mass X-ray binary Cygnus X-3 exhibits
a pivoting behavior in the `low' (as well as `hard') state, correlated to the
radio emission. The time scale of the soft and hard X-rays' anti-correlation,
which gave rise to the pivoting feature, was found to be less than a day from
the monitoring observations by RXTE--ASM and CGRO--BATSE. In this Letter we
report the detection of a lag of 1000s in the anti-correlation of
the hard X-ray emission (20--50 keV) to that of the soft X-ray emission (2--7
keV), which may be attributed to the viscous time scale of flow of matter in
the accretion disk. This suggests the geometrical picture of a truncated
accretion disc with a Compton cloud inside the disc, the relative sizes of
which determine the spectral shape. Any change in the disc structure will take
place in a viscous time scale, with corresponding anti-correlated change in the
Compton cloud. We also report the pivoting in the spectra in one span of a
pointed observation when an episode of the rearranging of the accretion system
is serendipitously observed. This is the first such observation of hard X-ray
delay seen in the persistent Galactic microquasars, within the precincts of the
hard state.Comment: Accepted in The Astrophysical Journal (Letters): in pres
Discovery of KiloHertz Quasi-Periodic Oscillations in 4U 1735-44
We discovered a single kHz quasi-periodic oscillation (QPO) near 1150 Hz in
the Rossi X-ray Timing Explorer X-ray light curve of the low mass X-ray binary
and atoll source 4U 1735-44. The rms amplitude of this peak was 2-3%, and the
FWHM 6-40 Hz. There are indications that the kHz QPO frequency decreased from
1160 Hz to 1145 Hz when the count rate increased, which would be quite
different from what is observed in other atoll sources for which kHz QPOs have
been discovered. In the X-ray color-color diagram and hardness-intensity
diagram the source traced out the curved branch (the so-called banana branch)
which has been found by previous instruments. The kHz QPO was only detected
when the source was at the lowest count rates during our observations, i.e. on
the lower part of the banana branch. When 4U 1735-44 was at higher count rates,
i.e. on the upper part of the banana branch and at higher inferred mass
accretion rate with respect to that on the lower part of the banana branch, the
QPO was not detected.
Besides the kHz QPO we discovered a low frequency QPO with a frequency near
67 Hz, together with a complex broad peaked noise component below 30 Hz. This
67 Hz QPO may be related to the magnetospheric beat-frequency QPO, which is
observed on the horizontal branch of Z sources. This idea is supported by the
(peaked) noise found in both 4U 1735-44 and Z sources at frequencies just below
the QPO frequency.Comment: 9 pages, including 2 figures. Accepted for publication in ApJ Letter
On the dimension of subspaces with bounded Schmidt rank
We consider the question of how large a subspace of a given bipartite quantum
system can be when the subspace contains only highly entangled states. This is
motivated in part by results of Hayden et al., which show that in large d x
d--dimensional systems there exist random subspaces of dimension almost d^2,
all of whose states have entropy of entanglement at least log d - O(1). It is
also related to results due to Parthasarathy on the dimension of completely
entangled subspaces, which have connections with the construction of
unextendible product bases. Here we take as entanglement measure the Schmidt
rank, and determine, for every pair of local dimensions dA and dB, and every r,
the largest dimension of a subspace consisting only of entangled states of
Schmidt rank r or larger. This exact answer is a significant improvement on the
best bounds that can be obtained using random subspace techniques. We also
determine the converse: the largest dimension of a subspace with an upper bound
on the Schmidt rank. Finally, we discuss the question of subspaces containing
only states with Schmidt equal to r.Comment: 4 pages, REVTeX4 forma
``Canonical'' Black Hole States in the Superluminal Source GRO J1655-40
We analyze RXTE/PCA observations of the black hole candidate and galactic
superluminal source GRO J1655-40 during its recent outburst. We show that
during its decay to quiescence, GRO J1655-40 goes through the high,
intermediate, and low state (and that at the beginning of its decay it might
have even shown signatures of a very high state), just like other black hole
candidates. This is the first time that such a transition is observed in a
galactic superluminal source. We discuss what are the implications of these
results on the hypothesis that the spin of the black hole in superluminal
sources is much higher than in other black hole candidates.Comment: To appear in ApJL. AAS LaTex v4.0 (9 pages, 2 ps-figures
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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