170,427 research outputs found
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
Evolution of Spectral States of Aql X-1 during the 2000 Outburst
We present the results of detailed analysis of X-ray data in 3-20 keV range
from a ~70 day outburst of the neutron star transient Aquila X-1 during
September-November 2000. Optical monitoring with the YALO 1m telescope was used
to trigger X-ray observations with Rossi X-ray Timing Explorer (RXTE) in order
to follow the outburst from a very early stage. In this paper we discuss the
correlated evolution in time of features in the spectral and temporal domains,
for the entire outburst. The state transition from low/hard state to high/soft
state during the rise of the outburst occurs at higher luminosity than the
transition back to low/hard state during the decay, as has also been observed
in other outbursts. Fourier power spectra at low frequencies show a broken
power law continuum during the rising phase, with the break frequency
increasing with time. During the decline from maximum the source evolves to a
position in the hardness-intensity plane as well as in the color-color diagram
which is similar to, but distinct from, the canonical high/soft state. High
frequency quasi-periodic oscillations from 636-870 Hz were seen only during
this transitional state.Comment: 25 pages, 11 figures. Accepted for publication in Ap
Accretion-ejection instability and QPO in black hole binaries. I. Observations
This is the first of two papers in which we address the physics of the
low-frequency Quasi-Periodic Oscillation (QPO) of X-ray binaries, in particular
those hosting a black hole. We discuss and repeat the recent analysis and
spectral modelling of the micro-quasar GRO J1655 by Sobczak et al (2000,
hereafter SMR), and compare it with GRS 1915; this leads us to confirm and
analyze in more detail the different behavior noted by SMR, between GRO J1655
and other sources, when comparing the correlation between the QPO frequency and
the disk inner radius. In a companion paper (Varniere et al., 2002, hereafter
Paper II) we will show that these opposite behaviors can be explained in the
context of the Accretion-Ejection Instability recently presented by Tagger and
Pellat (1999).
We thus propose that the difference between GRO J1655 and other sources comes
from the fact that in the former, observed in a very high state, the disk inner
radius always stays close to the Last Stable Orbit.
In the course of this analysis, we also indicate interesting differences
between the source properties, when the spectral fits give an anomalously low
inner disk radius. This might indicate the presence of a spiral shock or a hot
point in the disk.Comment: accepted by A&
Correlated Timing and Spectral Variations of the Soft X-ray Transient Aquila X-1: Evidence for an Atoll classification
Based on Rossi X-ray Timing Explorer data, we discuss the classification of
the soft X-ray transient Aquila X-1 in the Z/atoll scheme, and the relation of
its kilohertz quasi-periodic oscillations (kHz QPO) properties to the X-ray
colors. The color-color diagram shows one elongated ("banana") structure and
several "islands" of data points. The power spectra of the island are best
represented by a broken power-law, whereas those of the banana by a power-law
below ~ 1 Hz plus an exponentially cut-off component at intermediate
frequencies (30-60 Hz). The parameters of these two components change in
correlation with the position of the source in the color-color diagram. Based
on the pattern that the source shows in the color-color diagram and its
aperiodic variability we conclude that Aquila X-1 is an atoll source. We have
also investigated the possible correlation between the frequency of the kHz QPO
and the position of the source in the color-color diagram. The complexity seen
in the frequency versus count rate diagram is reduced to a single track when
the frequency is plotted against hard or soft color.Comment: 9 pages, 5 figures. Accepted for publication in Ap
Temporal And Spatial Turbulent Spectra Of MHD Plasma And An Observation Of Variance Anisotropy
The nature of magnetohydrodynamic (MHD) turbulence is analyzed through both temporal and spatial magnetic fluctuation spectra. A magnetically turbulent plasma is produced in the MHD wind tunnel configuration of the Swarthmore Spheromak Experiment. The power of magnetic fluctuations is projected into directions perpendicular and parallel to a local mean field; the ratio of these quantities shows the presence of variance anisotropy which varies as a function of frequency. Comparisons among magnetic, velocity, and density spectra are also made, demonstrating that the energy of the turbulence observed is primarily seeded by magnetic fields created during plasma production. Direct spatial spectra are constructed using multi-channel diagnostics and are used to compare to frequency spectra converted to spatial scales using the Taylor hypothesis. Evidence for the observation of dissipation due to ion inertial length scale physics is also discussed, as well as the role laboratory experiments can play in understanding turbulence typically studied in space settings such as the solar wind. Finally, all turbulence results are shown to compare fairly well to a Hall-MHD simulation of the experiment
The color of sea level: importance of spatial variations in spectral shape for assessing the significance of trends
We investigate spatial variations in the shape of the spectrum of sea level variability, based on a homogeneously-sampled 12-year gridded altimeter dataset. We present a method of plotting spectral information as color, focusing on periods between 2 and 24 weeks, which shows that significant spatial variations in the spectral shape exist,
and contain useful dynamical information. Using the Bayesian Information Criterion, we determine that, typically, a 5th order autoregressive model is needed to capture the structure in the spectrum. Using this model, we show that statistical errors in fitted local trends range between less than 1 and more than 5 times what would be calculated assuming “white” noise, and the time needed to detect a 1 mm/yr trend ranges between about 5 years and many decades. For global-mean sea level, the statistical error reduces to 0.1 mm/yr over 12 years, with only 2 years needed to detect a 1 mm/yr trend. We find significant regional differences in trend from the global mean. The
patterns of these regional differences are indicative of a sea level trend dominated by dynamical ocean processes, over this perio
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
Correlated X-ray Spectral and Timing Behavior of the Black Hole Candidate XTE J1550-564: A New Interpretation of Black Hole States
We present an analysis of RXTE data of the X-ray transient XTE J1550-564. The
source went through several states, which were divided into spectrally soft and
hard states. These states showed up as distinct branches in the color-color
diagram, forming a structure with a comb-like topology; the soft state branch
forming the spine and the hard state branches forming the teeth. Variability
was strongly correlated with the position on the branches. The broad band noise
became stronger, and changed from power law like to band limited, as the
spectrum became harder. Three types of QPOs were found: 1-18 Hz and 102-284 Hz
QPOs on the hard branches, and 16-18 Hz QPOs on and near the soft branch. The
frequencies of the high and low frequency QPOs on the hard branches were
correlated with each other, and anti-correlated with spectral hardness. The
changes in QPO frequency suggest that the inner disc radius only increases by a
factor of 3-4 as the source changes from a soft to a hard state. Our results on
XTE J1550-564 strongly favor a 2-dimensional description of black hole
behavior, where the regions near the spine of the comb in the color-color
diagram can be identified with the high state, and the teeth with transitions
from the high state, via the intermediate state (which includes the very high
state) to the low state, and back. The two physical parameters underlying this
behavior vary to a large extent independently and could for example be the mass
accretion rate through the disk and the size of a Comptonizing region.Comment: 49 pages (inlcuding 26 figures and 4 tables), accepted for
publication in ApJ Supplement
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