2,361 research outputs found
Coronal--Temporal Correlations in GX339-4: Hysteresis, Possible Reflection Changes, and Implications for ADAFs
We present spectral fits and timing analysis of Rossi X-ray Timing Explorer
observations of GX339-4. These observations were carried out over a span of
more than two years and encompassed both the soft/high and hard/low states.
Hysteresis in the soft state/hard state transition is observed. The hard state
exhibits a possible anti-correlation between coronal compactness (i.e.,
spectral hardness) and the covering fraction of cold, reflecting material. The
correlation between `reflection fraction' and soft X-ray flux, however, appears
to be more universal. Furthermore, low flux, hard state observations - taken
over a decline into quiescence- show that the Fe line, independent of
`reflection fraction', remains broad and at a roughly constant equivalent
width, counter to expectations from ADAF models. All power spectral densities
(PSD) of the hard state X-ray lightcurves are describable as the sum of just a
few broad, quasi-periodic features with frequencies that roughly scale as
coronal compactness to the -3/2 power. Similar to observations of Cyg X-1, time
lags between soft and hard variability anti-correlate with coronal compactness.
A stronger correlation is seen between the time lags and the `reflection
fraction'.Comment: 29 Pages, 17 Figures, 6 Tables. Accepted for Publication in MNRAS.
(Abstract Abridged
The column density towards LMC X-1
We measure the neutral absorption towards the black hole X-ray binary system
LMC X-1 from six archival soft X-ray spectra obtained with the gratings and/or
CCD detectors on Chandra, XMM-Newton, and Swift. Four spectral models for the
soft continuum have been investigated. While the powerlaw model may
overestimate NH considerably, the others give consistent results. Taking the
lower metalicity of the Large Magellanic Cloud into account, we find equivalent
hydrogen column densities of N_H = (1.0-1.3)*10^22 cm^-2, with a systematic
dependence on the orbital phase. This variation of the neutral absorption can
nearly explain the orbital modulation of the soft X-ray flux recently detected
with the All Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE).Comment: 4 pages, accepted for publication as a Letter in Astronomy and
Astrophysic
RXTE Observation of Cygnus X-1: II. Timing Analysis
We present timing analysis for a Rossi X-ray Timing Explorer observation of
Cygnus X-1 in its hard/low state. This was the first RXTE observation of Cyg
X-1 taken after it transited back to this state from its soft/high state.
RXTE's large effective area, superior timing capabilities, and ability to
obtain long, uninterrupted observations have allowed us to obtain measurements
of the power spectral density (PSD), coherence function, and Fourier time lags
to a decade lower in frequency and half a decade higher in frequency than
typically was achieved with previous instruments. Notable aspects of our
observations include a weak 0.005 Hz feature in the PSD coincident with a
coherence recovery; a `hardening' of the high-frequency PSD with increasing
energy; a broad frequency range measurement of the coherence function,
revealing rollovers from unity coherence at both low and high frequency; and an
accurate determination of the Fourier time lags over two and a half decades in
frequency. As has been noted in previous similar observations, the time delay
is approximately proportional to f^(-0.7), and at a fixed Fourier frequency the
time delay of the hard X-rays compared to the softest energy channel tends to
increase logarithmically with energy. Curiously, the 0.01-0.2 Hz coherence
between the highest and lowest energy bands is actually slightly greater than
the coherence between the second highest and lowest energy bands. We carefully
describe all of the analysis techniques used in this paper, and we make
comparisons of the data to general theoretical expectations. In a companion
paper, we make specific comparisons to a Compton corona model that we have
successfully used to describe the energy spectral data from this observation.Comment: To Be Published in the Astrophysical Journal. 18 pages. Uses
emulatepaj.st
Measurements of the Cosmological Evolution of Magnetic Fields with the Square Kilometre Array
We investigate the potential of the Square Kilometre Array (SKA) for
measuring the magnetic fields in clusters of galaxies via Faraday rotation of
background polarised sources. [...] We find that about 10 per cent of the sky
is covered by a significant extragalactic Faraday screen. Most of it has
rotation measures between 10 and 100 rad/m/m. We argue that the cluster centres
should have up to about 5000 rad/m/m. We show that the proposed mid frequency
aperture array of the SKA as well as the lowest band of the SKA dish array are
well suited to make measurements for most of these rotation measure values,
typically requiring a signal-to-noise of ten. We calculate the spacing of
sources forming a grid for the purpose of measuring foreground rotation
measures: it reaches a spacing of 36 arcsec for a 100 hour SKA observation per
field. We also calculate the statistics for background RM measurements in
clusters of galaxies. We find that a first phase of the SKA would allow us to
take stacking experiments out to high redshifts (>1), and provide improved
magnetic field structure measurements for individual nearby clusters. The full
SKA aperture array would be able to make very detailed magnetic field structure
measurements of clusters with more than 100 background sources per cluster up
to a redshift of 0.5 and more than 1000 background sources per cluster for
nearby clusters, and could for reasonable assumptions about future measurements
of electron densities in high redshift clusters constrain the power law index
for the magnetic field evolution to better than dm=0.4, if the magnetic field
in clusters should follow B ~ (1+z)^m.Comment: 12 pages, 10 figures, 3 tables, accepted by MNRAS, minor correction
to eq (5
Chandra X-ray spectroscopy of the focused wind in the Cygnus X-1 system. I. The non-dip spectrum in the low/hard state
We present analyses of a 50 ks observation of the supergiant X-ray binary
system Cygnus X-1/HDE 226868 taken with the Chandra High Energy Transmission
Grating Spectrometer (HETGS). Cyg X-1 was in its spectrally hard state and the
observation was performed during superior conjunction of the black hole,
allowing for the spectroscopic analysis of the accreted stellar wind along the
line of sight. A significant part of the observation covers X-ray dips as
commonly observed for Cyg X-1 at this orbital phase, however, here we only
analyze the high count rate non-dip spectrum. The full 0.5-10 keV continuum can
be described by a single model consisting of a disk, a narrow and a
relativistically broadened Fe Kalpha line, and a power law component, which is
consistent with simultaneous RXTE broad band data. We detect absorption edges
from overabundant neutral O, Ne and Fe, and absorption line series from highly
ionized ions and infer column densities and Doppler shifts. With emission lines
of He-like Mg XI, we detect two plasma components with velocities and densities
consistent with the base of the spherical wind and a focused wind. A simple
simulation of the photoionization zone suggests that large parts of the
spherical wind outside of the focused stream are completely ionized, which is
consistent with the low velocities (<200 km/s) observed in the absorption
lines, as the position of absorbers in a spherical wind at low projected
velocity is well constrained. Our observations provide input for models that
couple the wind activity of HDE 226868 to the properties of the accretion flow
onto the black hole.Comment: 16 pages, 15 figures, uses emulateapj, published as ApJ 690:330-346,
2009 January
On Estimating the High-Energy Cutoff in the X-ray Spectra of Black Holes via Reflection Spectroscopy
The fundamental parameters describing the coronal spectrum of an accreting
black hole are the slope of the power-law continuum and the energy
at which it rolls over. Remarkably, this parameter can be accurately
measured for values as high as 1 MeV by modeling the spectrum of X-rays
reflected from a black hole accretion disk at energies below 100 keV. This is
possible because the details in the reflection spectrum, rich in fluorescent
lines and other atomic features, are very sensitive to the spectral shape of
the hardest coronal radiation illuminating the disk. We show that fitting
simultaneous NuSTAR (3-79 keV) and low-energy (e.g., Suzaku) data with the most
recent version of our reflection model RELXILL, one can obtain reasonable
constraints on at energies from tens of keV up to 1 MeV, for a source
as faint as 1 mCrab in a 100 ks observation.Comment: Accepted for publication in ApJL, 6 pages, 5 figure
Multi-Satellite Observations of Cygnus X-1 to Study the Focused Wind and Absorption Dips
High-mass X-ray binary systems are powered by the stellar wind of their donor
stars. The X-ray state of Cygnus X-1 is correlated with the properties of the
wind which defines the environment of mass accretion. Chandra-HETGS
observations close to orbital phase 0 allow for an analysis of the photoionzed
stellar wind at high resolution, but because of the strong variability due to
soft X-ray absorption dips, simultaneous multi-satellite observations are
required to track and understand the continuum, too. Besides an earlier joint
Chandra and RXTE observation, we present first results from a recent campaign
which represents the best broad-band spectrum of Cyg X-1 ever achieved: On 2008
April 18/19 we observed this source with XMM-Newton, Chandra, Suzaku, RXTE,
INTEGRAL, Swift, and AGILE in X- and gamma-rays, as well as with VLA in the
radio. After superior conjunction of the black hole, we detect soft X-ray
absorption dips likely due to clumps in the focused wind covering >95 % of the
X-ray source, with column densities likely to be of several 10^23 cm^-2, which
also affect photon energies above 20 keV via Compton scattering.Comment: 10 pages, contributed talk at the 7th Microquasar Workshop, Foca,
Turkey, Sept. 1-5, 200
Andreev reflection at half-metal-superconductor interfaces with non-uniform magnetization
Andreev reflection at the interface between a half-metallic ferromagnet and a
spin-singlet superconductor is possible only if it is accompanied by a spin
flip. Here we calculate the Andreev reflection amplitudes for the case that the
spin flip originates from a spatially non-uniform magnetization direction in
the half metal. We calculate both the microscopic Andreev reflection amplitude
for a single reflection event and an effective Andreev reflection amplitude
describing the effect of multiple Andreev reflections in a ballistic thin film
geometry. It is shown that the angle and energy dependence of the Andreev
reflection amplitude strongly depends on the orientation of the gradient of the
magnetization with respect to the interface. Establishing a connection between
the scattering approach employed here and earlier work that employs the
quasiclassical formalism, we connect the symmetry properties of the Andreev
reflection amplitudes to the symmetry properties of the anomalous Green
function in the half metal.Comment: 13 pages, 4 figure
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