884 research outputs found
A Grid of Relativistic, non-LTE Accretion Disk Models for Spectral Fitting of Black Hole Binaries
Self-consistent vertical structure models together with non-LTE radiative
transfer should produce spectra from accretion disks around black holes which
differ from multitemperature blackbodies at levels which may be observed. High
resolution, high signal-to-noise observations warrant spectral modeling which
both accounts for relativistic effects, and treats the physics of radiative
transfer in detail. In Davis et al. (2005) we presented spectral models which
accounted for non-LTE effects, Compton scattering, and the opacities due to
ions of abundant metals. Using a modification of this method, we have tabulated
spectra for black hole masses typical of Galactic binaries. We make them
publicly available for spectral fitting as an Xspec model. These models
represent the most complete realization of standard accretion disk theory to
date. Thus, they are well suited for both testing the theory's applicability to
observed systems and for constraining properties of the black holes, including
their spins.Comment: 7 pages, emulate ApJ, accepted to Ap
Spin-Wave Spectrum in `Single-Domain' Magnetic Ground State of Triangular Lattice Antiferromagnet CuFeO2
By means of neutron scattering measurements, we have investigated spin-wave
excitation in a collinear four-sublattice (4SL) magnetic ground state of a
triangular lattice antiferromagnet CuFeO2, which has been of recent interest as
a strongly frustrated magnet, a spin-lattice coupled system and a multiferroic.
To avoid mixing of spin-wave spectrum from magnetic domains having three
different orientations reflecting trigonal symmetry of the crystal structure,
we have applied uniaxial pressure on [1-10] direction of a single crystal
CuFeO2. By elastic neutron scattering measurements, we have found that only 10
MPa of the uniaxial pressure results in almost 'single domain' state in the 4SL
phase. We have thus performed inelastic neutron scattering measurements using
the single domain sample, and have identified two distinct spin- wave branches.
The dispersion relation of the upper spin-wave branch cannot be explained by
the previous theoretical model [R. S. Fishman: J. Appl. Phys. 103 (2008)
07B109]. This implies the importance of the lattice degree of freedom in the
spin-wave excitation in this system, because the previous calculation neglected
the effect of the spin-driven lattice distortion in the 4SL phase. We have also
discussed relationship between the present results and the recently discovered
"electromagnon" excitation.Comment: 5 pages, 3 figures, accepted for publication in J. Phys. Soc. Jp
A Fourier-Based Algorithm for Modelling Aberrations in HETE-2's Imaging System
The High-Energy Transient Explorer (HETE-2), launched in October 2000, is a
satellite experiment dedicated to the study of gamma-ray bursts in a very wide
energy range from soft X-ray to gamma-ray wavelengths. The intermediate X-ray
range (2-30keV) is covered by the Wide-field X-ray Monitor WXM, a coded
aperture imager. In this article, an algorithm for reconstructing the positions
of gamma-ray bursts is described, which is capable of correcting systematic
aberrations to approximately 1 arcmin throughout the field of view.
Functionality and performance of this algorithm have been validated using data
from Monte Carlo simulations as well as from astrometric observations of the
X-ray source Scorpius X-1.Comment: 14 pages, 9 figures, 2 tables; Nucl.Instr.Meth., in pres
Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients
We propose a hybrid model to fit the X-ray spectra of atoll-type X-ray
transients in the soft and hard states. This model uniquely produces luminosity
tracks that are proportional to T^4 for both the accretion disk and boundary
layer. The model also indicates low Comptonization levels for the soft state,
gaining a similarity to black holes in the relationship between Comptonization
level and the strength of integrated rms variability in the power density
spectrum. The boundary layer appears small, with a surface area that is roughly
constant across soft and hard states. This result may suggestion that the NS
radius is smaller than its inner-most stable circular orbit.Comment: 15 pages, 15 figures, accepted for publication in the Ap
Kilohertz QPO Frequency and Flux Decrease in AQL X-1 and Effect of Soft X-ray Spectral Components
We report on an RXTE/PCA observation of Aql X-1 during its outburst in March
1997 in which, immediately following a Type-I burst, the broad-band 2-10 keV
flux decreased by about 10% and the kilohertz QPO frequency decreased from
813+-3 Hz to 776+-4 Hz. This change in kHz QPO frequency is much larger than
expected from a simple extrapolation of a frequency-flux correlation
established using data before the burst. Meanwhile a very low frequency noise
(VLFN) component in the broad-band FFT power spectra with a fractional
root-mean-square (rms) amplitude of 1.2% before the burst ceased to exist after
the burst. All these changes were accompanied by a change in the energy
spectral shape. If we characterize the energy spectra with a model composed of
two blackbody (BB) components and a power law component, almost all the
decrease in flux was in the two BB components. We attribute the two BB
components to the contributions from a region very near the neutron star or
even the neutron star itself and from the accretion disk, respectively.Comment: 12 pages with 4 figures, accepted for publication in ApJ Letters,
typos corrected and references update
Boundary layer on the surface of a neutron star
In an attempt to model the accretion onto a neutron star in low-mass X-ray
binaries, we present two-dimensional hydrodynamical models of the gas flow in
close vicinity of the stellar surface. First we consider a gas pressure
dominated case, assuming that the star is non-rotating. For the stellar mass we
take M_{\rm star}=1.4 \times 10^{-2} \msun and for the gas temperature K. Our results are qualitatively different in the case of a
realistic neutron star mass and a realistic gas temperature of
K, when the radiation pressure dominates. We show that to get the stationary
solution in a latter case, the star most probably has to rotate with the
considerable velocity.Comment: 7 pages, 7 figure
GRB Energetics in the Swift Era
We examine the rest frame energetics of 76 gamma-ray bursts (GRBs) with known
redshift that were detected by the Swift spacecraft and monitored by the
satellite's X-ray Telescope (XRT). Using the bolometric fluence values
estimated in Butler et al. 2007b and the last XRT observation for each event,
we set a lower limit the their collimation corrected energy Eg and find that a
68% of our sample are at high enough redshift and/or low enough fluence to
accommodate a jet break occurring beyond the last XRT observation and still be
consistent with the pre-Swift Eg distribution for long GRBs. We find that
relatively few of the X-ray light curves for the remaining events show evidence
for late-time decay slopes that are consistent with that expected from post jet
break emission. The breaks in the X-ray light curves that do exist tend to be
shallower and occur earlier than the breaks previously observed in optical
light curves, yielding a Eg distribution that is far lower than the pre-Swift
distribution. If these early X-ray breaks are not due to jet effects, then a
small but significant fraction of our sample have lower limits to their
collimation corrected energy that place them well above the pre-Swift Eg
distribution. Either scenario would necessitate a much wider post-Swift Eg
distribution for long cosmological GRBs compared to the narrow standard energy
deduced from pre-Swift observations. We note that almost all of the pre-Swift
Eg estimates come from jet breaks detected in the optical whereas our sample is
limited entirely to X-ray wavelengths, furthering the suggestion that the
assumed achromaticity of jet breaks may not extend to high energies.Comment: 30 pages, 10 figures, Accepted to Ap
Variable-Frequency QPOs from the Galactic Microquasar GRS 1915+105
We show that the galactic microquasar GRS 1915+105 exhibits quasi-periodic
oscillations (QPOs) whose frequency varies continuously from 1-15 Hz, during
spectrally hard dips when the source is in a flaring state. We report here
analyses of simultaneous energy spectra and power density spectra at 4 s
intervals. The energy spectrum is well fit at each time step by an optically
thick accretion disk plus power law model, while the power density spectrum
consists of a varying red noise component plus the variable frequency QPO. The
features of both spectra are strongly correlated with one another. The 1-15 Hz
QPOs appear when the power law component becomes hard and intense, and
themselves have an energy spectrum consistent with the power law component
(with root mean square amplitudes as high as 10%). The frequency of the
oscillations, however, is most strikingly correlated with the parameters of the
thermal disk component. The tightest correlation is between QPO frequency and
the disk X-ray flux. This fact indicates that the properties of the QPO are not
determined by solely a disk or solely a corona.Comment: Accepted to ApJ Letters, 12 pages, 3 figures, AASTEX forma
Electric Polarization Induced by a Proper Helical Magnetic Ordering in a Delafossite Multiferroic CuFe1-xAlxO2
Multiferroic CuFe1-xAlxO2 (x=0.02) exhibits a ferroelectric ordering
accompanied by a proper helical magnetic ordering below T=7K under zero
magnetic field. By polarized neutron diffraction and pyroelectric measurements,
we have revealed a one-to-one correspondence between the spin helicity and the
direction of the spontaneous electric polarization. This result indicates that
the spin helicity of the proper helical magnetic ordering is essential for the
ferroelectricity in CuFe1-xAlxO2. The induction of the electric polarization by
the proper helical magnetic ordering is, however, cannot be explained by the
Katsura-Nagaosa-Balatsky model, which successfully explains the
ferroelectricity in the recently explored ferroelectric helimagnets, such as
TbMnO3. We thus conclude that CuFe1-xAlxO2 is a new class of magnetic
ferroelectrics.Comment: 4 pages, 4 figure
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