516 research outputs found
X-ray reflected spectra from accretion disk models.II. Diagnostic tools for X-ray observations
We present a comprehensive study of the emission spectra from accreting
sources. We use our new reflection code to compute the reflected spectra from
an accretion disk illuminated by X-rays. This set of models covers different
values of ionization parameter, solar iron abundance and photon index for the
illuminating spectrum. These models also include the most complete and recent
atomic data for the inner-shell of the iron and oxygen isonuclear sequences. We
concentrate our analysis to the 2-10 keV energy region, and in particular to
the iron K-shell emission lines. We show the dependency of the equivalent width
(EW) of the Fe K with the ionization parameter. The maximum value of
the EW is eV for models with log , and decreases
monotonically as increases. For lower values of the Fe K EW
decreases to a minimum near log . We produce simulated CCD
observations based on our reflection models. For low ionized, reflection
dominated cases, the 2-10 keV energy region shows a very broad, curving
continuum that cannot be represented by a simple power-law. We show that in
addition to the Fe K-shell emission, there are other prominent features such as
the Si and S L lines, a blend of Ar {\sc viii-xi} lines, and the Ca
{\sc x} K line. In some cases the S {\sc xv} blends with the He-like Si
RRC producing a broad feature that cannot be reproduced by a simple Gaussian
profile. This could be used as a signature of reflection.Comment: 34 pages, 10 figures, 1 tabl
On the role of the UV and X-ray radiation in driving a disk wind in X-ray binaries
X-ray heating of the photosphere of an accretion disk is a possible mechanism
to produce strong, broad UV emission lines in low mass X-ray binaries (LMXBs).
However, detailed photoionization calculations show that this mechanism fails
to produce sufficient emission measure. We present the results of
hydrodynamical calculations of the disk photosphere irradiated by strong
X-rays. We attempt to determine whether LMXBs can harbor significant UV-driven
disk winds despite the effects of X-ray ionization. Such winds would be a
likely candidate for the site of emission of UV lines and may better explain
the observations than the X-ray heated disk photosphere. We find that the local
disk radiation cannot launch a wind from the disk because of strong ionizing
radiation from the central object. Unphysically high X-ray opacities would be
required to shield the UV emitting disk and allow the line force to drive a
disk wind. However the same X-ray radiation that inhibits line driving heats
the disk and can produce a hot bipolar wind or corona above the disk. To assess
the impact of X-ray heating upon driving of a disk wind by the line force in
any system with an accretion disk we derive analytic formulae. In particular,
we compare results of line-driven disk wind models for accretion disks in LMXBs
and active galactic nuclei. The latter show spectral features associated with a
strong and fast disk wind. The key parameter determining the role of the line
force is not merely the presence of the luminous UV zone in the disk and the
presence of the X-rays, but also the distance of this UV zone from the center.Comment: LaTeX, 34 pages, contains color figures, to appear in Ap
A Comprehensive X-ray Absorption Model for Atomic Oxygen
An analytical formula is developed to represent accurately the
photoabsorption cross section of O I for all energies of interest in X-ray
spectral modeling. In the vicinity of the Kedge, a Rydberg series expression is
used to fit R-matrix results, including important orbital relaxation effects,
that accurately predict the absorption oscillator strengths below threshold and
merge consistently and continuously to the above-threshold cross section.
Further minor adjustments are made to the threshold energies in order to
reliably align the atomic Rydberg resonances after consideration of both
experimental and observed line positions. At energies far below or above the
K-edge region, the formulation is based on both outer- and inner-shell direct
photoionization, including significant shake-up and shake-off processes that
result in photoionization-excitation and double photoionization contributions
to the total cross section. The ultimate purpose for developing a definitive
model for oxygen absorption is to resolve standing discrepancies between the
astronomically observed and laboratory measured line positions, and between the
inferred atomic and molecular oxygen abundances in the interstellar medium from
XSTAR and SPEX spectral models
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Real-Time Ellipsometry-Based Transmission Ultrasound Imaging
Ultrasonic imaging is a valuable tool for non-destructive evaluation and medical diagnosis. Reflection mode is exclusively used for medical imaging, and is most frequently used for nondestructive evaluation (NDE) because of the relative speed of acquisition. Reflection mode imaging is qualitative, yielding little information about material properties, and usually only about material interfaces. Transmission imaging can be used in 3D reconstructions to yield quantitative information: sound speed and attenuation. Unfortunately, traditional scanning methods of acquiring transmission data are very slow, requiring on the order of 20 minutes per image. The sensing of acoustic pressure fields as optical images can significantly speed data acquisition. An entire 2D acoustic pressure field can be acquired in under a second. The speed of data acquisition for a 2D view makes it feasible to obtain multiple views of an object. With multiple views, 3D reconstruction becomes possible. A fast, compact (no big magnets or accelerators), inexpensive, 3D imaging technology that uses no ionizing radiation could be a boon to the NDE and medical communities. 2D transmission images could be examined in real time to give the ultrasonic equivalent of a fluoroscope, or accumulated in such a way as to acquire phase and amplitude data over multiple views for 3D reconstruction (for breast cancer imaging, for example). Composite panels produced for the aircraft and automobile industries could be inspected in near real time, and inspection of attenuating materials such as ceramics and high explosives would be possible. There are currently three optical-readout imaging transmission ultrasound technologies available. One is based on frustrated total internal reflection (FTIR) [1,2], one on Fabry-Perot interferometry [3], and another on critical angle modulation [4]. Each of these techniques has its problems. The FTIR based system cannot currently be scaled to large aperture sizes, the Fabry-Perot system has never been fully implemented for area imaging, and the critical angle modulation system is not sensitive enough for medical imaging. We proposed an entirely new way of using acoustic pressure to modulate a light beam. This new technology should be sensitive enough to be useful for medical imaging and have a large enough aperture to speed acquisition by orders of magnitude over point sampling. Unfortunately, we were unable to bring this technology to fruition
The Suzaku view of 3C 382
We present a long Suzaku observation of 3C 382. A Swift BAT spectrum from the
58-month survey is also analyzed, together with an archival XMM-Newton EPIC
exposure. Our main result is the finding with Suzaku of a broad FeK line with a
relativistic profile consistent with emission from an accretion disk at tens of
gravitational radii from the central black hole. The XIS data indicate emission
from highly ionized iron and allow us to set tight, albeit model-dependent,
constraints on the inner and outer radii of the disk reflecting region,
r_in~10r_g and r_out~20r_g, respectively, and on the disk inclination, i~30deg.
Two ionized reflection components are possibly observed, with similar
contributions of ~10% to the total continuum. A highly ionized one, with
log_xi~3, which successfully models the relativistic line and a mildly ionized
one, with log_xi~1.5, which models the narrow Fe K line and high energy hump.
When both these components are included, there is no further requirement for an
additional black body soft excess below 2keV. The Suzaku data confirm the
presence of a warm absorber previously known from grating studies. After
accounting for all the spectral features, the intrinsic photon index of the
X-ray continuum is ~1.8 with a cutoff energy at ~200keV, consistent with
Comptonization models and excluding jet-related emission up to these energies.
Comparison of the X-ray properties of 3C 382 and other BLRGs to Seyferts
confirms the idea that the distinction between radio-loud and radio-quiet AGN
at X-rays is blurred.Comment: Accepted for publication in Ap
Carbon X-ray absorption in the local ISM: Fingerprintsin X-ray Novae spectra
We present a study of the C K-edge using high-resolution LETGS Chandraspectra of four novae during their super-soft-source (SSS) phase. We identified absorption lines due to C II Kα, C III Kα, and C III Kβ resonances. We used these astronomical observations to perform a benchmarking of the atomic data, which involves wavelength shifts of the resonances and photoionization cross-sections. We used improved atomic data to estimate the C II and C III column densities. The absence of physical shifts for the absorption lines, the consistence of the column densities between multiple observations, and the high temperature required for the SSS nova atmosphere modelling support our conclusion about an interstellar medium origin of the respective absorption lines. Assuming a collisional ionization equilibrium plasma the maximum temperature derived from the ratio of C II/C III column densities of the absorbers correspond to T_(max) < 3.05 × 10^4 K
Double-Peaked X-Ray Lines from the Oxygen/Neon-Rich Accretion Disk in 4U1626-67
We report on a 39 ks observation of the 7.7-s low-mass X-ray binary pulsar
4U1626-67 with the High Energy Transmission Grating Spectrometer (HETGS) on the
Chandra X-Ray Observatory. This ultracompact system consists of a
disk-accreting magnetic neutron star and a very low mass, hydrogen-depleted
companion in a 42-min binary. We have resolved the previously reported Ne/O
emission line complex near 1 keV into Doppler pairs of broadened (2500 km/s
FWHM) lines from highly ionized Ne and O. In most cases, the blue and red line
components are of comparable strength, with blueshifts of 1550-2610 km/s and
redshifts of 770-1900 km/s. The lines appear to originate in hot (10^6 K),
dense material just below the X-ray-heated skin of the outer Keplerian
accretion disk, or else possibly in a disk wind driven from the pulsar's
magnetopause. The observed photoelectric absorption edges of Ne and O appear
nearly an order of magnitude stronger than expected from interstellar material
and are likely formed in cool, metal-rich material local to the source. Based
on the inferred local abundance ratios, we argue that the mass donor in this
binary is probably the 0.02 M_sun chemically fractionated core of a C-O-Ne or
O-Ne-Mg white dwarf which has previously crystallized.Comment: 9 pages. Accepted for publication in ApJ. Table 2 correcte
Resonance line-profile calculations based on hydrodynamical models of cataclysmic variable winds
We present synthetic line profiles as predicted by the models of 2-D line-
driven disk winds due to Proga, Stone & Drew. We compare the model line
profiles with HST observations of the cataclysmic variable IX Vel. The model
wind consists of a slow outflow that is bounded on the polar side by a fast
stream. We find that these two components of the wind produce distinct spectral
features. The fast stream produces profiles which show features consistent with
observations. These include the appearance of the P-Cygni shape for a range of
inclinations, the location of the maximum depth of the absorption component at
velocities less than the terminal velocity, and the transition from absorption
to emission with increasing inclination. However the model profiles have too
little absorption or emission equivalent width. This quantitative difference
between our models and observations is not a surprise because the line-driven
wind models predict a mass loss rate that is lower than the rate required by
the observations. We note that the model profiles exhibit a double-humped
structure near the line center which is not echoed in observations. We identify
this structure with a non-negligible redshifted absorption which is formed in
the slow component of the wind where the rotational velocity dominates over
expansion velocity. We conclude that the next generation of disk wind models,
developed for application to CVs, needs to yield stronger wind driving out to
larger disk radii than do the present models.Comment: LaTeX, 19 pages, to appear in Ap
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