15052426
Thermal instability and photoionized X-ray reflection in accretion disks
We study the illumination of accretion disks in the vicinity of compact objects by an overlying X-ray source. Our approach differs from previous works of the subject in that we relax the simplifying assumption of constant gas density used in these studies; instead we determine the density from hydrostatic balance which is solved simultaneously with the ionization balance and the radiative transfer in a plane-parallel geometry. We find that the self-consistent density determination makes evident the presence of a thermal ionization instability. The main effect of this instability is to prevent the illuminated gas from attaining temperatures at which the gas is unstable to thermal perturbations. In sharp contrast to the constant density calculations that predict a continuous and rather smooth variation of the gas temperature in the illuminated material, we find that the temperature profile consists of several well defined thermally stable layers. In particular, the uppermost layers of the X-ray illuminated ...
36463478
MODELING THE OXYGEN K ABSORPTION IN THE INTERSTELLAR MEDIUM: AN XMM-NEWTON VIEW OF Sco X-1
We investigate the X-ray absorption structure of oxygen in the interstellar medium by analyzing XMM-Newton observations of the low-mass X-ray binary Sco X-1. Simple models based on the O i atomic photoabsorption cross section from different sources are used to fit the data and evaluate the impact of the atomic data on the interpretation of the observations. We show that relatively small differences in the atomic calculations can yield spurious results, and that the most complete and accurate set of atomic cross sections successfully reproduce the observed data in the 21.0–24.5 A wavelength region of the spectrum. Our fits indicate that the absorption is mainly due to neutral gas with an ionization parameter of ξ = 10 −4 erg cm s −1 and an oxygen column density of NO ≈ (8–10) × 10 17 cm −2 . The models are able to reproduce both the K edge and the Kα absorption line from O i which are the two main features in this region. We find no conclusive evidence for absorption by anything other than atomic oxygen.
51898691
On the Structure of the Iron K Edge
It is shown that the commonly held view of sharp Fe K edges must be modified if the decay pathways of the resonances converging to the K thresholds are adequately taken into account. These resonances display damped Lorentzian profiles of nearly constant widths along the spectroscopic series that are smeared to impose continuity across the threshold. By modeling the effects of K damping on opacities, it is found that the broadening of the K edge grows with the ionization level of the plasma and that the appearance at high ionization of a localized absorption feature around 7.2 keV is identified as the Kβ unresolved transition array.
119097166
Dynamics of line-driven disk winds in active galactic nuclei
We present the results of axisymmetric time-dependent hydrodynamic calculations of line-driven winds from accretion disks in active galactic nuclei (AGNs). We assume the disk is flat, Keplerian, geometrically thin, and optically thick, radiating according to the ?-disk prescription. The central engine of the AGN is a source of both ionizing X-rays and wind-driving UV photons. To calculate the radiation force, we take into account radiation from the disk and the central engine. The gas temperature and ionization state in the wind are calculated self-consistently from the photoionization and heating rate of the central engine. We find that a disk accreting onto a 108 M? black hole at the rate of 1.8 M? yr-1 can launch a wind at ~1016 cm from the central engine. The X-rays from the central object are significantly attenuated by the disk atmosphere so they cannot prevent the local disk radiation from pushing matter away from the disk. However, in the supersonic portion of the flow high above the disk, the X-rays can overionize the gas and decrease the wind terminal velocity. For a reasonable X-ray opacity, e.g., ?X = 40 g-1 cm2, the disk wind can be accelerated by the central UV radiation to velocities of up to 15,000 km s-1 at a distance of ~1017 cm from the central engine. The covering factor of the disk wind is ~0.2. The wind is unsteady and consists of an opaque, slow vertical flow near the disk that is bounded on the polar side by a high-velocity stream. A typical column density through the fast stream is a few times 1023 cm-2 so the stream is optically thin to the UV radiation. This low column density is precisely why gas can be accelerated to high velocities. The fast stream contributes nearly 100% to the total wind mass-loss rate of 0.5 M? yr-1.
5037228
Photoionization and High Density Gas
We present results of calculations using the XSTAR version 2 computer code. This code is loosely based on the XSTAR version 1 code, which has been available for public use for some time. However, it represents an improvement and update in several major respects, including atomic data, code structure, user interface, and improved physical description of ionization/excitation. In particular, it now is applicable to high-density situations in which significant excited atomic level populations are likely to occur. We describe the computational techniques and assumptions and present sample runs with particular emphasis on high-density situations.
120148896
The effects of X-ray illumination on accretion disk interiors
The structure of accretion disks illuminated by X-ray from a central compact object. At radii larger than a certain value, X-ray heating can dominate the viscous heating and determine the disk structure in a standard alpha-disk model. The disk structure resulting from X-ray illumination is calculated both analytically and numerically for low-mass X-ray binaries as well as for active galactic nuclei. X-ray illumination does have important effects on the disk structures, under a variety of circumstances; instabilities may arise under significant X-ray heating. 53 refs.
14320234
Studies of EGRET sources with a novel image restoration technique
We have developed an image restoration technique based on the Richardson‐Lucy algorithm optimized for GLAST‐LAT image analysis. Our algorithm is original since it utilizes the PSF (point spread function) that is calculated for each event. This is critical for EGRET and GLAST‐LAT image analysis since the PSF depends on the energy and angle of incident gamma‐rays and varies by more than one order of magnitude. EGRET and GLAST‐LAT image analysis also faces Poisson noise due to low photon statistics. Our technique incorporates wavelet filtering to minimize noise effects. We present studies of EGRET sources using this novel image restoration technique for possible identification of extended gamma‐ray sources.