Constraints on the Velocity and Spatial Distribution of Helium-like Ions in the Wind of SMC X-1 from Observations with XMM-Newton/RGS

We present here X-ray spectra of the HMXB SMC X-1 obtained in an observation with the XMM observatory beginning before eclipse and ending near the end of eclipse. With the Reflection Grating Spectrometers (RGS) on board XMM, we observe emission lines from hydrogen-like and helium-like ions of nitrogen, oxygen, neon, magnesium, and silicon. Though the resolution of the RGS is sufficient to resolve the helium-like n=2->1 emission into three line components, only one of these components, the intercombination line, is detected in our data. The lack of flux in the forbidden lines of the helium-like triplets is explained by pumping by ultraviolet photons from the B0 star and, from this, we set an upper limit on the distance of the emitting ions from the star. The lack of observable flux in the resonance lines of the helium-like triplets indicate a lack of enhancement due to resonance line scattering and, from this, we derive a new observational constraint on the distribution of the wind in SMC X-1 in velocity and coordinate space. We find that the solid angle subtended by the volume containing the helium-like ions at the neutron star multiplied by the velocity dispersion of the helium-like ions must be less than 4pi steradians km/s. This constraint will be satisfied if the helium-like ions are located primarily in clumps distributed throughout the wind or in a thin layer along the surface of the B0 star.Comment: 22 pages, 17 figures, ApJ accepted, discussion of relevant other work adde

Modeling of Photoionized Plasmas

In this paper I review the motivation and current status of modeling of plasmas exposed to strong radiation fields, as it applies to the study of cosmic X-ray sources. This includes some of the astrophysical issues which can be addressed, the ingredients for the models, the current computational tools, the limitations imposed by currently available atomic data, and the validity of some of the standard assumptions. I will also discuss ideas for the future: challenges associated with future missions, opportunities presented by improved computers, and goals for atomic data collection.Comment: 17 pages, 8 figures, to appear in the proceedings of Xray2010, Utrecht, the Netherlands, March 15-17 201

X-ray reflected spectra from accretion disk models. III. A complete grid of ionized reflection calculations

We present a new and complete library of synthetic spectra for modeling the component of emission that is reflected from an illuminated accretion disk. The spectra were computed using an updated version of our code XILLVER that incorporates new routines and a richer atomic data base. We offer in the form of a table model an extensive grid of reflection models that cover a wide range of parameters. Each individual model is characterized by the photon index \Gamma of the illuminating radiation, the ionization parameter \xi at the surface of the disk (i.e., the ratio of the X-ray flux to the gas density), and the iron abundance A_{Fe} relative to the solar value. The ranges of the parameters covered are: 1.2 \leq \Gamma \leq 3.4, 1 \leq \xi \leq 10^4, and 0.5 \leq A_{Fe} \leq 10. These ranges capture the physical conditions typically inferred from observations of active galactic nuclei, and also stellar-mass black holes in the hard state. This library is intended for use when the thermal disk flux is faint compared to the incident power-law flux. The models are expected to provide an accurate description of the Fe K emission line, which is the crucial spectral feature used to measure black hole spin. A total of 720 reflection spectra are provided in a single FITS file{\url{http://hea-www.cfa.harvard.edu/~javier/xillver/}} suitable for the analysis of X-ray observations via the atable model in XSPEC. Detailed comparisons with previous reflection models illustrate the improvements incorporated in this version of XILLVER.Comment: 70 pages, 21 figures, submitted to Ap

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