47 research outputs found
Momentum Transfer by Laser Ablation of Irregularly Shaped Space Debris
Proposals for ground-based laser remediation of space debris rely on the
creation of appropriately directed ablation-driven impulses to either divert
the fragment or drive it into an orbit with a perigee allowing atmospheric
capture. For a spherical fragment, the ablation impulse is a function of the
orbital parameters and the laser engagement angle. If, however, the target is
irregularly shaped and arbitrarily oriented, new impulse effects come into
play. Here we present an analysis of some of these effects.Comment: 8 pages, Proceedings of the 2010 International High-Power Laser
Ablation Conferenc
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
The Physics of Wind-Fed Accretion
We provide a brief review of the physical processes behind the radiative
driving of the winds of OB stars and the Bondi-Hoyle-Lyttleton capture and
accretion of a fraction of the stellar wind by a compact object, typically a
neutron star, in detached high-mass X-ray binaries (HMXBs). In addition, we
describe a program to develop global models of the radiatively-driven
photoionized winds and accretion flows of HMXBs, with particular attention to
the prototypical system Vela X-1. The models combine XSTAR photoionization
calculations, HULLAC emission models appropriate to X-ray photoionized plasmas,
improved models of the radiative driving of photoionized winds, FLASH
time-dependent adaptive-mesh hydrodynamics calculations, and Monte Carlo
radiation transport. We present two- and three-dimensional maps of the density,
temperature, velocity, ionization parameter, and emissivity distributions of
representative X-ray emission lines, as well as synthetic global Monte Carlo
X-ray spectra. Such models help to better constrain the properties of the winds
of HMXBs, which bear on such fundamental questions as the long-term evolution
of these binaries and the chemical enrichment of the interstellar medium.Comment: 9 pages including 5 color encapsulated postscript figures; accepted
for inclusion in the proceedings of "Cool Discs, Hot Flows: The Varying Faces
of Accreting Compact Objects," ed. M. Axelsson (New York: AIP); minor
revision which addresses the referee's comments; added Fig. 1 and removed
Fig. 3 and the associated tex
Atomic X-Ray Spectra of Accretion Disk Atmospheres in the Kerr Metric
We calculate the atmospheric structure of an accretion disk around a Kerr
black hole and obtain its X-ray spectrum, which exhibits prominent atomic
transitions under certain circumstances. The gravitational and Doppler
(red)shifts of the C V, C VI, O VII, O VIII, and Fe I-XXVI emission lines are
observable in active galaxies. We quantify the line emissivities as a function
of radius, to identify the effects of atmospheric structure, and to determine
the usefulness of these lines for probing the disk energetics. The line
emissivities do not always scale linearly with the incident radiative energy,
as in the case of Fe XXV and Fe XXVI. Our model incorporates photoionization
and thermal balance for the plasma, the hydrostatic approximation perpendicular
to the plane of the disk, and general relativistic tidal forces. We include
radiative recombination rates, fluorescence yields, Compton scattering, and
photoelectric opacities for the most abundant elements.Comment: 4 pages, 1 figure, to appear in the Proc. of the 10th Marcel
Grossmann Meeting on General Relativity, World Scientific, Rio de Janeiro,
July 20-26, 200
Fe L-Shell Density Diagnostics in Theory and Practice
We provide a discussion of the density and photoexcitation sensitivity of the
X-ray spectra of Fe L-shell ions (Fe XVII-Fe XXIV) calculated with the
Livermore X-ray Spectral Synthesizer, a suite of IDL codes that calculates
spectral models of highly charged ions based primarily on HULLAC atomic data.
These models are applicable to collisionally ionized laboratory or cosmic
plasmas with electron temperatures T_e ~ 2-45 MK (0.2-4 keV) and electron
densities n_e > 1E11 cm^{-3}. Potentially useful density diagnostics are
identified for Fe XVII and Fe XIX-Fe XXIII, with the most straightforward being
the Fe XVII I(17.10 A)/I(17.05 A) line ratio and the Fe XXII I(11.92 A)/I(11.77
A) line ratio. Applying these models to the Chandra X-ray Observatory High
Energy Transmission Grating spectrum of the intermediate polar EX Hya, we find
that the strength of all the Fe L-shell lines are consistent with electron
densities n_e > 1E14 cm^{-3}. Specifically, from the observed Fe XVII I(17.10
A)/I(17.05 A) line ratio, we infer an electron density n_e > 2E14 cm^{-3} at
the 3 sigma level, while from the observed Fe XXII I(11.92 A)/I(11.77 A) line
ratio, we infer n_e = 1.0^{+2.0}_{-0.5}E14 cm^{-3} at the 1 sigma level and n_e
> 2E13 cm^{-3} at the 3 sigma level.Comment: 11 pages including 9 encapsulated postscript figures; LaTeX format,
uses aipproc.cls, aip-6d.clo, and aipxfm.sty; to appear in the proceedings of
X-ray Diagnostics for Astrophysical Plasmas: Theory, Experiment, and
Observation, ed. R. K. Smith (Melville: AIP
High-Resolution X-Ray Spectroscopy of the Accretion Disk Corona Source 4U 1822-37
We present a preliminary analysis of the X-ray spectrum of the accretion disk
corona source, 4U 1822-37, obtained with the High Energy Transmission Grating
Spectrometer onboard the Chandra X-ray Observatory. We detect discrete emission
lines from photoionized iron, silicon, magnesium, neon, and oxygen, as well as
a bright iron fluorescence line. Phase-resolved spectroscopy suggests that the
recombination emission comes from an X-ray illuminated bulge located at the
predicted point of impact between the disk and the accretion stream. The
fluorescence emission originates in an extended region on the disk that is
illuminated by light scattered from the corona.Comment: 12 pages, 6 figures; Accepted for publication in ApJ Letter