15,763 research outputs found
The Extended Power Law as Intrinsic Signature For a Black Hole
We analyze the exact general relativistic exact integro-differential equation
of radiative transfer describing the interaction of low energy photons with a
Maxwellian distribution of hot electrons in gravitational field of a
Schwarzschild black hole. We prove that due to Comptonization an initial
arbitrary spectrum of low energy photons unavoidably results in spectra
characterized by an extended power-law feature. We examine the spectral index
by using both analytical and numerical methods for a variety of physical
parameters as such the plasma temperature and the mass accretion rate. The
presence of the event horizon as well as the behaviour of the null geodesics in
its vicinity largely determine the dependence of the spectral index on the flow
parameters. We come to the conclusion that the bulk motion of a converging flow
is more efficient in upscattering photons than thermal Comptonization provided
that the electron temperature in the flow is of order of a few keV or less. In
this case, the spectrum observed at infinity consists of a soft component
produced by those input photons that escape after a few scatterings without any
significant energy change and of hard component (described by a power law)
produced by the photons that underwent significant upscattering. The luminosity
of the power-law component is relatively small compared to that of the soft
component. For accretion into black hole the spectral energy index of the
power-law is always higher than one for plasma temperature of order of a few
keV. This result suggests that the bulk motion Comptonization might be
responsible for the power-law spectra seen in the black-hole X-ray sources.Comment: 31 pages, 3 figures; Astrophysical Journal accepte
The use of imaging systems to monitor shoreline dynamics
The development of imaging systems is nowadays established as one of the most powerful and reliable tools for monitoring beach morphodynamics. Two different techniques for shoreline detection are presented here and, in one case, applied to the study of beach width oscillations on a sandy beach (Pauanui Beach, New Zealand). Results indicate that images can provide datasets whose length and sample interval are accurate enough to resolve inter-annual and seasonal oscillations, and long-term trends. Similarly, imaging systems can be extremely useful in determining the statistics of rip current occurrence. Further improvements in accuracy and reliability are expected with the recent introduction of digital systems
Compton scattering in a converging fluid flow - I. The transfer equation
The equations describing Compton scattering in an optically thick fluid flow are derived under the diffusion approximation. The relative importance of the bulk and random motions of the scattering electrons is discussed. In a converging fluid flow of speed u, bulk acceleration of photons dominates thermal Comptonization when u âł (12kT_e/m_e)^(1/2)â
Compton scattering in a converging fluid flow - III Spherical supercritical accretion
Radiative transfer in spherical, supercritical accretion on to a massive black hole is considered. Particular emphasis is placed on the case of non-adiabatic flow in which electron scattering in the converging flow is the dominant source of opacity and photon heating. In escaping diffusively, the photons, which are produced mainly at the trapping radius, undergo âŒ(c/u_(tr))^2 scatterings (where utr is the velocity of the flow at the trapping radius), each one giving a secular fractional energy increase âŒ(u_(tr)/c)^2 and a total average increase of order unity. The emitted radiation spectrum will be a superposition of the locally produced spectra at low frequencies and a power-law at high frequencies. For gas accreting radially with the free-fall speed, the spectral index observed in the outflowing radiation is α~2. Both thermal and non-thermal emission processes are discussed. The conditions under which bulk heating of the photons is important are specified
Hydromagnetic flows from accretion discs and the production of radio jets
We examine the possibility that energy and angular momentum are removed magnetically from accretion discs, by field lines that leave the disc surface and extend to large distances. We illustrate this mechanism by solving the equations of magnetohydrodynamics, assuming infinite conductivity, for axially symmetric, self-similar, cold magnetospheric flow from a Keplerian accretion disc in which the field strength B scales with radius r as B âr^(-5/4). We show that a centrifugally driven outflow of matter from the disc is possible, if the poloidal component of the magnetic field makes an angle of less than 60° with the disc surface. At large distances from the disc, the toroidal component of the magnetic field becomes important and collimates the outflow into a pair of anti-parallel jets moving perpendicular to the disc. Close to the disc, the flow is probably driven by gas pressure in a hot magnetically dominated corona. In this way, magnetic stresses can extract the angular momentum from a thin accretion disc and thus enable matter to be accreted, independently of the presence of viscosity. These jet solutions have the property that most of the power is concentrated within a central core, while most of the angular momentum and magnetic flux is carried near the jet walls. The relevance of this mechanism for the evolution of accretion discs around massive black holes in galactic nuclei and the production of jets in extragalactic radio sources is described
Compton scattering in a converging fluid flow - II. Radiation-dominated shock
The problem of Compton scattering in an optically thick fluid flow in which bulk motion is the dominant source of photon heating is illustrated by analysing a radiation-dominated, plane-parallel shock of speed u with photon to electron ratio greatly exceeding âŒ(m_p/m_e)â . In traversing the shock (of thickness âŒ(c/u) Thomson optical depths), a typical photon experiences (c/u)2 scatterings, each one giving a secular fractional energy increase âŒ(u/c)^2 and a total average increase of order unity. In a converging fluid flow, an exponentially small number of photons are accelerated to an exponentially large energy. Thus, a power-law spectrum will be transmitted at high frequencies. For a shock of Mach number M, bulk acceleration produces a spectral index α = (M^2 â Âœ)(M^2 + 6)(M^2 â 1)^(â2), which tends to unity for a strong shock. The applicability of these results to quasars and the microwave background is briefly discussed
Dynamic range and mass accuracy of wide-scan direct infusion nanoelectrospray fourier transform ion cyclotron resonance mass spectrometry-based metabolomics increased by the spectral stitching method
Direct infusion nanoelectrospray Fourier transform ion cyclotron resonance mass spectrometry (DI nESI FT-ICR MS)offers high mass accuracy and resolution for analyzing complex metabolite mixtures. High dynamic range across a wide mass range, however, can only be achieved at the expense of mass accuracy, since the large numbers of ions entering the ICR detector induce adverse spacecharge effects. Here we report an optimized strategy for wide-scan DI nESI FT-ICR MS that increases dynamic range but maintains high mass accuracy. It comprises the collection if multiple adjacent selected ion monitoring (SIM) windows that are stitched together using novel algorithms. The final SIM-stitching method, derived from several optimization experiments, comprises 21 adjoining SIM windows each of width m/z 30 (from m/z 70 to 500; adjacent windows overlap by m/z 10) with an automated gain control (AGC) target of 1 105 charges. SIMstitching and wide-scan range (WSR; Thermo Electron)were compared using a defined standard to assess mass accuracy and a liver extract to assess peak count and dynamic range. SIM-stitching decreased the maximum mass error by 1.3- and 4.3-fold, and increased the peak count by 5.3- and 1.8-fold, versus WSR (AGC targets of 1 x 105 and 5 x 105, respectively). SIM-stitching achieved an rms mass error of 0.18 ppm and detected over 3000 peaks in liver extract. This novel approach increases metabolome coverage, has very high mass accuracy, and at 5.5 min/sample is conducive for high- throughput metabolomics
X-ray Spectral Formation in a Converging Fluid Flow: Spherical Accretion into Black Holes
We study Compton upscattering of low-frequency photons in a converging flow
of thermal plasma. The photons escape diffusively and electron scattering is
the dominant source of opacity. We solve numerically and approximately
analytically the equation of radiative transfer in the case of spherical,
steady state accretion into black holes. Unlike previous work on this subject,
we consider the inner boundary at a finite radius and this has a significant
effect on the emergent spectrum. It is shown that the bulk motion of the
converging flow is more efficient in upscattering photons than thermal
Comptonization, provided that the electron temperature in the flow is of order
a few keV or less. In this case, the spectrum observed at infinity consists of
a soft component coming from those input photons which escaped after a few
scatterings without any significant energy change and of a power law which
extends to high energies and is made of those photons which underwent
significant upscattering. The luminosity of the power law is relatively small
compared to that of the soft component. The more reflective the inner boundary
is, the flatter the power-law spectrum becomes. The spectral energy power-law
index for black-hole accretion is always higher than 1 and it is approximately
1.5 for high accretion rates. This result tempts us to say that bulk motion
Comptonization might be the mechanism behind the power-law spectra seen in
black-hole X-ray sources.Comment: 37 pages, LaTex, AAS Macros, 8 ps figures, to appear in Ap
Nuclear Corrections to Hyperfine Structure in Light Hydrogenic Atoms
Hyperfine intervals in light hydrogenic atoms and ions are among the most
accurately measured quantities in physics. The theory of QED corrections has
recently advanced to the point that uncalculated terms for hydrogenic atoms and
ions are probably smaller than 0.1 parts per million (ppm), and the experiments
are even more accurate. The difference of the experiments and QED theory is
interpreted as the effect on the hyperfine interaction of the (finite) nuclear
charge and magnetization distributions, and this difference varies from tens to
hundreds of ppm. We have calculated the dominant component of the 1s hyperfine
interval for deuterium, tritium and singly ionized helium, using modern
second-generation potentials to compute the nuclear component of the hyperfine
splitting for the deuteron and the trinucleon systems. The calculated nuclear
corrections are within 3% of the experimental values for deuterium and tritium,
but are about 20% discrepant for singly ionized helium. The nuclear corrections
for the trinucleon systems can be qualitatively understood by invoking SU(4)
symmetry.Comment: 26 pages, 1 figure, latex - submitted to Physical Review
Ensemble density-functional theory for ab-initio molecular dynamics of metals and finite-temperature insulators
A new method is presented for performing first-principles molecular-dynamics
simulations of systems with variable occupancies. We adopt a matrix
representation for the one-particle statistical operator Gamma, to introduce a
``projected'' free energy functional G that depends on the Kohn-Sham orbitals
only and that is invariant under their unitary transformations. The Liouville
equation [ Gamma , H ] = 0 is always satisfied, guaranteeing a very efficient
and stable variational minimization algorithm that can be extended to
non-conventional entropic formulations or fictitious thermal distributions.Comment: 5 pages, two-column style with 2 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#nm_meta
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