92,910 research outputs found
Resonant Inverse Compton Scattering Spectra from Highly-magnetized Neutron Stars
Hard, non-thermal, persistent pulsed X-ray emission extending between 10 keV
and keV has been observed in nearly ten magnetars. For
inner-magnetospheric models of such emission, resonant inverse Compton
scattering of soft thermal photons by ultra-relativistic charges is the most
efficient production mechanism. We present angle-dependent upscattering spectra
and pulsed intensity maps for uncooled, relativistic electrons injected in
inner regions of magnetar magnetospheres, calculated using collisional
integrals over field loops. Our computations employ a new formulation of the
QED Compton scattering cross section in strong magnetic fields that is
physically correct for treating important spin-dependent effects in the
cyclotron resonance, thereby producing correct photon spectra. The spectral
cut-off energies are sensitive to the choices of observer viewing geometry,
electron Lorentz factor, and scattering kinematics. We find that electrons with
energies MeV will emit most of their radiation below 250 keV,
consistent with inferred turnovers for magnetar hard X-ray tails. More
energetic electrons still emit mostly below 1 MeV, except for viewing
perspectives sampling field line tangents. Pulse profiles may be singly- or
doubly-peaked dependent upon viewing geometry, emission locale, and observed
energy band. Magnetic pair production and photon splitting will attenuate
spectra to hard X-ray energies, suppressing signals in the Fermi-LAT band. The
resonant Compton spectra are strongly polarized, suggesting that hard X-ray
polarimetry instruments such as X-Calibur, or a future Compton telescope, can
prove central to constraining model geometry and physics.Comment: 43 pages, 12 figures; accepted for publication in ApJ; v3 fixes typos
and updates some reference
The effect of realistic geometries on the susceptibility-weighted MR signal in white matter
Purpose: To investigate the effect of realistic microstructural geometry on
the susceptibility-weighted magnetic resonance (MR) signal in white matter
(WM), with application to demyelination.
Methods: Previous work has modeled susceptibility-weighted signals under the
assumption that axons are cylindrical. In this work, we explore the
implications of this assumption by considering the effect of more realistic
geometries. A three-compartment WM model incorporating relevant properties
based on literature was used to predict the MR signal. Myelinated axons were
modeled with several cross-sectional geometries of increasing realism: nested
circles, warped/elliptical circles and measured axonal geometries from electron
micrographs. Signal simulations from the different microstructural geometries
were compared to measured signals from a Cuprizone mouse model with varying
degrees of demyelination.
Results: Results from simulation suggest that axonal geometry affects the MR
signal. Predictions with realistic models were significantly different compared
to circular models under the same microstructural tissue properties, for
simulations with and without diffusion.
Conclusion: The geometry of axons affects the MR signal significantly.
Literature estimates of myelin susceptibility, which are based on fitting
biophysical models to the MR signal, are likely to be biased by the assumed
geometry, as will any derived microstructural properties.Comment: Accepted March 4 2017, in publication at Magnetic Resonance in
Medicin
Controllers for imposing continuum-to-molecular boundary conditions in arbitrary fluid flow geometries
We present a new parallelised controller for steering an arbitrary geometric region of a molecular dynamics (MD) simulation towards a desired thermodynamic and hydrodynamic state. We show that the controllers may be applied anywhere in the domain to set accurately an initial MD state, or solely at boundary regions to prescribe non-periodic boundary conditions (PBCs) in MD simulations. The mean molecular structure and velocity autocorrelation function remain unchanged (when sampled a few molecular diameters away from the constrained region) when compared with those distributions measured using PBCs. To demonstrate the capability of our new controllers, we apply them as non-PBCs in parallel to a complex MD mixing nano-channel and in a hybrid MD continuum simulation with a complex coupling region. The controller methodology is easily extendable to polyatomic MD fluids
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