463 research outputs found
Efficient algorithm for current spectral density calculation in single-electron tunneling and hopping
This write-up describes an efficient numerical method for the Monte Carlo
calculation of the spectral density of current in the multi-junction
single-electron devices and hopping structures. In future we plan to expand
this write-up into a full-size paper.Comment: 4 page
Multifrequency Observations of Giant Radio Pulses from the Millisecond Pulsar B1937+21
Giant pulses are short, intense outbursts of radio emission with a power-law
intensity distribution that have been observed from the Crab Pulsar and PSR
B1937+21. We have undertaken a systematic study of giant pulses from PSR
B1937+21 using the Arecibo telescope at 430, 1420, and 2380 MHz. At 430 MHz,
interstellar scattering broadens giant pulses to durations of secs,
but at higher frequencies the pulses are very short, typically lasting only
-secs. At each frequency, giant pulses are emitted only in narrow
(\lsim10 \mus) windows of pulse phase located -sec after the
main and interpulse peaks. Although some pulse-to-pulse jitter in arrival times
is observed, the mean arrival phase appears stable; a timing analysis of the
giant pulses yields precision competitive with the best average profile timing
studies. We have measured the intensity distribution of the giant pulses,
confirming a roughly power-law distribution with approximate index of -1.8,
contributing \gsim0.1% to the total flux at each frequency. We also find that
the intensity of giant pulses falls off with a slightly steeper power of
frequency than the ordinary radio emission.Comment: 21 pages, 10 Postscript figures; LaTeX with aaspp4.sty and epsf.tex;
submitted to Ap
A Numerical Study of Transport and Shot Noise at 2D Hopping
We have used modern supercomputer facilities to carry out extensive Monte
Carlo simulations of 2D hopping (at negligible Coulomb interaction) in
conductors with the completely random distribution of localized sites in both
space and energy, within a broad range of the applied electric field and
temperature , both within and beyond the variable-range hopping region. The
calculated properties include not only dc current and statistics of localized
site occupation and hop lengths, but also the current fluctuation spectrum.
Within the calculation accuracy, the model does not exhibit noise, so
that the low-frequency noise at low temperatures may be characterized by the
Fano factor . For sufficiently large samples, scales with conductor
length as , where , and
parameter is interpreted as the average percolation cluster length. At
relatively low , the electric field dependence of parameter is
compatible with the law which follows from directed
percolation theory arguments.Comment: 17 pages, 8 figures; Fixed minor typos and updated reference
A Numerical Study of Coulomb Interaction Effects on 2D Hopping Transport
We have extended our supercomputer-enabled Monte Carlo simulations of hopping
transport in completely disordered 2D conductors to the case of substantial
electron-electron Coulomb interaction. Such interaction may not only suppress
the average value of hopping current, but also affect its fluctuations rather
substantially. In particular, the spectral density of current
fluctuations exhibits, at sufficiently low frequencies, a -like increase
which approximately follows the Hooge scaling, even at vanishing temperature.
At higher , there is a crossover to a broad range of frequencies in which
is nearly constant, hence allowing characterization of the current
noise by the effective Fano factor F\equiv S_I(f)/2e \left. For
sufficiently large conductor samples and low temperatures, the Fano factor is
suppressed below the Schottky value (F=1), scaling with the length of the
conductor as . The exponent is significantly
affected by the Coulomb interaction effects, changing from when such effects are negligible to virtually unity when they are
substantial. The scaling parameter , interpreted as the average
percolation cluster length along the electric field direction, scales as when Coulomb interaction effects are negligible
and when such effects are substantial, in
good agreement with estimates based on the theory of directed percolation.Comment: 19 pages, 7 figures. Fixed minor typos and updated reference
Sub-electron Charge Relaxation via 2D Hopping Conductors
We have extended Monte Carlo simulations of hopping transport in completely
disordered 2D conductors to the process of external charge relaxation. In this
situation, a conductor of area shunts an external capacitor
with initial charge . At low temperatures, the charge relaxation process
stops at some "residual" charge value corresponding to the effective threshold
of the Coulomb blockade of hopping. We have calculated the r.m.s value
of the residual charge for a statistical ensemble of capacitor-shunting
conductors with random distribution of localized sites in space and energy and
random , as a function of macroscopic parameters of the system. Rather
unexpectedly, has turned out to depend only on some parameter
combination: for negligible Coulomb interaction
and for substantial interaction. (Here
is the seed density of localized states, while is the
dielectric constant.) For sufficiently large conductors, both functions
follow the power law , but with different
exponents: for negligible and
for significant Coulomb interaction. We have been able to derive this law
analytically for the former (most practical) case, and also explain the scaling
(but not the exact value of the exponent) for the latter case. In conclusion,
we discuss possible applications of the sub-electron charge transfer for
"grounding" random background charge in single-electron devices.Comment: 12 pages, 5 figures. In addition to fixing minor typos and updating
references, the discussion has been changed and expande
An axisymmetric hydrodynamical model for the torus wind in AGN. III: Spectra from 3D radiation transfer calculations
We calculate a series of synthetic X-ray spectra from outflows originating
from the obscuring torus in active galactic nuclei (AGN). Such modeling
includes 2.5D hydrodynamical simulations of an X-ray excited torus wind,
including the effects of X-ray heating, ionization, and radiation pressure. 3D
radiation transfer calculations are performed in the 3D Sobolev approximation.
Synthetic X-ray line spectra and individual profiles of several strong lines
are shown at different inclination angles, observing times, and for different
characteristics of the torus.
Our calculations show that rich synthetic warm absorber spectra from 3D
modeling are typically observed at a larger range of inclinations than was
previously inferred from simple analysis of the transmitted spectra. In
general, our results are supportive of warm absorber models based on the
hypothesis of an "X-ray excited funnel flow" and are consistent with
characteristics of such flows inferred from observations of warm absorbers from
Seyfert 1 galaxies.Comment: 31 pages, 10 figure
The Soft X-ray Spectrum from NGC 1068 Observed with LETGS on Chandra
Using the combined spectral and spatial resolving power of the Low Energy
Transmission Grating (LETGS) on board Chandra, we obtain separate spectra from
the bright central source of NGC 1068 (Primary region), and from a fainter
bright spot 4" to the NE (Secondary region). Both spectra are dominated by line
emission from H- and He-like ions of C through S, and from Fe L-shell ions, but
also include narrow radiative recombination continua, indicating that most of
the soft X-ray emission arises in low-temperature (kT few eV) photoionized
plasma. We confirm the conclusions of Kinkhabwala et al. (2002), based on
XMM-Newton RGS observations, that the entire nuclear spectrum can be explained
by recombination/radiative cascade following photoionization, and radiative
decay following photoexcitation, with no evidence for hot, collisionally
ionized plasma. In addition, this model also provides an excellent fit to the
spectrum of the Secondary region, albeit with radial column densities a factor
of three lower, as would be expected given its distance from the source of the
ionizing continuum. The remarkable overlap and kinematical agreement of the
optical and X-ray line emission, coupled with the need for a distribution of
ionization parameter to explain the X-ray spectra, collectively imply the
presence of a distribution of densities (over a few orders of magnitude) at
each radius in the ionization cone. Relative abundances of all elements are
consistent with Solar abundance, except for N, which is 2-3 times Solar. The
long wavelength spectrum beyond 30 A is rich of L-shell transitions of Mg, Si,
S, and Ar, and M-shell transitions of Fe. The velocity dispersion decreases
with increasing ionization parameter, as deduced from these long wavelength
lines and the Fe-L shell lines.Comment: 12 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
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