2,082 research outputs found
Efficient computation of high index Sturm-Liouville eigenvalues for problems in physics
Finding the eigenvalues of a Sturm-Liouville problem can be a computationally
challenging task, especially when a large set of eigenvalues is computed, or
just when particularly large eigenvalues are sought. This is a consequence of
the highly oscillatory behaviour of the solutions corresponding to high
eigenvalues, which forces a naive integrator to take increasingly smaller
steps. We will discuss some techniques that yield uniform approximation over
the whole eigenvalue spectrum and can take large steps even for high
eigenvalues. In particular, we will focus on methods based on coefficient
approximation which replace the coefficient functions of the Sturm-Liouville
problem by simpler approximations and then solve the approximating problem. The
use of (modified) Magnus or Neumann integrators allows to extend the
coefficient approximation idea to higher order methods
Average output entropy for quantum channels
We study the regularized average Renyi output entropy \bar{S}_{r}^{\reg} of
quantum channels. This quantity gives information about the average noisiness
of the channel output arising from a typical, highly entangled input state in
the limit of infinite dimensions. We find a closed expression for
\beta_{r}^{\reg}, a quantity which we conjecture to be equal to \Srreg. We
find an explicit form for \beta_{r}^{\reg} for some entanglement-breaking
channels, and also for the qubit depolarizing channel as a
function of the parameter . We prove equality of the two quantities in
some cases, in particular we conclude that for both are
non-analytic functions of the variable .Comment: 32 pages, several plots and figures; positivity condition added for
Theorem on entanglement breaking channels; new result for entrywise positive
channel
Partial covering of emission regions of Q 0528-250 by intervening H clouds
We present an analysis of the molecular hydrogen absorption system at z = 2.811 in the spectrum of the blazar Q0528-250. We demonstrate that the
molecular cloud does not cover the background source completely. The partial
coverage reveals itself as a residual flux in the bottom of saturated H_2
absorption lines. This amounts to about (2.220.54)% of the continuum and
does not depend on the wavelength. This value is small and it explains why this
effect has not been detected in previous studies of this quasar spectrum.
However, it is robustly detected and significantly higher than the zero flux
level in the bottom of saturated lines of the Ly-alpha forest,
(-0.210.22)%. The presence of the residual flux could be caused by
unresolved quasar multicomponents, by light scattered by dust, and/or by
jet-cloud interaction. The H absorption system is very well described by a
two-component model without inclusion of additional components when we take
partial coverage into account. The derived total column densities in the H
absorption components A and B are logN(H)[cm] = 18.100.02 and
17.820.02, respectively. HD molecules are present only in component B.
Given the column density, logN(HD)= 13.330.02, we find
N(HD)/2N(H)=(1.480.10)x10, significantly lower than previous
estimations. We argue that it is crucial to take into account partial coverage
effects for any analysis of H bearing absorption systems, in particular
when studying the physical state of high-redshift interstellar medium.Comment: Accepted for MNRA
Neutral chlorine and molecular hydrogen at high redshift
Chlorine and molecular hydrogen are known to be tightly linked together in
the cold phase of the local interstellar medium through rapid chemical
reactions. We present here the first systematic study of this relation at high
redshifts using H-bearing damped Ly systems (DLAs) detected along
quasar lines of sight. Using high-resolution spectroscopic data from VLT/UVES
and Keck/HIRES, we report the detection of ClI in 9 DLAs (including 5 new
detections) out of 18 high- DLAs with Hcm
(including a new H detection at towards J21000641) and
present upper limits for the remaining 9 systems. We find a 5
correlation between (ClI) and (H) with only 0.2dex
dispersion over the range 18.1log(H)20.1, thus probing
column densities 10 times lower those seen towards nearby stars, roughly
following the relation (ClIH.
This relation between column densities is surprisingly the same at low and high
redshift suggesting that the physical and chemical conditions are similar for a
given H (or ClI) column density. In turn, the N({Cl\,I})/N({\rm
H_2}) ratio is found to be uncorrelated with the overall metallicity in the
DLA. Our results confirm that neutral chlorine is an excellent tracer of
molecule-rich gas and show that the molecular fraction or/and metallicity in
the H-bearing component of DLA could possibly be much higher than the
line-of-sight average values usually measured in DLAs.Comment: 5 pages, 3 figures, Accepted for publication in A&A Letter
CO-dark molecular gas at high redshift: very large H content and high pressure in a low metallicity damped Lyman-alpha system
We present a detailed analysis of a H-rich, extremely strong intervening
Damped Ly- Absorption system (DLA) at towards the
quasar J0843+0221, observed with the Ultraviolet and Visual Echelle
Spectrograph on the Very Large Telescope. The total column density of molecular
(resp. atomic) hydrogen is (H)= (resp. (HI)=), making it to be the first case in quasar absorption
lines studies with H column density as high as what is seen in
CO-selected clouds in the Milky-Way.
We find that this system has one of the lowest metallicity detected among
H-bearing DLAs, with . This can be the
reason for the marked differences compared to systems with similar H column
densities in the local Universe: the kinetic temperature, 120~K,
derived from the H rotational levels is at least twice higher than
expected; there is little dust extinction with A; no
CO molecules are detected, putting a constraint on the factor
cm/(km/s\,K), in the very low metallicity
gas. Low CO and high H contents indicate that this system represents
"CO-dark/faint" gas.
We investigate the physical conditions in the H-bearing gas using the
fine-structure levels of CI, CII, SiII and the rotational levels of
HD and H. We find the number density to be about cm, implying a high thermal pressure of cmK. We further identify a trend of increasing pressure with
increasing total hydrogen column density. This independently supports the
suggestion that extremely strong DLAs (with N(H) ) probe
high-z galaxies at low impact parameters.Comment: 21 pages, 21 figures. Accepted for publication in MNRA
Intervening Metal Systems in GRB and QSO sight-lines: The Mgii and Civ Question
Prochter et al. 2006 recently found that the number density of strong
intervening 0.5<z<2 MgII absorbers detected in gamma-ray burst (GRB) afterglow
spectra is nearly 4 times larger than in QSO spectra. We have conducted a
similar study using CIV absorbers. Our CIV sample, consisting of a total of 20
systems, is drawn from 3 high resolution and high to moderate S/N VLT/UVES
spectra of 3 long-duration GRB afterglows, covering the redshift interval 1.6<
z<3.1. The column density distribution and number density of this sample do not
show any statistical difference with the same quantities measured in QSO
spectra. We discuss several possibilities for the discrepancy between CIV and
MgII absorbers and conclude that a higher dust extinction in the MgII QSO
samples studied up to now would give the most straightforward solution.
However, this effect is only important for the strong MgII absorbers.
Regardless of the reasons for this discrepancy, this result confirms once more
that GRBs can be used to detect a side of the universe that was unknown before,
not necessarily connected with GRBs themselves, providing an alternative and
fundamental investigative tool of the cosmic evolution of the universe.Comment: 21 pages, 4 figures, ApJ accepted, Revised after Referee Repor
Phase space measure concentration for an ideal gas
We point out that a special case of an ideal gas exhibits concentration of
the volume of its phase space, which is a sphere, around its equator in the
thermodynamic limit. The rate of approach to the thermodynamic limit is
determined. Our argument relies on the spherical isoperimetric inequality of
L\'{e}vy and Gromov.Comment: 15 pages, No figures, Accepted by Modern Physics Letters
Silicon nanoparticles and interstellar extinction
To examine a recently proposed hypothesis that silicon nanoparticles are the
source of extended red emission (ERE) in the interstellar medium, we performed
a detailed modeling of the mean Galactic extinction in the presence of silicon
nanoparticles. For this goal we used the appropriate optical constants of
nanosized Si, essentially different from those of bulk Si due to quantum
confinement. It was found that a dust mixture of silicon nanoparticles, bare
graphite grains, silicate core-organic refractory mantle grains and three-layer
silicate-water ice-organic refractory grains works well in explaining the
extinction and, in addition, results in the acceptable fractions of UV/visible
photons absorbed by silicon nanoparticles: 0.071-0.081. Since these fractions
barely agree with the fraction of UV/visible photons needed to excite the
observed ERE, we conclude that the intrinsic photon conversion efficiency of
the photoluminescence by silicon nanoparticles must be near 100%, if they are
the source of the ERE.Comment: Latex2e, uses emulateapj.sty (included), multicol.sty, epsf.sty, 6
pages, 3 figures (8 Postscript files), accepted for publication in ApJ
Letters, complete Postscript file is also available at
http://physics.technion.ac.il/~zubko/eb.html#SNP
Deuterium at high-redshift: Primordial abundance in the zabs = 2.621 damped Ly-alpha system towards CTQ247
The detection of neutral deuterium in the low-metallicity damped
Lyman-{\alpha} system at zabs = 2.621 towards the quasar CTQ247 is reported.
Using a high signal-to-noise and high spectral resolution (R = 60000) spectrum
from the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph, we
precisely measure the deuterium-to-oxygen ratio log N(DI)/N(OI) = 0.74+/-0.04,
as well as the overall oxygen abundance, log N(OI)/N(HI)=-5.29+/-0.10 (or
equivalently [O/H]=-1.99+/-0.10 with respect to the solar value). Assuming
uniform metallicity throughout the system, our measurement translates to (D/H)
= (2.8+0.8 -0.6)x10^-5. This ratio is consistent within errors (<0.4sigma) with
the primordial ratio, (D/H)p = (2.59+/-0.15)x10^-5, predicted by standard
Big-Bang Nucleosynthesis using the WMAP7 value of the cosmological density of
baryons (100 Omega_b h^2 = 2.249+/-0.056). The DI absorption lines are observed
to be broader than the OI absorption lines. From a consistent fit of the
profiles we derive the turbulent broadening to be 5.2 km/s and the temperature
of the gas to be T = 8800+/-1500 K, corresponding to a warm neutral medium.Comment: Accepted for publication in A&A Letter
Torsional nodeless vibrations of quaking neutron star restored by combined forces of shear elastic and magnetic field stresses
Within the framework of Newtonian magneto-solid-mechanics, relying on
equations appropriate for a perfectly conducting elastic continuous medium
threaded by a uniform magnetic field, the asteroseismic model of a neutron star
undergoing axisymmetric global torsional nodeless vibrations under the combined
action of Hooke's elastic and Lorentz magnetic forces is considered with
emphasis on a toroidal Alfv\'en mode of differentially rotational vibrations
about the dipole magnetic moment axis of the star. The obtained spectral
equation for frequency is applied to -pole identification of
quasi-periodic oscillations (QPOs) of X-ray flux during the giant flares of SGR
1806-20 and SGR 1900+14. Our calculations suggest that detected QPOs can be
consistently interpreted, within the framework of this model, as produced by
global torsional nodeless vibrations of quaking magnetar if they are considered
to be restored by the joint action of bulk forces of shear elastic and magnetic
field stresses.Comment: 18 pages, 5 figures; accepted in Ap
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