8,745 research outputs found
Is turbulent mixing a self convolution process ?
Experimental results for the evolution of the probability distribution
function (PDF) of a scalar mixed by a turbulence flow in a channel are
presented. The sequence of PDF from an initial skewed distribution to a sharp
Gaussian is found to be non universal. The route toward homogeneization depends
on the ratio between the cross sections of the dye injector and the channel. In
link with this observation, advantages, shortcomings and applicability of
models for the PDF evolution based on a self-convolution mechanisms are
discussed.Comment: 4 page
Systematic Errors in the Estimation of Black Hole Masses by Reverberation Mapping
The mass of the central black hole in many active galactic nuclei has been
estimated on the basis of the assumption that the dynamics of the broad
emission line gas are dominated by the gravity of the black hole. The most
commonly-employed method is to estimate a characteristic size-scale from
reverberation mapping experiments and combine it with a characteristic velocity
taken from the line profiles; the inferred mass is then estimated by . We critically discuss the evidence supporting the assumption of
gravitational dynamics and find that the arguments are still inconclusive. We
then explore the range of possible systematic error if the assumption of
gravitational dynamics is granted. Inclination relative to a flattened system
may cause a systematic underestimate of the central mass by a factor , where is the aspect ratio of the flattening. The coupled
effects of a broad radial emissivity distribution, an unknown angular radiation
pattern of line emission, and sub-optimal sampling in the reverberation
experiment can cause additional systematic errors as large as a factor of 3 or
more in either direction.Comment: 19 pages, 4 figures, AASLaTeX, accepted by Ap
Dynamics of the Lyman alpha and C IV emitting gas in 3C 273
In this paper we study the variability properties of the Lyman alpha and C IV
emission lines in 3C273 using archival IUE observations. Our data show for the
first time the existence of variability on time scales of several years. We
study the spatial distribution and the velocity field of the emitting gas by
performing detailed analyses on the line variability using correlations, 1D and
2D response functions, and principal component analysis. In both lines we find
evidence for two components, one which has the dynamic properties of gas in
Keplerian motion around a black hole with a mass of the order of 10^9 Mo, and
one which is characterized by high, blue-shifted velocities at large lag. There
is no indication of the presence of optically thick emission medium neither in
the Lya, nor in the Civ response functions. The component characterized by
blue-shifted velocities, which is comparatively much stronger in Civ than in
Lya, is more or less compatible with being the result of gas falling towards
the central black hole with free-fall acceleration. We propose however that the
line emission at high, blue-shifted velocities is better explained in terms of
entrainment of gas clouds by the jet. This gas is therefore probably
collisionally excited as a result of heating due to the intense infrared
radiation from the jet, which would explain the strength of this component in
Civ relative to Lya. This phenomenon might be a signature of disk-jet
interaction.Comment: 16 pages, 10 figures. Accepted for publication in ApJ. Uses aaste
Near-field interaction between domain walls in adjacent Permalloy nanowires
The magnetostatic interaction between two oppositely charged transverse
domain walls (DWs)in adjacent Permalloy nanowires is experimentally
demonstrated. The dependence of the pinning strength on wire separation is
investigated for distances between 13 and 125 nm, and depinning fields up to 93
Oe are measured. The results can be described fully by considering the
interaction between the full complex distribution of magnetic charge within
rigid, isolated DWs. This suggests the DW internal structure is not appreciably
disturbed by the pinning potential, and that they remain rigid although the
pinning strength is significant. This work demonstrates the possibility of
non-contact DW trapping without DW perturbation and full continuous flexibility
of the pinning potential type and strength. The consequence of the interaction
on DW based data storage schemes is evaluated.Comment: 4 pages, 4 figures, 1 page supplimentary material (supporting.ps
Spin-dynamics of the low-dimensional magnet (CH3)2NH2CuCl3
Dimethylammonium copper (II) chloride (also known as DMACuCl3 or MCCL) is a
low dimensional S=1/2 quantum spin system proposed to be an alternating
ferro-antiferromagnetic chain with similar magnitude ferromagnetic (FM) and
antiferromagnetic (AFM) exchange interactions. Subsequently, it was shown that
the existing bulk measurements could be adequately modeled by considering
DMACuCl3 as independent AFM and FM dimer spin pairs. We present here new
inelastic neutron scattering measurements of the spin-excitations in single
crystals of DMACuCl3. These results show significant quasi-one-dimensional
coupling, however the magnetic excitations do not propagate along the expected
direction. We observe a band of excitations with a gap of 0.95 meV and a
bandwidth of 0.82 meV.Comment: 3 pages, 2 figures included in text, submitted to proceedings of
International Conference on Neutron Scattering, December 200
How many trafficked people are there in Greater New Orleans? lessons in measurement
In an effort to develop a model for estimating prevalence in a city or region of the United States, this study employed Multiple Systems Estimation, a statistical approach that uses data on known cases collected from individual agencies to estimate the number not known, with the ultimate aim of estimating the prevalence of trafficking in a region. Utilizing de-identified data provided by local non-profits and law enforcement agencies, the researchers estimated the prevalence of trafficking in the New Orleans-Metairie metropolitan statistical area. This represents one of the first attempts to use Multiple Systems Estimation to quantify human trafficking in a United States context. The article provides an account of the impediments to and limitations of conducting such an estimate, given the definitional variance and political dynamics that are endemic to anti-trafficking efforts in the United States. The authors provide recommendations for data collection and prevalence analysis that could be applied in other cities or regions of the United States as well as in other similarly-resourced environments
Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Akn 564. II. Ultraviolet Continuum and Emission-line Variability
We present results of an intensive two-month campaign of approximately daily
spectrophotometric monitoring of the narrow-line Seyfert 1 galaxy Akn 564 with
HST. The fractional variability amplitude of the continuum variations between
1365-3000 A is ~6%, about a factor 3 less than that found in typical Seyfert 1
galaxies over a similar period of time. However, large amplitude, short
time-scale flaring behavior is evident, with trough-to-peak flux changes of
about 18% in approximately 3 days. We present evidence for wavelength-dependent
continuum time delays, with the variations at 3000 A lagging behind those at
1365 A by about 1 day. These delays may be interpreted as evidence for a
stratified continuum reprocessing region, possibly an accretion-disk structure.
The Lyman-alpha 1216 emission-line exhibits flux variations of about 1%
amplitude.Comment: 27 pages, 14 figures. Accepted by Astrophysical Journa
Application of energy and angular momentum balance to gravitational radiation reaction for binary systems with spin-orbit coupling
We study gravitational radiation reaction in the equations of motion for
binary systems with spin-orbit coupling, at order (v/c)^7 beyond Newtonian
gravity, or O(v/c)^2 beyond the leading radiation reaction effects for
non-spinning bodies. We use expressions for the energy and angular momentum
flux at infinity that include spin-orbit corrections, together with an
assumption of energy and angular momentum balance, to derive equations of
motion that are valid for general orbits and for a class of coordinate gauges.
We show that the equations of motion are compatible with those derived earlier
by a direct calculation.Comment: 12 pages, submitted to General Relativity and Gravitatio
- …