4,703 research outputs found
Investigating the Structure of the Windy Torus in Quasars
Thermal mid-infrared emission of quasars requires an obscuring structure that
can be modeled as a magneto-hydrodynamic wind in which radiation pressure on
dust shapes the outflow. We have taken the dusty wind models presented by
Keating and collaborators that generated quasar mid-infrared spectral energy
distributions (SEDs), and explored their properties (such as geometry, opening
angle, and ionic column densities) as a function of Eddington ratio and X-ray
weakness. In addition, we present new models with a range of magnetic field
strengths and column densities of the dust-free shielding gas interior to the
dusty wind. We find this family of models -- with input parameters tuned to
accurately match the observed mid-IR power in quasar SEDs -- provides
reasonable values of the Type 1 fraction of quasars and the column densities of
warm absorber gas, though it does not explain a purely luminosity-dependent
covering fraction for either. Furthermore, we provide predictions of the
cumulative distribution of E(B-V) values of quasars from extinction by the wind
and the shape of the wind as imaged in the mid-infrared. Within the framework
of this model, we predict that the strength of the near-infrared bump from hot
dust emission will be correlated primarily with L/L_Edd rather than luminosity
alone, with scatter induced by the distribution of magnetic field strengths.
The empirical successes and shortcomings of these models warrant further
investigations into the composition and behaviour of dust and the nature of
magnetic fields in the vicinity of actively accreting supermassive black holes.Comment: 11 pages, 6 figures, accepted for publication in MNRA
Assessing Financial Vulnerability in the Nonprofit Sector
Effective nonprofit governance relies upon understanding an organization's financial condition and vulnerabilities. However, financial vulnerability of nonprofit organizations is a relatively new area of study. In this paper, we compare two models used to forecast bankruptcy in the corporate sector (Altman 1968 and Ohlson 1980) with the model used by nonprofit researchers (Tuckman and Chang 1991). We find that the Ohlson model has higher explanatory power than either Tuckman and Chang's or Altman's in predicting four different measures of financial vulnerability. However, we show that none of the models, individually or combined, are effective in predicting financial distress. We then propose a more comprehensive model of financial vulnerability by adding two new variables to represent reliance on commercial-type activities to generate revenues and endowment sufficiency. We find that this model outperforms Ohlson's model and performs substantially better in explaining and predicting financial vulnerability. Hence, the expanded model can be used as a guide for understanding the drivers of financial vulnerability and for identifying more effective proxies for nonprofit sector financial distress for use in future research. This publication is Hauser Center Working Paper No. 27. The Hauser Center Working Paper Series was launched during the summer of 2000. The Series enables the Hauser Center to share with a broad audience important works-in-progress written by Hauser Center scholars and researchers
Confinement and electron correlation effects in photoionization of atoms in endohedral anions: Ne@C60^{z-}
Trends in resonances, termed confinement resonances, in photoionization of
atoms A in endohedral fullerene anions A@C60^{z-} are theoretically studied and
exemplified by the photoionization of Ne in Ne@C{60}^{z-}. Remarkably, above a
particular nl ionization threshold of Ne in neutral Ne@C60 (I_{nl}^{z=0}),
confinement resonances in corresponding partial photoionization cross sections
sigma_{nl} of Ne in any charged Ne@C60^{z-} remain almost intact by a charge z
on the carbon cage, as a general phenomenon. At lower photon energies, omega <
I_{nl}^{z=0}, the corresponding photoionization cross sections develop
additional, strong, z-dependent resonances, termed Coulomb confinement
resonances, as a general occurrence. Furthermore, near the innermost 1s
ionization threshold, the 2p photoionization cross section sigma_{2p} of the
outermost 2p subshell of thus confined Ne is found to inherit the confinement
resonance structure of the 1s photoionization spectrum, via interchannel
coupling. As a result, new confinement resonances emerge in the 2p
photoionization cross section of the confined Ne atom at photoelectron energies
which exceed the 2p threshold by about a thousand eV, i.e., far above where
conventional wisdom said they would exist. Thus, the general possibility for
confinement resonances to resurrect in photoionization spectra of encapsulated
atoms far above thresholds is revealed, as an interesting novel general
phenomenon.Comment: 6 pages, 4 figures, Latex2e, jpconf.cls styl
Revealing Cosmic Rotation
Cosmological Birefringence (CB), a rotation of the polarization plane of
radiation coming to us from distant astrophysical sources, may reveal parity
violation in either the electromagnetic or gravitational sectors of the
fundamental interactions in nature. Until only recently this phenomenon could
be probed with only radio observations or observations at UV wavelengths.
Recently, there is a substantial effort to constrain such non-standard models
using observations of the rotation of the polarization plane of cosmic
microwave background (CMB) radiation. This can be done via measurements of the
-modes of the CMB or by measuring its TB and EB correlations which vanish in
the standard model. In this paper we show that correlations-based
estimator is the best for upcoming polarization experiments. The based
estimator surpasses other estimators because it has the smallest noise and of
all the estimators is least affected by systematics. Current polarimeters are
optimized for the detection of -mode polarization from either primordial
gravitational waves or by large scale structure via gravitational lensing. In
the paper we also study optimization of CMB experiments for the detection of
cosmological birefringence, in the presence of instrumental systematics, which
by themselves are capable of producing correlations; potentially mimicking
CB.Comment: 10 pages, 3 figures, 2 table
On the multiplicativity of quantum cat maps
The quantum mechanical propagators of the linear automorphisms of the
two-torus (cat maps) determine a projective unitary representation of the theta
group, known as Weil's representation. We prove that there exists an
appropriate choice of phases in the propagators that defines a proper
representation of the theta group. We also give explicit formulae for the
propagators in this representation.Comment: Revised version: proof of the main theorem simplified. 21 page
B polarization of the CMB from Faraday rotation
We study the effect of Faraday rotation due to a homogeneous magnetic field
on the polarization of the cosmic microwave background (CMB). Scalar
fluctuations give rise only to parity-even E-type polarization of the CMB.
However in the presence of a magnetic field, a non-vanishing parity-odd B-type
polarization component is produced through Faraday rotation. We derive the
exact solution for the E and B modes generated by scalar perturbations
including the Faraday rotation effect of a uniform magnetic field, and evaluate
their cross-correlations with temperature anisotropies. We compute the angular
autocorrelation function of the B-modes in the limit that the Faraday rotation
is small. We find that primordial magnetic fields of present strength around
G rotate E-modes into B-modes with amplitude comparable to those
due to the weak gravitational lensing effect at frequencies around
GHz. The strength of B-modes produced by Faraday rotation scales as
. We evaluate also the depolarizing effect of Faraday rotation upon
the cross correlation between temperature anisotropy and E-type polarization.Comment: 11 pages, 4 figures. Minor changes to match the published versio
Spectral determinants and zeta functions of Schr\"odinger operators on metric graphs
A derivation of the spectral determinant of the Schr\"odinger operator on a
metric graph is presented where the local matching conditions at the vertices
are of the general form classified according to the scheme of Kostrykin and
Schrader. To formulate the spectral determinant we first derive the spectral
zeta function of the Schr\"odinger operator using an appropriate secular
equation. The result obtained for the spectral determinant is along the lines
of the recent conjecture.Comment: 16 pages, 2 figure
Universal quantum signature of mixed dynamics in antidot lattices
We investigate phase coherent ballistic transport through antidot lattices in
the generic case where the classical phase space has both regular and chaotic
components. It is shown that the conductivity fluctuations have a non-Gaussian
distribution, and that their moments have a power-law dependence on a
semiclassical parameter, with fractional exponents. These exponents are
obtained from bifurcating periodic orbits in the semiclassical approximation.
They are universal in situations where sufficiently long orbits contribute.Comment: 7 page
Some recursive formulas for Selberg-type integrals
A set of recursive relations satisfied by Selberg-type integrals involving
monomial symmetric polynomials are derived, generalizing previously known
results. These formulas provide a well-defined algorithm for computing
Selberg-Schur integrals whenever the Kostka numbers relating Schur functions
and the corresponding monomial polynomials are explicitly known. We illustrate
the usefulness of our results discussing some interesting examples.Comment: 11 pages. To appear in Jour. Phys.
- …