6,969 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
Nonisomorphic curves that become isomorphic over extensions of coprime degrees
We show that one can find two nonisomorphic curves over a field K that become
isomorphic to one another over two finite extensions of K whose degrees over K
are coprime to one another.
More specifically, let K_0 be an arbitrary prime field and let r and s be
integers greater than 1 that are coprime to one another. We show that one can
find a finite extension K of K_0, a degree-r extension L of K, a degree-s
extension M of K, and two curves C and D over K such that C and D become
isomorphic to one another over L and over M, but not over any proper
subextensions of L/K or M/K.
We show that such C and D can never have genus 0, and that if K is finite, C
and D can have genus 1 if and only if {r,s} = {2,3} and K is an odd-degree
extension of F_3. On the other hand, when {r,s}={2,3} we show that genus-2
examples occur in every characteristic other than 3.
Our detailed analysis of the case {r,s} = {2,3} shows that over every finite
field K there exist nonisomorphic curves C and D that become isomorphic to one
another over the quadratic and cubic extensions of K.
Most of our proofs rely on Galois cohomology. Without using Galois
cohomology, we show that two nonisomorphic genus-0 curves over an arbitrary
field remain nonisomorphic over every odd-degree extension of the base field.Comment: LaTeX, 32 pages. Further references added to the discussion in
Section 1
Percolation, depinning, and avalanches in capillary condensation of gases in disordered porous solids
We propose a comprehensive theoretical description of hysteresis in capillary
condensation of gases in mesoporous disordered materials. Applying mean-field
density functional theory to a coarse-grained lattice-gas model, we show that
the morphology of the hysteresis loops is influenced by out-of-equilibrium
transitions that are different on filling and on draining. In particular,
desorption may be associated to a depinning process and be percolation-like
without explicit pore-blocking effects.Comment: 4 pages, 5 figure
A Superluminal Subway: The Krasnikov Tube
The ``warp drive'' metric recently presented by Alcubierre has the problem
that an observer at the center of the warp bubble is causally separated from
the outer edge of the bubble wall. Hence such an observer can neither create a
warp bubble on demand nor control one once it has been created. In addition,
such a bubble requires negative energy densities. One might hope that
elimination of the first problem might ameliorate the second as well. We
analyze and generalize a metric, originally proposed by Krasnikov for two
spacetime dimensions, which does not suffer from the first difficulty. As a
consequence, the Krasnikov metric has the interesting property that although
the time for a one-way trip to a distant star cannot be shortened, the time for
a round trip, as measured by clocks on Earth, can be made arbitrarily short. In
our four dimensional extension of this metric, a ``tube'' is constructed along
the path of an outbound spaceship, which connects the Earth and the star.
Inside the tube spacetime is flat, but the light cones are opened out so as to
allow superluminal travel in one direction. We show that, although a single
Krasnikov tube does not involve closed timelike curves, a time machine can be
constructed with a system of two non-overlapping tubes. Furthermore, it is
demonstrated that Krasnikov tubes, like warp bubbles and traversable wormholes,
also involve unphysically thin layers of negative energy density, as well as
large total negative energies, and therefore probably cannot be realized in
practice.Comment: 20 pages, LATEX, 5 eps figures, uses \eps
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