3,685 research outputs found
Light Attenuation in Back Bay, Virginia
In order to help assess the cause of the recent decline in submersed macrophytes, light attenuation was measured at selected stations in Back Bay, Virginia, in July 1987 and April 1988, using an underwater spectroradiometer. Secchi depth and concentrations of total suspended solids and chlorophyll-a were measured simultaneously. In July 1987, extinction coefficients ranged from 2.7 to 5.7 m-1 and Secchi depths ranged from 0.26 to 0.44 m. Total suspended solids ranged from 27 to 64 mg/L--37 to 80% of the suspended material was organic matter. Chlorophyll-a concentrations ranged from 43 to 71 μ g/L; indicating the presence of large numbers of algae. Water clarity was least in North Bay and greatest at the North Carolina border. In April 1988, during a period of strong wind, total suspended solids were extremely high, ranging from 78 to 214 mg/L, whereas the organic fraction ranged from 20 to 30%. Chlorophyll-a concentration ranged from 34.5 to 88 μ g/L. Secchi depth ranged from 0.16 to 0.33 m and K ranged from 3.7 m-1 at the North Carolina line to 19.9 m-1 in a canal near Pellitory Point. Comparison of the conditions in Back Bay in 1986-88 with those in the tidal Potomac River and Estuary indicate that the decline in submersed macrophyte, in Back Bay is related to high light attenuation
The spectrum of the Broad Line Region and the high-energy emission of powerful blazars
High-energy emission (from the X-ray through the gamma-ray band) of Flat
Spectrum Radio Quasars is widely associated with the inverse Compton (IC)
scattering of ambient photons, produced either by the accretion disk or by the
Broad Line Region, by high-energy electrons in a relativistic jet. In the
modelling of the IC spectrum one usually adopts a simple black-body
approximation for the external radiation field, though the real shape is
probably more complex. The knowledge of the detailed spectrum of the external
radiation field would allow to better characterize the soft-medium X-ray IC
spectrum, which is crucial to address several issues related to the study of
these sources. Here we present a first step in this direction, calculating the
IC spectra expected by considering a realistic spectrum for the external
radiation energy density produced by the BLR, as calculated with the
photoionization code CLOUDY. We find that, under a wide range of the physical
parameters characterizing the BLR clouds, the IC spectrum calculated with the
black-body approximation reproduces quite well the exact spectrum for energies
above few keV. In the soft energy band, instead, the IC emission calculated
using the BLR emission shows a complex shape, with a moderate excess with
respect to the approximate spectrum, which becomes more important for
decreasing values of the peak frequency of the photoionizing continuum. We also
show that the high-energy spectrum shows a marked steepening, due to the energy
dependence of the scattering cross section, above a characteristic energy of
10-20 GeV, quasi independent on the Lorentz factor of the jet.Comment: 10 pages, 9 figures, accepted for publication in MNRA
Two-photon transitions in primordial hydrogen recombination
The subject of cosmological hydrogen recombination has received much
attention recently because of its importance to predictions for and
cosmological constraints from CMB observations. While the central role of the
two-photon decay 2s->1s has been recognized for many decades, high-precision
calculations require us to consider two-photon decays from the higher states
ns,nd->1s (n>=3). Simple attempts to include these processes in recombination
calculations have suffered from physical problems associated with sequences of
one-photon decays, e.g. 3d->2p->1s, that technically also produce two photons.
These correspond to resonances in the two-photon spectrum that are optically
thick, necessitating a radiative transfer calculation. We derive the
appropriate equations, develop a numerical code to solve them, and verify the
results by finding agreement with analytic approximations to the radiative
transfer equation. The related processes of Raman scattering and two-photon
recombination are included using similar machinery. Our results show that early
in recombination the two-photon decays act to speed up recombination, reducing
the free electron abundance by 1.3% relative to the standard calculation at
z=1300. However we find that some photons between Ly-alpha and Ly-beta are
produced, mainly by 3d->1s two-photon decay and 2s->1s Raman scattering. At
later times these photons redshift down to Ly-alpha, excite hydrogen atoms, and
act to slow recombination. Thus the free electron abundance is increased by
1.3% relative to the standard calculation at z=900. The implied correction to
the CMB power spectrum is neligible for the recently released WMAP and ACBAR
data, but at Fisher matrix level will be 7 sigma for Planck. [ABRIDGED]Comment: Matches PRD accepted version. 28 pages, 12 figure
3-Body Dynamics in a (1+1) Dimensional Relativistic Self-Gravitating System
The results of our study of the motion of a three particle, self-gravitating
system in general relativistic lineal gravity is presented for an arbitrary
ratio of the particle masses. We derive a canonical expression for the
Hamiltonian of the system and discuss the numerical solution of the resulting
equations of motion. This solution is compared to the corresponding
non-relativistic and post-Newtonian approximation solutions so that the
dynamics of the fully relativistic system can be interpretted as a correction
to the one-dimensional Newtonian self-gravitating system. We find that the
structure of the phase space of each of these systems yields a large variety of
interesting dynamics that can be divided into three distinct regions: annulus,
pretzel, and chaotic; the first two being regions of quasi-periodicity while
the latter is a region of chaos. By changing the relative masses of the three
particles we find that the relative sizes of these three phase space regions
changes and that this deformation can be interpreted physically in terms of the
gravitational interactions of the particles. Furthermore, we find that many of
the interesting characteristics found in the case where all of the particles
share the same mass also appears in our more general study. We find that there
are additional regions of chaos in the unequal mass system which are not
present in the equal mass case. We compare these results to those found in
similar systems.Comment: latex, 26 pages, 17 figures, high quality figures available upon
request; typos and grammar correcte
Distributed Computing in the Asynchronous LOCAL model
The LOCAL model is among the main models for studying locality in the
framework of distributed network computing. This model is however subject to
pertinent criticisms, including the facts that all nodes wake up
simultaneously, perform in lock steps, and are failure-free. We show that
relaxing these hypotheses to some extent does not hurt local computing. In
particular, we show that, for any construction task associated to a locally
checkable labeling (LCL), if is solvable in rounds in the LOCAL model,
then remains solvable in rounds in the asynchronous LOCAL model.
This improves the result by Casta\~neda et al. [SSS 2016], which was restricted
to 3-coloring the rings. More generally, the main contribution of this paper is
to show that, perhaps surprisingly, asynchrony and failures in the computations
do not restrict the power of the LOCAL model, as long as the communications
remain synchronous and failure-free
Chaos in a Relativistic 3-body Self-Gravitating System
We consider the 3-body problem in relativistic lineal gravity and obtain an
exact expression for its Hamiltonian and equations of motion. While
general-relativistic effects yield more tightly-bound orbits of higher
frequency compared to their non-relativistic counterparts, as energy increases
we find in the equal-mass case no evidence for either global chaos or a
breakdown from regular to chaotic motion, despite the high degree of
non-linearity in the system. We find numerical evidence for a countably
infinite class of non-chaotic orbits, yielding a fractal structure in the outer
regions of the Poincare plot.Comment: 9 pages, LaTex, 3 figures, final version to appear in Phys. Rev. Let
Predicted FeII Emission-Line Strengths from Active Galactic Nuclei
We present theoretical FeII emission line strengths for physical conditions
typical of Active Galactic Nuclei with Broad-Line Regions. The FeII line
strengths were computed with a precise treatment of radiative transfer using
extensive and accurate atomic data from the Iron Project. Excitation mechanisms
for the FeII emission included continuum fluorescence, collisional excitation,
self-fluorescence amoung the FeII transitions, and fluorescent excitation by
Lyman-alpha and Lyman-beta. A large FeII atomic model consisting of 827 fine
structure levels (including states to E ~ 15 eV) was used to predict fluxes for
approximately 23,000 FeII transitions, covering most of the UV, optical, and IR
wavelengths of astrophysical interest. Spectral synthesis for wavelengths from
1600 Angstroms to 1.2 microns is presented. Applications of present theoretical
templates to the analysis of observations are described. In particular, we
discuss recent observations of near-IR FeII lines in the 8500 Angstrom -- 1
micron region which are predicted by the Lyman-alpha fluorescence mechanism. We
also compare our UV spectral synthesis with an empirical iron template for the
prototypical, narrow-line Seyfert galaxy I Zw 1. The theoretical FeII template
presented in this work should also applicable to a variety of objects with FeII
spectra formed under similar excitation conditions, such as supernovae and
symbiotic stars.Comment: 33 pages, 15 postscript figure
Radio and gamma-ray constraints on dark matter annihilation in the Galactic center
We determine upper limits on the dark matter (DM) self-annihilation cross
section for scenarios in which annihilation leads to the production of
electron--positron pairs. In the Galactic centre (GC), relativistic electrons
and positrons produce a radio flux via synchroton emission, and a gamma ray
flux via bremsstrahlung and inverse Compton scattering. On the basis of
archival, interferometric and single-dish radio data, we have determined the
radio spectrum of an elliptical region around the Galactic centre of extent 3
degrees semi-major axis (along the Galactic plane) and 1 degree semi-minor axis
and a second, rectangular region, also centered on the GC, of extent 1.6
degrees x 0.6 degrees. The radio spectra of both regions are non-thermal over
the range of frequencies for which we have data: 74 MHz -- 10 GHz. We also
consider gamma-ray data covering the same region from the EGRET instrument
(about GeV) and from HESS (around TeV). We show how the combination of these
data can be used to place robust constraints on DM annihilation scenarios, in a
way which is relatively insensitive to assumptions about the magnetic field
amplitude in this region. Our results are approximately an order of magnitude
more constraining than existing Galactic centre radio and gamma ray limits. For
a DM mass of m_\chi =10 GeV, and an NFW profile, we find that the
velocity-averaged cross-section must be less than a few times 10^-25 cm^3 s^-1.Comment: 14 pages, 9 figures. Version accepted for publication in PRD.
Reference section updated/extended
The dissipative effect of thermal radiation loss in high-temperature dense plasmas
A dynamical model based on the two-fluid dynamical equations with energy
generation and loss is obtained and used to investigate the self-generated
magnetic fields in high-temperature dense plasmas such as the solar core. The
self-generation of magnetic fields might be looked at as a
self-organization-type behavior of stochastic thermal radiation fields, as
expected for an open dissipative system according to Prigogine's theory of
dissipative structures.Comment: 4 pages, 1 postscript figure included; RevTeX3.0, epsf.tex neede
Statistical Mechanics of Relativistic One-Dimensional Self-Gravitating Systems
We consider the statistical mechanics of a general relativistic
one-dimensional self-gravitating system. The system consists of -particles
coupled to lineal gravity and can be considered as a model of
relativistically interacting sheets of uniform mass. The partition function and
one-particle distitrubion functions are computed to leading order in
where is the speed of light; as results for the
non-relativistic one-dimensional self-gravitating system are recovered. We find
that relativistic effects generally cause both position and momentum
distribution functions to become more sharply peaked, and that the temperature
of a relativistic gas is smaller than its non-relativistic counterpart at the
same fixed energy. We consider the large-N limit of our results and compare
this to the non-relativistic case.Comment: latex, 60 pages, 22 figure
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