622 research outputs found
Constraining Radiatively Inefficient Accretion Flows with Polarization
The low-luminosity black hole Sgr A* provides a testbed for models of
Radiatively Inefficient Accretion Flows (RIAFs). Recent sub-millimeter linear
polarization measurements of Sgr A* have provided evidence that the electrons
in the accretion flow are relativistic over a large range of radii. Here, we
show that these high temperatures result in elliptical plasma normal modes.
Thus, polarized millimeter and sub-millimeter radiation emitted within RIAFs
will undergo generalized Faraday rotation, a cyclic conversion between linear
and circular polarization. This effect will not depolarize the radiation even
if the rotation measure is extremely high. Rather, the beam will take on the
linear and circular polarization properties of the plasma normal modes. As a
result, polarization measurements of Sgr A* in this frequency regime will
constrain the temperature, density and magnetic profiles of RIAF models.Comment: 4 pages, 3 figures, accepted by ApJ Letter
Torque-Limited Growth of Massive Black Holes in Galaxies Across Cosmic Time
We combine cosmological hydrodynamic simulations with analytic models to
evaluate the role of galaxy-scale gravitational torques on the evolution of
massive black holes at the centers of star-forming galaxies. We confirm and
extend our earlier results to show that torque-limited growth yields black
holes and host galaxies evolving on average along the Mbh-Mbulge relation from
early times down to z = 0 and that convergence onto the scaling relation occurs
independent of the initial conditions and with no need for mass averaging
through mergers or additional self-regulation processes. Smooth accretion
dominates the long-term evolution, with black hole mergers with mass ratios
>1:5 representing typically a small fraction of the total growth. Winds from
the accretion disk are required to eject significant mass to suppress black
hole growth, but there is no need for coupling this wind to galactic-scale gas
to regulate black holes in a non-linear feedback loop. Torque-limited growth
yields a close-to-linear relation for the star formation rate and the black
hole accretion rate averaged over galaxy evolution time scales. However, the
SFR-AGN connection has significant scatter owing to strong variability of black
hole accretion at all resolved time scales. Eddington ratios can be described
by a broad lognormal distribution with median value evolving roughly as (1 +
z)^1.9, suggesting a main sequence for black hole growth similar to the cosmic
evolution of specific SFRs. Our results offer an attractive scenario consistent
with available observations in which cosmological gas infall and transport of
angular momentum in the galaxy by gravitational instabilities regulate the
long-term co-evolution of black holes and star-forming galaxies.Comment: 26 pages, 15 figures, replaced by published versio
X-ray Images of Hot Accretion Flows
We consider the X-ray emission due to bremsstrahlung processes from hot, low
radiative-efficiency accretion flows around supermassive and galactic black
holes. We calculate surface brightness profiles and Michelson visibility
functions for a range of density profiles, rho ~ r^(-3/2+p), with 0 < p < 1, to
allow for the presence of outflows. We find that although the 1 keV emitting
region in these flows can always extend up to 10^6 Schwarzschild radii (R_S),
their surface brightness profiles and visibility functions are strongly
affected by the specific density profile. The advection-dominated solutions
with no outflows (p=0) lead to centrally peaked profiles with characteristic
sizes of only a few tens of R_S. Solutions with strong outflows (p~1) lead to
flat intensity profiles with significantly larger characteristic sizes of up to
10^6 R_S. This implies that low luminosity galactic nuclei, such as M87, may
appear as extended X-ray sources when observed with current X-ray imaging
instruments. We show that X-ray brightness profiles and their associated
visibility functions may be powerful probes for determining the relevant mode
of accretion and, in turn, the properties of hot accretion flows. We discuss
the implications of our results for observations with the Chandra X-ray
Observatory and the planned X-ray interferometer MAXIM.Comment: 14 pages, 4 figures, accepted by The Astrophysical Journal, minor
change
Toward a better understanding of tool wear effect through a comparison between experiments and SPH numerical modelling of machining hard materials
The aim of this study is to improve the general understanding of tungsten carbide (WCâCo) tool wear under dry machining of the hard-to-cut titanium alloy Ti6Al4V. The chosen approach includes experimental and numerical tests. The experimental part is designed to identify wear mechanisms using cutting force measurements, scanning electron microscope observations and optical profilometer analysis. Machining tests were conducted in the orthogonal cutting framework and showed a strong evolution of the cutting forces and the chip profiles with tool wear. Then, a numerical method has been used in order to model the machining process with both new and worn tools. The use of smoothed particle hydrodynamics model (SPH model) as a numerical tool for a better understanding of the chip formation with worn tools is a key aspect of this work. The redicted chip morphology and the cutting force evolution with respect to the tool wear are qualitatively compared with experimental trends. The chip formation mechanisms during dry cutting process are shown to be quite dependent from the worn tool geometry. These mechanisms explain the high variation of the experimental and numerical feed force between new and worn tools
Biotransformation of Chemicals at the WaterâSediment InterfaceâToward a Robust Simulation Study Setup
Studying aquatic biotransformation of chemicals in laboratory experiments, i.e., OECD 308 and OECD 309 studies, is required by international regulatory frameworks to prevent the release of persistent chemicals into natural water bodies. Here, we aimed to address several previously described shortcomings of OECD 308/309 studies regarding their variable outcomes and questionable environmental relevance by broadly testing and characterizing a modified biotransformation test system in which an aerated water column covers a thin sediment layer. Compared to standard OECD 308/309 studies, the modified system showed little inter-replicate variability, improved observability of biotransformation, and consistency with first-order biotransformation kinetics for the majority of 43 test compounds, including pharmaceuticals, pesticides, and artificial sweeteners. To elucidate the factors underlying the decreased inter-replicate variability compared to OECD 309 outcomes, we used multidimensional flow cytometry data and a machine learning-based cell type assignment pipeline to study cell densities and cell type diversities in the sediment and water compartments. Our here presented data on cell type composition in both water and sediment allows, for the first time, to study the behavior of microbial test communities throughout different biotransformation simulation studies. We found that sediment-associated microbial communities were generally more stable throughout the experiments and exhibited higher cell type diversity than the water column-associated communities. Consistently, our data indicate that aquatic biotransformation of chemicals can be most robustly studied in test systems providing a sufficient amount of sediment-borne biomass. While these findings favor OECD 308-type systems over OECD 309-type systems to study biotransformation at the waterâsediment interface, our results suggest that the former should be modified toward lower sedimentâwater ratios to improve observability and interpretability of biotransformation.
KEYWORDS- biotransformation micropollutants chemical persistence waterâsediment systems phenotypic microbial community composition cell type diversity OECD 308/309 studie
Spectroscopic Constraints on the Surface Magnetic Field of the Accreting Neutron Star EXO 0748-676
Gravitationally redshifted absorption lines of Fe XXVI, Fe XXV, and O VIII
were inferred recently in the X-ray spectrum of the bursting neutron star EXO
0748-676. We place an upper limit on the stellar magnetic field based on the
iron lines. The oxygen absorption feature shows a multiple component profile
that is consistent with Zeeman splitting in a magnetic field of ~(1-2)x10^9
gauss, and for which the corresponding Zeeman components of the iron lines are
expected to be blended together. In other systems, a field strength >5x10^{10}
gauss could induce a blueshift of the line centroids that would counteract
gravitational redshift and complicate the derivation of constraints on the
equation of state of the neutron star.Comment: 5 pages, submitted to Phys. Rev. Let
Quark matter in compact stars?
Ozel, in a recent reanalysis of EXO 0748-676 observational data
(astro-ph/0605106), concluded that quark matter probably does not exist in the
center of compact stars. We show that the data is actually consistent with the
presence of quark matter in compact stars.Comment: 4 pages, LaTeX; New title and overall rewrite to reflect version
published in Nature. Conclusions unchange
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