2,718 research outputs found
Baryon Loading of AGN Jets Mediated by Neutrons
Plasmas of geometrically thick, black hole (BH) accretion flows in active
galactic nuclei (AGNs) are generally collisionless for protons, and involve
magnetic field turbulence. Under such conditions a fraction of protons can be
accelerated stochastically and create relativistic neutrons via nuclear
collisions. These neutrons can freely escape from the accretion flow and decay
into protons in dilute polar region above the rotating BH to form relativistic
jets. We calculate geometric efficiencies of the neutron energy and mass
injections into the polar region, and show that this process can deposit
luminosity as high as L_j ~ 2e-3 dot{M} c^2 and mass loading dot{M}_j ~ 6e-4
dot{M} for the case of the BH mass M ~ 1e8 M_sun, where dot{M} is mass
accretion rate. The terminal Lorentz factors of the jets are Gamma ~ 3, and
they may explain the AGN jets having low luminosities. For higher luminosity
jets, which can be produced by additional energy inputs such as Poynting flux,
the neutron decay still can be a dominant mass loading process, leading to
e.g., Gamma ~ 50 for L_{j,tot} ~ 3e-2 dot{M}c^2.Comment: 7 pages, 6 figures; accepted for publication in Ap
Measuring Black Hole Spin by the Continuum-Fitting Method: Effect of Deviations from the Novikov-Thorne Disc Model
The X-ray spectra of accretion discs of eight stellar-mass black holes have
been analyzed to date using the thermal continuum fitting method, and the
spectral fits have been used to estimate the spin parameters of the black
holes. However, the underlying model used in this method of estimating spin is
the general relativistic thin-disc model of Novikov & Thorne, which is only
valid for razor-thin discs. We therefore expect errors in the measured values
of spin due to inadequacies in the theoretical model. We investigate this issue
by computing spectra of numerically calculated models of thin accretion discs
around black holes, obtained via three-dimensional general relativistic
magnetohydrodynamic (GRMHD) simulations. We apply the continuum fitting method
to these computed spectra to estimate the black hole spins and check how
closely the values match the actual spin used in the GRMHD simulations. We find
that the error in the dimensionless spin parameter is up to about 0.2 for a
non-spinning black hole, depending on the inclination. For black holes with
spins of 0.7, 0.9 and 0.98, the errors are up to about 0.1, 0.03 and 0.01
respectively. These errors are comparable to or smaller than those arising from
current levels of observational uncertainty. Furthermore, we estimate that the
GRMHD simulated discs from which these error estimates are obtained correspond
to effective disc luminosities of about 0.4-0.7 Eddington, and that the errors
will be smaller for discs with luminosities of 0.3 Eddington or less, which are
used in the continuum-fitting method. We thus conclude that use of the
Novikov-Thorne thin-disc model does not presently limit the accuracy of the
continuum-fitting method of measuring black hole spin.Comment: 13 pages, 7 figures, accepted for publication in MNRAS. v2: fixed
typo in author name, updated acknowledgment
A mechanism for biogenic production and emission of MEK from MVK decoupled from isoprene biosynthesis
Methyl ethyl ketone (MEK) is an important compound in atmospheric chemistry.
While attention has been paid mostly to anthropogenic sources of MEK,
recently it has been shown that biogenic sources are globally as important as
anthropogenic ones. However, the origin of biogenic MEK has yet to be
completely elucidated. We present the full mechanism by which within-plant
transformation of methyl vinyl ketone (MVK) and, to a minor extent, of
2-butanol and 3-buten-2-ol, is a source of biogenic MEK. Such transformation
is observed in red oak for both exogenous MVK, taken up from the atmosphere,
and endogenous MVK generated within a plant when it experiences stress (e.g. heat stress).
Endogenous MVK emitted by plants is typically explained by within-plant
oxidation of isoprene caused by oxidative stress. In this study we show that
MVK and MEK emissions caused by heat stress are not related to isoprene in
isoprene-emitting plants, implying that the massive carbon investment that
plants commit to isoprene production is not explained by a direct antioxidant
role. The presented mechanism can be important for inclusion in plant
emission and in plant–atmosphere interaction models.</p
Resistive and magnetized accretion flows with convection
We considered the effects of convection on the radiatively inefficient
accretion flows (RIAF) in the presence of resistivity and toroidal magnetic
field. We discussed the effects of convection on transports of angular momentum
and energy. We established two cases for the resistive and magnetized RIAFs
with convection: assuming the convection parameter as a free parameter and
using mixing-length theory to calculate convection parameter. A self-similar
method was used to solve the integrated equations that govern the behavior of
the presented model. The solutions showed that the accretion and rotational
velocities decrease by adding the convection parameter, while the sound speed
increases. Moreover, by using mixing-length theory to calculate convection
parameter, we found that the convection can be important in RIAFs with magnetic
field and resistivity.Comment: 7 pages, 3 figures, accepted by Ap&S
A case-control study of oesophageal adenocarcinoma in women: a preventable disease
The incidence of adenocarcinoma of the oesophagus in British women is among the highest in the world. To investigate its aetiology, we conducted a multi-centre, population based case–control study in four regions in England and Scotland. We included 74 incident cases in women with histologically confirmed diagnoses of adenocarcinoma of the oesophagus, and 74 female controls matched by age and general practice. High body mass index (BMI) around the age of 20 years (highest vs lowest quartile, adjusted odds ratio (OR) = 6.04, 95% confidence interval (CI) 1.28–28.52) and low consumption of fruit (highest vs lowest quartile, adjusted OR = 0.08, 95% Cl 0.01–0.49) were associated with increases in risk. Breastfeeding by women was associated with reduced risk of their subsequently developing this cancer (ever vs never, adjusted OR = 0.41, 95% CI 0.20–0.82) and there was a significant dose–response effect with total duration of breastfeeding. The summary population attributable risk from these three factors was 96% (90% if breastfeeding is excluded). We conclude that high BMI in early adulthood and low consumption of fruit are important risk factors for adenocarcinoma of the oesophagus. Breastfeeding may confer a protective effect but this needs confirmation. This cancer is a largely preventable disease in women. © 2000 Cancer Research Campaig
Spontaneous DC Current Generation in a Resistively Shunted Semiconductor Superlattice Driven by a TeraHertz Field
We study a resistively shunted semiconductor superlattice subject to a
high-frequency electric field. Using a balance equation approach that
incorporates the influence of the electric circuit, we determine numerically a
range of amplitude and frequency of the ac field for which a dc bias and
current are generated spontaneously and show that this region is likely
accessible to current experiments. Our simulations reveal that the Bloch
frequency corresponding to the spontaneous dc bias is approximately an integer
multiple of the ac field frequency.Comment: 8 pages, Revtex, 3 Postscript figure
The rarity of terrestrial gamma-ray flashes
We report on the first search for Terrestrial Gamma-ray Flashes (TGFs) from altitudes where they are thought to be produced. The Airborne Detector for Energetic Lightning Emissions (ADELE), an array of gamma-ray detectors, was flown near the tops of Florida thunderstorms in August/September 2009. The plane passed within 10 km horizontal distance of 1213 lightning discharges and only once detected a TGF. If these discharges had produced TGFs of the same intensity as those seen from space, every one should have been seen by ADELE. Separate and significant nondetections are established for intracloud lightning, negative cloud-to-ground lightning, and narrow bipolar events. We conclude that TGFs are not a primary triggering mechanism for lightning. We estimate the TGF-to-flash ratio to be on the order of 10^(−2) to 10^(−3) and show that TGF intensities cannot follow the well-known power-law distribution seen in earthquakes and solar flares, due to our limits on the presence of faint events
Bose-Einstein condensation in variable dimensionality
We introduce dimensional perturbation techniques to Bose-Einstein
condensation of inhomogeneous alkali gases (BEC). The perturbation parameter is
delta=1/kappa, where kappa depends on the effective dimensionality of the
condensate and on the angular momentum quantum number. We derive a simple
approximation that is more accurate and flexible than the N -> infinity
Thomas-Fermi ground state approximation (TFA) of the Gross-Pitaevskii equation.
The approximation presented here is well-suited for calculating properties of
states in three dimensions and in low effective dimensionality, such as vortex
states in a highly anisotropic trap
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