2,447 research outputs found
Structure and Magnetic Fields in the Precessing Jet System SS 433 II. Intrinsic Brightness of the Jets
Deep Very Large Array imaging of the binary X-ray source SS 433, sometimes
classified as a microquasar, has been used to study the intrinsic brightness
distribution and evolution of its radio jets. The intrinsic brightness of the
jets as a function of age at emission of the jet material tau is recovered by
removal of the Doppler boosting and projection effects. We find that
intrinsically the two jets are remarkably similar when compared for equal tau,
and that they are best described by Doppler boosting of the form D^{2+alpha},
as expected for continuous jets. The intrinsic brightnesses of the jets as
functions of age behave in complex ways. In the age range 60 < tau < 150 days,
the jet decays are best represented by exponential functions of tau, but linear
or power law functions are not statistically excluded. This is followed by a
region out to tau ~ 250 days during which the intrinsic brightness is
essentially constant. At later times the jet decay can be fit roughly as
exponential or power law functions of tau.Comment: 30 Pages, 11 Figures, Submitted to Ap
A database of schemes that prioritize sites and species based on their conservation value: focusing business on biodiversity
<p>Abstract</p> <p>Background</p> <p>Biodiversity offsets are conservation projects used mainly by business to counterbalance the environmental impacts of their operations, with the aim of achieving a net neutral or even beneficial outcome for biodiversity. Companies considering offsets need to know: (1) if there are areas of such biological importance that no impact is acceptable, and outside of these <it>no-go areas</it>, (2) the relative importance of biodiversity in the impacted site versus the site(s) proposed for protection, to ensure that the offset is of equal or greater status than that lost through the company's operations. We compiled a database of 40 schemes that use various methods to assess conservation priorities, and we examined if the schemes would allow companies to answer the above questions.</p> <p>Description</p> <p>Overall, schemes tend to be designed to guide conservation organizations in their own priority setting or they categorize species based on conservation status. Generally, the schemes do not provide all the necessary information for offsets because they operate at a broad spatial scale or with low spatial resolution, which make it difficult to assess sites at the project level. Furthermore, most schemes do not explicitly incorporate <it>threat</it>, which we consider key to assessing whether offsets protect habitats or species that would otherwise be lost (<it>i.e</it>., provide <it>additionality</it>). The schemes are useful, however, for identifying the major conservation issues in different ecosystems around the globe.</p> <p>Conclusion</p> <p>Companies can proceed by first avoiding, reducing, and mitigating impacts, and then using existing schemes to identify i) no-go areas and ii) appropriate offsets to compensate for any unavoidable loss in biodiversity. If existing schemes are inadequate, then companies should use integrated conservation planning techniques to define offset options within the region of their operations.</p
Two-loop self-energy correction in high-Z hydrogen-like ions
A complete evaluation of the two-loop self-energy diagrams to all orders in
Z\alpha is presented for the ground state of H-like ions with Z\ge 40.Comment: RevTeX, 5 figures, 1 tabl
Measurement of the 6S-7S transition polarizablility in atomic cesium and an improved test of the standard model
The ratio of the off-diagonal hyperfine amplitude to the tensor transition
polarizability (Mhf/beta) for the 6S-7S transition in cesium has been measured.
The value of beta=27.024(43)(expt)(67)(theory)a_0^3 is then obtained using an
accurate semi-empirical value of Mhf. This is combined with a previous
measurement of parity nonconservation in atomic cesium and previous atomic
structure calculations to determine the value of the weak charge. The
uncertainties in the atomic structure calculations are updated (and reduced) in
light of new experimental tests. The result Q_W=-72.06(28)(expt) (34)(theory)
differs from the prediction of the standard model of elementary particle
physics by 2.5 sigma.Comment: 12 pages, 1 figur
Breit interaction correction to the hyperfine constant of an external s-electron in many-electron atom
Correction to the hyperfine constant of an external s-electron in
many-electron atom caused by the Breit interaction is calculated analytically:
. Physical mechanism for this correction is
polarization of the internal electronic shells (mainly shell) by the
magnetic field of the external electron. This mechanism is similar to the
polarization of vacuum considered by Karplus and Klein long time ago. The
similarity is the reason why in both cases (Dirac sea polarization and internal
atomic shells polarization) the corrections have the same dependence on the
nuclear charge and fine structure constant.
In conclusion we also discuss corrections to the parity violation
effects in atoms.Comment: 8 pages, 2 figure
Why do gallium clusters have a higher melting point than the bulk?
Density functional molecular dynamical simulations have been performed on
Ga and Ga clusters to understand the recently observed
higher-than-bulk melting temperatures in small gallium clusters [Breaux {\em et
al.}, Phys. Rev. Lett. {\bf 91}, 215508 (2003)]. The specific-heat curve,
calculated with the multiple-histogram technique, shows the melting temperature
to be well above the bulk melting point of 303 K, viz. around 650 K and 1400 K
for Ga and Ga, respectively. The higher-than-bulk melting
temperatures are attributed mainly to the covalent bonding in these clusters,
in contrast with the covalent-metallic bonding in the bulk.Comment: 4 pages, including 6 figures. accepted for publication in Phys. Rev.
Let
XMM-Newton observations of the eastern jet of SS433
The radio supernova remnant W50 hosts at its center the peculiar galactic
X-ray binary SS 433. It shows a central spherical structure with two `ears'
which are supposed to be formed by the interaction of the precessing jets of SS
433 with the supernova shell. In two pointings in September/October 2004 for 30
ks each the eastern jet of SS 433 was observed with XMM-Newton to study the
outermost parts of the `ear' and the X-ray bright emission region about 35
arcmin from SS 433. The spectra consist of two components: a non-thermal power
law with photon index \Gamma ~ 2.17+/-0.02 and a thermal component at a typical
temperature of kT ~ 0.3 keV. The X-ray emission seems to fill the whole
interior region of the radio remnant W50. The jet terminates in the eastern
`ear' in a ring-like terminal shock which indicates a flow with a kind of
hollow-cone morphology. The spatial coincidence of X-ray and radio emission
suggests physical conditions similar to those found at the outer shocks of
ordinary supernova remnants. The bright emission region closer to SS 433
radiates non-thermally in a spatially well confined geometry at higher X-ray
energies. At soft X-rays the shape of the region gets blurred, centered on the
hard lenticular emission. The shape of this region and the bend in the jet
propagation direction might be caused by the interaction of a re-collimated jet
with the outer, non homogeneous interstellar matter distribution. The physical
conditions leading to the re-collimation of the jet and the peculiar emission
morphology are far from being understood and require deeper observations as
well as a detailed modeling of the interaction of a jet with its surroundings.Comment: 10 pages, 8 figures, to appear in A&
An analytically solvable model of the effect of magnetic breakdown on angle-dependent magnetoresistance in a quasi-two-dimensional metal
We have developed an analytical model of angle-dependent magnetoresistance
oscillations (AMROs) in a quasi-two-dimensional metal in which magnetic
breakdown occurs. The model takes account of all the contributions from
quasiparticles undergoing both magnetic breakdown and Bragg reflection at each
junction and allows extremely efficient simulation of data which can be
compared with recent experimental results on the organic metal
kappa-ET2Cu(NCS)2. AMROs resulting from both closed and open orbits emerge
naturally at low field, and the model enables the transition to breakdown-AMROs
with increasing field to be described in detail.Comment: 4 pages, 3 figure
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