847 research outputs found
The 67 Hz Feature in the Black Hole Candidate GRS 1915+105 as a Possible ``Diskoseismic'' Mode
The Rossi X-ray Timing Explorer (RXTE) has made feasible for the first time
the search for high-frequency (~ 100 Hz) periodic features in black hole
candidate (BHC) systems. Such a feature, with a 67 Hz frequency, recently has
been discovered in the BHC GRS 1915+105 (Morgan, Remillard, & Greiner). This
feature is weak (rms variability ~0.3%-1.6%), stable in frequency (to within ~2
Hz) despite appreciable luminosity fluctuations, and narrow (quality factor Q ~
20). Several of these properties are what one expects for a ``diskoseismic''
g-mode in an accretion disk about a 10.6 M_sun (nonrotating) - 36.3 M_sun
(maximally rotating) black hole (if we are observing the fundamental mode
frequency). We explore this possibility by considering the expected luminosity
modulation, as well as possible excitation and growth mechanisms---including
turbulent excitation, damping, and ``negative'' radiation damping. We conclude
that a diskoseismic interpretation of the observations is viable.Comment: 4 Pages, Latex (emulateapj.sty included), to Appear in ApJ Letters,
Vol. 477, Final Version with Updated Reference
Surface and volume effects in the photoabsorption of nuclei
Recent experimental results for meson photoproduction from nuclei obtained
with TAPS at MAMI are analyzed in view of the suppression of the second nucleon
resonance region in total photoabsorption. The cross sections can be split into
a component from the low density surface region of nuclei and a component which
scales more like the nuclear volume. The energy dependence of the surface
component is similar to the deuteron cross section, it shows a clear signal for
the second resonance peak assigned to the excitation of the P11(1440),
D13(1520), and S11(1535). The volume component behaves differently, it is
lacking the second resonance peak and shows an enhancement at intermediate
photon energies.Comment: accepted for publication in Eur. J. Phys.
AnĂĄlise da composição fĂsico-quĂmica de salames coloniais e bacons comercializados no municĂpio de ConcĂłrdia, Santa Catarina.
Projeto/Plano de Ação: 01.06.06.001
Photoproduction of pi0-mesons from nuclei
Photoproduction of neutral pions from nuclei (carbon, calcium, niobium, lead)
has been studied for incident photon energies from 200 MeV to 800 MeV with the
TAPS detector using the Glasgow photon tagging spectrometer at the Mainz MAMI
accelerator. Data were obtained for the inclusive photoproduction of neutral
pions and the partial channels of quasifree single pi0, double pi0, and
pi0pi+/- photoproduction. They have been analyzed in terms of the in-medium
behavior of nucleon resonances and the pion - nucleus interaction. They are
compared to earlier measurements from the deuteron and to the predictions of a
Boltzmann-Uehling-Uhlenbeck (BUU) transport model for photon induced pion
production from nuclei.Comment: 15 pages, 22 figures, accepted for publication in EPJ
Relativistic Diskoseismology. I. Analytical Results for 'Gravity Modes'
We generalize previous calculations to a fully relativistic treatment of
adiabatic oscillations which are trapped in the inner regions of accretion
disks by non-Newtonian gravitational effects of a black hole. We employ the
Kerr geometry within the scalar potential formalism of Ipser and Lindblom,
neglecting the gravitational field of the disk. This approach treats
perturbations of arbitrary stationary, axisymmetric, perfect fluid models. It
is applied here to thin accretion disks. Approximate analytic eigenfunctions
and eigenfrequencies are obtained for the most robust and observable class of
modes, which corresponds roughly to the gravity (internal) oscillations of
stars. The dependence of the oscillation frequencies on the mass and angular
momentum of the black hole is exhibited. These trapped modes do not exist in
Newtonian gravity, and thus provide a signature and probe of the strong-field
structure of black holes. Our predictions are relevant to observations which
could detect modulation of the X-ray luminosity from stellar mass black holes
in our galaxy and the UV and optical luminosity from supermassive black holes
in active galactic nuclei.Comment: 31 pages, 6 figures, uses style file aaspp4.sty, prepared with the
AAS LATEX macros v4.0, significant revision of earlier submission to include
modes with axial index m>
Elaboração de embutido fermentado adicionado de carne de frango.
Projeto/Plano de Ação: 11.11.11.111
Status Inconsistency, Status Aspiration, Task Mobility, and Preferences for Specialization and Despecialization of Group Task Structure
This technical report is a further elaboration of the exchange-based theory of status consistency and individualsâ reactions to various patterns that was presented in technical report 21. The extended theory predicts mobility and preferences for more and less differentiated task structures as outcomes of different patterns of inconsistency. Experimental test results were generally confirmatory
Modes of Status Equilibration
This is a revision of TR#23, focusing on the experimental tests
Effect of applied field and rate of voltage rise on surface breakdown of oil-immersed polymers
In sub-systems of high-voltage, pulsed-power machines, the introduction of a solid into bulk liquid insulation located between two conductors is often necessary to provide mechanical support. Breakdown events on or around the surface of the solid can result in permanent damage to the insulation system. Described in the present paper are experimental results pertaining to surface breakdown of five different solid dielectrics held between plane-parallel electrodes immersed in mineral oil. The effect of varying level of peak applied field from 200 kV/cm (dV/dt 70 kV/”s) to 1 MV/cm (dV/dt 350 kV/”s) is investigated, and the breakdown voltages and times to breakdown are compared to those for an open oil gap. The time to breakdown is shown to be reduced by the introduction of a solid spacer into the gap. Rexolite and Torlon samples suffered significant mechanical damage, and consistently showed lower breakdown voltage than the other materials - average streamer propagation velocity up to 125 km/s was implied by the short times to breakdown. Although ultra-high molecular weight polyethylene yielded the longest times to breakdown of the five types of liquid-solid gap, breakdown events could be initiated at lower levels of applied field for spacers of this material than those with permittivity closely matched to that of the surrounding mineral oil. Polypropylene and low-density polyethylene are concluded to provide the most stable performance in mineral oil. Due to the similarity of the applied voltage wave-shape (1/6.5 ”s) to short-tail lightning impulses, the results may also be of interest to high-voltage system designers in the power industry
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