6,761 research outputs found
Atomic spin sensitive dissipation on magnetic surfaces
We identify the mechanism of energy dissipation relevant to spin-sensitive
nanomechanics including the recently introduced magnetic exchange force
microscopy, where oscillating magnetic tips approach surface atomic spins. The
tip-surface exchange couples spin and atom coordinates, leading to a
spin-phonon problem with Caldeira-Leggett type dissipation. In the overdamped
regime, that can lead to a hysteretic flip of the local spin with a large
spin-dependent dissipation, even down to the very low experimental tip
oscillation frequencies, describing recent observations for Fe tips on NiO. A
phase transition to an underdamped regime with dramatic drop of magnetic tip
dissipation should in principle be possible by tuning tip-surface distance.Comment: 4 pages, 4 figure
On characters of Chevalley groups vanishing at the non-semisimple elements
Let G be a finite simple group of Lie type. In this paper we study characters
of G that vanish at the non-semisimple elements and whose degree is equal to
the order of a maximal unipotent subgroup of G. Such characters can be viewed
as a natural generalization of the Steinberg character. For groups G of small
rank we also determine the characters of this degree vanishing only at the
non-identity unipotent elements.Comment: Dedicated to Lino Di Martino on the occasion of his 65th birthda
Cooling flows and quasars: II. Detailed models of feedback modulated accretion flows
Most elliptical galaxies contain central black holes (BHs), and most also
contain significant amounts of hot gas capable of accreting on to the central
BH due to cooling times short compared to the Hubble time. Why therefore do we
not see AGNs at the center of most elliptical galaxies rather than in only (at
most) a few per cent of them? We propose here the simple idea that feedback
from accretion events heats the ambient gas retarding subsequent infall. In
this context, we present a class of 1D hydrodynamical evolutionary sequences
for the gas flows in elliptical galaxies with a massive central BH. The
resulting evolution is characterized by strong oscillations, in which very fast
and energetic bursts of the BH are followed by longer periods in which the
X-ray galaxy emission comes from the coronal gas. We also allow for departures
from spherical symmetry by examining scenarios in which the central engine is
either an ADAF or a more conventional accretion disk that is optically thick
except for a polar region. In all cases the duty cycle (fraction of the time
that the system will be seen as an AGN) is quite small and in the range 10^{-4}
- 10^{-3}. Thus, for any reasonable value of the efficiency, the presence of a
massive BH at the center of a galaxy seems to be incompatible with the presence
of a long-lived cooling flow.Comment: 43 pages, 10 figures. Main additions concern observed Compton
temperatures and few extra numerical models. Conclusions unchanged. 1 new
table and 3 new figures. Accepted for publication on ApJ (main journal
Structure and Feedback in 30 Doradus I: Observations
We have completed a a new optical imaging and spectrophotometric survey of a
140 x 80 pc region of 30 Doradus centered on R136, covering key optical
diagnostic emission lines including \Ha, \Hb, \Hg, [O III]
4363, 4959, 5007, [N II] 6548, 6584, [S II]
6717, 6731 [S III] 6312 and in some locations [S III]
9069. We present maps of fluxes and intensity ratios for these lines,
and catalogs of isolated ionizing stars, elephant-trunk pillars, and edge-on
ionization fronts. The final science-quality spectroscopic data products are
available to the public. Our analysis of the new data finds that, while stellar
winds and supernovae undoubtedly produce shocks and are responsible for shaping
the nebula, there are no global spectral signatures to indicate that shocks are
currently an important source of ionization. We conclude that the considerable
region covered by our survey is well described by photoionization from the
central cluster where the ionizing continuum is dominated by the most massive O
stars. We show that if 30 Dor were viewed at a cosmological distance, its
integrated light would be dominated by its extensive regions of lower
surface-brightness rather than by the bright, eye-catching arcs.Comment: 42 pages, 16 figures, Accepted for publication in ApJ
Non Markovian Quantum Repeated Interactions and Measurements
A non-Markovian model of quantum repeated interactions between a small
quantum system and an infinite chain of quantum systems is presented. By
adapting and applying usual pro jection operator techniques in this context,
discrete versions of the integro-differential and time-convolutioness Master
equations for the reduced system are derived. Next, an intuitive and rigorous
description of the indirect quantum measurement principle is developed and a
discrete non Markovian stochastic Master equation for the open system is
obtained. Finally, the question of unravelling in a particular model of
non-Markovian quantum interactions is discussed.Comment: 22 page
Chandra Observations of Low Mass X-ray Binaries and Diffuse Gas in the Early-Type Galaxies NGC 4365 and NGC 4382 (M85)
(Abridged) We used the Chandra X-ray Observatory ACIS S3 to image the X-ray
faint elliptical galaxy NGC 4365 and lenticular galaxy NGC 4382. The
observations resolve much of the X-ray emission into 99 and 58 sources,
respectively, most of which are low-mass X-ray binaries (LMXBs) associated with
each of the galaxies. We identify 18 out of the 37 X-ray sources in a central
field in NGC 4365 with globular clusters. The luminosity functions of the
resolved sources for both galaxies are best fit with cutoff power-laws whose
cutoff luminosity is ergs s. These
luminosities are much larger than those previously measured for similar
galaxies; we do not find evidence for a break in the luminosity function at the
Eddington luminosity of a 1.4 neutron star. The spatial distributions
of the resolved sources for both galaxies are broader than the distribution of
optical stars. In both galaxies, a hard power-law model fits the summed
spectrum of all of the sources. The unresolved emission is best fit by the sum
of a soft mekal model representing emission from diffuse gas, and a hard
power-law, presumed to be from unresolved LMXBs. A standard beta model fits the
radial distribution of the diffuse gas in both galaxies. In the elliptical NGC
4365, the best-fit core radius is very small, while the S0 galaxy NGC 4382 has
a larger core radius. This may indicate that the gas in NGC 4382 is rotating
significantly.Comment: Astrophysical Journal, accepted: 38 pages with 20 embedded reduced
resolution Postscript figure
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