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$^2$ region of 30 Doradus centered on R136, covering key optical diagnostic emission lines including \Ha, \Hb, \Hg, [O III] $\lambda\lambda$4363, 4959, 5007, [N II] $\lambda\lambda$6548, 6584, [S II] $\lambda\lambda$6717, 6731 [S III] $\lambda$6312 and in some locations [S III] $\lambda$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 $\approx 0.9 - 3.1 \times 10^{39}$ ergs s$^{-1}$. 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 $M_\odot$ 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