Oscillation modes of relativistic slender tori

Accretion flows with pressure gradients permit the existence of standing waves which may be responsible for observed quasi-periodic oscillations (QPO's) in X-ray binaries. We present a comprehensive treatment of the linear modes of a hydrodynamic, non-self-gravitating, polytropic slender torus, with arbitrary specific angular momentum distribution, orbiting in an arbitrary axisymmetric spacetime with reflection symmetry. We discuss the physical nature of the modes, present general analytic expressions and illustrations for those which are low order, and show that they can be excited in numerical simulations of relativistic tori. The mode oscillation spectrum simplifies dramatically for near Keplerian angular momentum distributions, which appear to be generic in global simulations of the magnetorotational instability. We discuss our results in light of observations of high frequency QPO's, and point out the existence of a new pair of modes which can be in an approximate 3:2 ratio for arbitrary black hole spins and angular momentum distributions, provided the torus is radiation pressure dominated. This mode pair consists of the axisymmetric vertical epicyclic mode and the lowest order axisymmetric breathing mode.Comment: submitted to MNRA

6 hr tide seen in sporadic E layers

The GPS radio occultation technique is used to study sporadic E (Es) layer plasma irregularities of the Earth’s ionosphere on a global scale using COSMIC/FORMOSAT-3 satellite constellation GPS signal-to-noise ratio (SNR) profiles. The maximum deviation from the mean SNR is attributed to the height of the Es layer. Es are produced by ion convergence due to vertical wind shear in the presence of a horizontal component of the Earth magnetic field, while the wind shear is provided mainly by solar tides. Indeed, close correlation between Es and wind shear phases have already been found for the semidiurnal and terdiurnal tidal components. Here, we present the global distribution of quarterdiurnal (QDT) signatures in Es occurrence rates. We find that, in accordance with upward energy flux, negative vertical phase gradients of QDT Es signatures are observed. The maximum signal of QDT Es is found at altitudes above 100 km. In the southern hemisphere, maximum QDT Es occurrence rates are found in winter and during equinoxes. In the northern hemisphere, however, at altitudes above 100 km strong QDT activity is also visible.Die GPS-Radiookkultationstechnik wird verwendet, um ionosphärische Plasmairregularitäten in Verbindung mit sporadischen E- (Es) Schichten auf globaler Skala zu untersuchen. Verwendet werden Signal-Rauschverhältnis- (SNR-) Profile. Die maximale Abweichung vom mittleren SNR wird der Höhe der Es-Schicht zugeordnet. Es werden durch Ionenkonvergenz aufgrund von vertikaler Windscherung in Anwesenheit einer horizontalen Komponente des Erdmagnetfeldes hervorgerufen, wobei die Scherung hauptsächlich durch solare Gezeiten verursacht wird. Tatsächlich wurden schon früher deutliche Übereinstimmungen zwischen dem Auftreten von Es und den Phasen der halb- und dritteltägigen Gezeiten gefunden. Hier stellen wir die globale Verteilung der vierteltägigen (QDT) Signaturen in Es- Auftretensraten vor. Es zeigt sich dass, in Übereinstimmung mit einem aufwärts gerichteten Energietransport, negative vertikale Phasengradienten der QDT in Es auftreten. Die maximale Auftretenswahrscheinlichkeit liegt bei Höhen oberhalb von 100 km. Auf der Südhemisphäre fällt das Maximum der QDT in Es-Auftretensraten in den Winter, während auf der Nordhemisphäre oberhalb von 100 km auch im Sommer starke QDT-Aktivität zu verzeichnen ist

Strain and correlation of self-organized Ge_(1-x)Mn_x nanocolumns embedded in Ge (001)

We report on the structural properties of Ge_(1-x)Mn_x layers grown by molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray scattering, atomic force and transmission electron microscopy to study the structural properties of the columns. We demonstrate how the elastic deformation of the matrix (as calculated using atomistic simulations) around the columns, as well as the average inter-column distance can account for the shape of the diffusion around Bragg peaks.Comment: 9 pages, 7 figure

Migrating and nonmigrating tidal signatures in sporadic E layer occurrence rates

We analyse sporadic E (ES) layer occurrence rates (OR) obtained from ionospheric GPS radio occultation measurements by the FORMOSAT-3/COSMIC constellation. Maximum OR are seen at 95–105 km altitude. Midlatitude ES layers are mainly due to wind shear in the presence of tides, and the strongest signals are the migrating diurnal and semidiurnal components. Especially in the Southern Hemisphere, nonmigrating components such as a diurnal westward wave 2 and a semidiurnal westward wave 1 are also visible, especially at higher latitudes. Near the equator, a strong diurnal eastward wavenumber 3 component and a semidiurnal eastward wavenumber 2 component occur in summer and autumn. Terdiurnal and quarterdiurnal components are weaker than the diurnal and semidiurnal ones.</p

Hydrogen Burning on Magnetar Surfaces

We compute the rate of diffusive nuclear burning for hydrogen on the surface of a "magnetar" (Soft Gamma-Ray Repeater or Anomalous X-Ray Pulsar). We find that hydrogen at the photosphere will be burned on an extremely rapid timescale of hours to years, depending on composition of the underlying material. Improving on our previous studies, we explore the effect of a maximally thick "inert" helium layer, previously thought to slow down the burning rate. Since hydrogen diffuses faster in helium than through heavier elements, we find this helium buffer actually increases the burning rate for magnetars. We compute simple analytic scalings of the burning rate with temperature and magnetic field for a range of core temperature. We conclude that magnetar photospheres are very unlikely to contain hydrogen. This motivates theoretical work on heavy element atmospheres that are needed to measure effective temperature from the observed thermal emission and constrains models of AXPs that rely on magnetar cooling through thick light element envelopes.Comment: 4 pages, 2 figures, To be published in ApJ Letter

Can Parity Violation in Neutrino Transport Lead to Pulsar Kicks?

In magnetized proto-neutron stars, neutrino cross sections depend asymmetrically on the neutrino momenta due to parity violation. However, these asymmetric opacities do not induce any asymmetric flux in the bulk interior of the star where neutrinos are nearly in thermal equilibrium. Consequently, parity violation in neutrino absorption and scattering can only give rise to asymmetric neutrino flux above the neutrino-matter decoupling layer. The kick velocity is substantially reduced from previous estimates, requiring a dipole field $B \sim 10^{16}$~G to get $v_{kick}$ of order a few hundred km~s$^{-1}$.Comment: REVTEX, 4 pages, no figures. Submitted to Phys. Rev. Letter

R-modes in Neutron Stars with Crusts: Turbulent Saturation, Spin-down, and Crust Melting

Rossby waves (r-modes) have been suggested as a means to regulate the spin periods of young or accreting neutron stars, and also to produce observable gravitational wave radiation. R-modes involve primarily transverse, incompressive motions of the star's fluid core. However, neutron stars gain crusts early in their lives: therefore, r-modes also imply shear in the fluid beneath the crust. We examine the criterion for this shear layer to become turbulent, and derive the rate of dissipation in the turbulent regime. Unlike dissipation from a viscous boundary layer, turbulent energy loss is nonlinear in mode energy and can therefore cause the mode to saturate at amplitudes typically much less than unity. This energy loss also reappears as heat below the crust. We study the possibility of crust melting as well as its implications for the spin evolution of low-mass X-ray binaries. Lastly, we identify some universal features of the spin evolution that may have observational consequences.Comment: 12 pages, 4 figures, submitted to Ap