Parallel Performance for a Real Time Lattice Boltzmann Code

The paper will present the details of a Lattice Boltzmann solver running in real time for unsteady wake computations. In addition to algorithmic implementation, computational results, single core and parallel optimization of the methods are also discussed

Linear Two-Dimensional MHD of Accretion Disks: Crystalline structure and Nernst coefficient

We analyse the two-dimensional MHD configurations characterising the steady state of the accretion disk on a highly magnetised neutron star. The model we describe has a local character and represents the extension of the crystalline structure outlined in Coppi (2005), dealing with a local model too, when a specific accretion rate is taken into account. We limit our attention to the linearised MHD formulation of the electromagnetic back-reaction characterising the equilibrium, by fixing the structure of the radial, vertical and azimuthal profiles. Since we deal with toroidal currents only, the consistency of the model is ensured by the presence of a small collisional effect, phenomenologically described by a non-zero constant Nernst coefficient (thermal power of the plasma). Such an effect provides a proper balance of the electron force equation via non zero temperature gradients, related directly to the radial and vertical velocity components. We show that the obtained profile has the typical oscillating feature of the crystalline structure, reconciled with the presence of viscosity, associated to the differential rotation of the disk, and with a net accretion rate. In fact, we provide a direct relation between the electromagnetic reaction of the disk and the (no longer zero) increasing of its mass per unit time. The radial accretion component of the velocity results to be few orders of magnitude below the equatorial sound velocity. Its oscillating-like character does not allow a real matter in-fall to the central object (an effect to be searched into non-linear MHD corrections), but it accounts for the out-coming of steady fluxes, favourable to the ring-like morphology of the disk.Comment: 15 pages, 1 figure, accepted for publication on Modern Physics Letters

The alignment of disk and black hole spins in active galactic nuclei

The inner parts of an accretion disk around a spinning black hole are forced to align with the spin of the hole by the Bardeen-Petterson effect. Assuming that any jet produced by such a system is aligned with the angular momentum of either the hole or the inner disk, this can, in principle provide a mechanism for producing steady jets in AGN whose direction is independent of the angular momentum of the accreted material. However, the torque which aligns the inner disk with the hole, also, by Newton's third law, tends to align the spin of the hole with the outer accretion disk. In this letter, we calculate this alignment timescale for a black hole powering an AGN, and show that it is relatively short. This timescale is typically much less than the derived ages for jets in radio loud AGN, and implies that the jet directions are not in general controlled by the spin of the black hole. We speculate that the jet directions are most likely controlled either by the angular momentum of the accreted material or by the gravitational potential of the host galaxy.Comment: 4 pages, LateX file, accepted for publication in ApJ Letter

Variability Profiles of Millisecond X-Ray Pulsars: Results of Pseudo-Newtonian 3D MHD Simulations

We model the variability profiles of millisecond period X-ray pulsars. We performed three-dimensional magnetohydrodynamic simulations of disk accretion to millisecond period neutron stars with a misaligned magnetic dipole moment, using the pseudo-Newtonian Paczynski-Wiita potential to model general relativistic effects. We found that the shapes of the resulting funnel streams of accreting matter and the hot spots on the surface of the star are quite similar to those for more slowly rotating stars obtained from earlier simulations using the Newtonian potential. The funnel streams and hot spots rotate approximately with the same angular velocity as the star. The spots are bow-shaped (bar-shaped) for small (large) misalignment angles. We found that the matter falling on the star has a higher Mach number when we use the Paczynski-Wiita potential than in the Newtonian case. Having obtained the surface distribution of the emitted flux, we calculated the variability curves of the star, taking into account general relativistic, Doppler and light-travel-time effects. We found that general relativistic effects decrease the pulse fraction (flatten the light curve), while Doppler and light-travel-time effects increase it and distort the light curve. We also found that the light curves from our hot spots are reproduced reasonably well by spots with a gaussian flux distribution centered at the magnetic poles. We also calculated the observed image of the star in a few cases, and saw that for certain orientations, both the antipodal hot spots are simultaneously visible, as noted by earlier authors.Comment: 9 pages, 10 figures, accepted for publication in ApJ; corrected some typo

Two-dimensional radiation-hydrodynamic model for limit-cycle oscillations of luminous accretion disks

We investigate the time evolution of luminous accretion disks around black holes, conducting the two-dimensional radiation-hydrodynamic simulations. We adopt the alpha prescription for the viscosity. The radial-azimuthal component of viscous stress tensor is assumed to be proportional to the total pressure in the optically thick region, while the gas pressure in the optically thin regime. The viscosity parameter, alpha, is taken to be 0.1. We find the limit-cycle variation in luminosity between high and low states. When we set the mass input rate from the outer disk boundary to be 100 L_E/c^2, the luminosity suddenly rises from 0.3L_E to 2L_E, where L_E is the Eddington luminosity. It decays after retaining high value for about 40 s. Our numerical results can explain the variation amplitude and duration of the recurrent outbursts observed in microquasar, GRS 1915+105. We show that the multi-dimensional effects play an important role in the high-luminosity state. In this state, the outflow is driven by the strong radiation force, and some part of radiation energy dissipated inside the disk is swallowed by the black hole due to the photon-trapping effects. This trapped luminosity is comparable to the disk luminosity. We also calculate two more cases: one with a much larger accretion rate than the critical value for the instability and the other with the viscous stress tensor being proportional to the gas pressure only even when the radiation pressure is dominant. We find no quasi-periodic light variations in these cases. This confirms that the limit-cycle behavior found in the simulations is caused by the disk instability.Comment: 6 pages, 4 figures, accepted for publication in ApJ (ApJ 01 April 2006, v640, 2 issue

Boundary layer on the surface of a neutron star

In an attempt to model the accretion onto a neutron star in low-mass X-ray binaries, we present two-dimensional hydrodynamical models of the gas flow in close vicinity of the stellar surface. First we consider a gas pressure dominated case, assuming that the star is non-rotating. For the stellar mass we take M_{\rm star}=1.4 \times 10^{-2} \msun and for the gas temperature $T=5 \times 10^6$ K. Our results are qualitatively different in the case of a realistic neutron star mass and a realistic gas temperature of $T\simeq 10^8$ K, when the radiation pressure dominates. We show that to get the stationary solution in a latter case, the star most probably has to rotate with the considerable velocity.Comment: 7 pages, 7 figure

Proteolytic processing of QSOX1A ensures efficient secretion of a potent disulfide catalyst

QSOX1 (quiescin sulfhydryl oxidase 1) efficiently catalyses the insertion of disulfide bonds into a wide range of proteins. The enzyme is mechanistically well characterized, but its subcellular location and the identity of its protein substrates remain ill-defined. The function of QSOX1 is likely to involve disulfide formation in proteins entering the secretory pathway or outside the cell. In the present study, we show that this enzyme is efficiently secreted from mammalian cells despite the presence of a transmembrane domain. We identify internal cleavage sites and demonstrate that the protein is processed within the Golgi apparatus to yield soluble enzyme. As a consequence of this efficient processing, QSOX1 is probably functional outside the cell. Also, QSOX1 forms a dimer upon cleavage of the C-terminal domain. The processing of QSOX1 suggests a novel level of regulation of secretion of this potent disulfide catalyst and producer of hydrogen peroxide

Megamaser Disks in Active Galactic Nuclei

Recent spectroscopic and VLBI-imaging observations of bright extragalactic water maser sources have revealed that the megamaser emission often originates in thin circumnuclear disks near the centers of active galactic nuclei (AGNs). Using general radiative and kinematic considerations and taking account of the observed flux variability, we argue that the maser emission regions are clumpy, a conclusion that is independent of the detailed mechanism (X-ray heating, shocks, etc.) driving the collisionally pumped masers. We examine scenarios in which the clumps represent discrete gas condensations (i.e., clouds) and do not merely correspond to velocity irregularities in the disk. We show that even two clouds that overlap within the velocity coherence length along the line of sight could account (through self-amplification) for the entire maser flux of a high-velocity ``satellite'' feature in sources like NGC 4258 and NGC 1068, and we suggest that cloud self-amplification likely contributes also to the flux of the background-amplifying ``systemic'' features in these objects. Analogous interpretations have previously been proposed for water maser sources in Galactic star-forming regions. We argue that this picture provides a natural explanation of the time-variability characteristics of extragalactic megamaser sources and of their apparent association with Seyfert 2-like galaxies. We also show that the requisite cloud space densities and internal densities are consistent with the typical values of nuclear (broad emission-line region-type) clouds.Comment: 55 pages, 7 figures, AASTeX4.0, to appear in The Astrophysical Journal (1999 March 1 issue

On the Relative Surface Density Change of Thermally Unstable Accretion Disks

The relations among the relative changes of surface density, temperature, disk height and vertical integrated pressure in three kinds of thermally unstable accretion disks were quantitatively investigated by assuming local perturbations. The surface density change was found to be very small in the long perturbation wavelength case but can not be ignored in the short wavelength case. It becomes significant in an optically thin, radiative cooling dominated disk when the perturbation wavelength is shorter than 15H (H is the scale height of disk) and in a geometrically thin, optically thick and radiation pressure dominated disk when the perturbation wavelength is shorter than 50H. In an optically thick, advection-dominated disk, which is thermally unstable against short wavelength perturbations, the relative surface density change is much larger. We proved the positive correlation between the changes of surface density and temperature in an optically thick, advection- dominated disk which was previously claimed to be the essential point of its thermal instability. Moreover, we found an anticorrelation between the changes of disk height and temperature in an optically thick, advection-dominated disk. This is the natural result of the absence of appreciable vertical integrated pressure change.Comment: 10 pages AAS LaTex file, 3 figures, accepted for publication in Ap

Evaluation of the DigiBete App, a Self-Management App for Type 1 Diabetes: Experiences of Young People, Families and Healthcare Professionals

Abstract: Type 1 Diabetes (T1DM) is a public health issue for children, young people, and families (CYPF) requiring innovative interventions. The DigiBete app is a self-management and educa-tional app to help CYPF and healthcare professionals (HCPs) manage T1DM and features edu-cational advice and resources such as, guidance, quizzes, and educational and instructional videos on how to manage T1DM. To assess the impact and implementation of the app, the service level evaluation deployed a mixed-methods design. App data was captured via the DigiBete platform and an online survey with a non-probability sample of HCPs (N=178) and CYPF (N=1,165) = 1,343. Overall, 55.7% (n=512/919) of app users were female and 4,855 videos had been viewed across the participating areas, with an average of 1,213 videos per site (range 776-1,679) and 4.4 videos per app user. The most popular videos were how to give a glucagon injection and ‘My Sick Day Rules’ showing what to do when CYPF were unwell due to T1DM. Interviews (n=63) were undertaken with 38 CYPF and 25 HCPs. The findings indicate that CYPF and HCPs found the app an essential tool in the management of T1DM. CYPF. HCPs felt the app provided a val-uable educational resource in a central location and was invaluable in an emergency or unknown situation. The app was a trusted and bona-fide source of information that could be accessed at any time. HCPs validated DigiBete in helping CYPF to manage their T1DM. At the same time, the app saved services time and money and helped CYPF take back some of the control for managing their diabetes