The Origin of Warped, Precessing Disks in X-Ray Binaries

The radiation-driven warping instability discovered by Pringle holds considerable promise as the mechanism responsible for producing warped, precessing accretion disks in X-ray binaries. This instability is an inherently global mode of the disk, thereby avoiding the difficulties with earlier models for the precession. Here we follow up earlier work to study the linear behavior of the instability in the specific context of a binary system. We treat the influence of the companion as an orbit-averaged, quadrupole torque on the disk. The presence of this external torque allows the existence of solutions in which the direction of precession of the warp is retrograde with respect to disk rotation, in addition to the prograde solutions which exist in the absence of external torques.Comment: 12 pages, 3 figure

Strong Evidence for a Buried AGN in UGC 5101: Implications for LINER-Type Ultra-Luminous Infrared Galaxies

We report on the results of 3--4 $\mu$m spectroscopy of the ultra-luminous infrared galaxy (ULIRG) UGC 5101. It has a cool far-infrared color and a LINER-type optical spectrum, and so, based on a view gaining some currency, would be regarded as dominated by star formation. However, we find that it has strong 3.4 $\mu$m carbonaceous dust absorption, low-equivalent-width 3.3 $\mu$m polycyclic aromatic hydrocarbon (PAH) emission, and a small 3.3 $\mu$m PAH to far-infrared luminosity ratio. This favors an alternative scenario, in which an energetically dominant AGN is present behind obscuring dust. The AGN is plausibly obscured along all lines of sight (a `buried AGN'), rather than merely obscured along our particular line of sight. Such buried AGNs have previously been found in thermal infrared studies of the ULIRGs IRAS 08572+3915 and IRAS F00183$-$7111, both classified optically as LINERs. We argue that buried AGNs can produce LINER-type optical spectra, and that at least some fraction of LINER-type ULIRGs are predominantly powered by buried AGNs.Comment: 11 pages, 1 figure, accepted by ApJ Lette

A Chandra observation of the H2O megamaser IC2560

A short Chandra ACIS-S observation of the Seyfert 2 galaxy IC 2560, which hosts a luminous nuclear water megamaser, shows: 1) the X-ray emission is extended; 2) the X-ray spectrum shows emission features in the soft (E<2 keV) X-ray band; this is the major component of the extended emission; and 3) a very strong (EW~3.6 keV) iron K line at 6.4 keV on a flat continuum. This last feature clearly indicates that the X-ray source is hidden behind Compton-thick obscuration, so that the intrinsic hard X-ray luminosity must be much higher than observed, probably close to ~3e42 erg/s. We briefly discuss the implications for powering of the maser emission and the central source.Comment: 5 pages, MNRAS in pres

Evaluating Habitat-based Niche Requirements and Potential Recruitment Bottlenecks for Imperiled Bluehead Sucker (Catostomus discobolus)

Changes to rivers that alter physical and thermal habitat may cause fish population abundance to decline, due to fewer individuals maturing and entering the adult population. The Weber River has become highly degraded with many dams and diversions altering fish habitat, river volume, velocity, and temperature, and limiting movement between reaches. Bluehead suckers (Catostomus discobolus) occupy only 47% of their historical range and the genetically-distinct Weber River (northern UT) population is declining and contains few young, juvenile fish. My objectives were to determine whether spawning and rearing habitat available in the Weber River may be limiting bluehead sucker reproductive success and population growth. I used reach-based surveys to locate and quantify spawning habitat in the Weber River and Ferron Creek (central Utah), a relatively unaltered river for comparison. I sampled slow-water backwaters near (\u3c 1 km) spawning reaches for juvenile sucker and surveyed habitat characteristics. I conducted laboratory experiments to evaluate the effect different temperature and velocity treatments (12-19°C, 0.004-0.18 m/s) have on juvenile bluehead sucker growth. In the Weber River and Ferron Creek, reaches with gravel (4-64 mm diameter), cobble (64-256 mm diameter), and pools (6-26 pools/km) were used by spawning bluehead sucker. In Weber River backwaters, deeper backwaters contained significantly more juvenile sucker (18-378 sucker; range: 19-87 cm max depth). Laboratory results indicated that juvenile bluehead sucker growth was greatest in the cooler temperature and slower velocity treatments. Collectively these results suggest spawning habitat is limited by the availability of small, rocky substrate and pools and rearing habitat is limited by the availability of deep, slow backwaters at the optimal temperature. By evaluating factors that may limit bluehead sucker population growth, this study will provide a template for future restoration efforts directed at recovering this imperiled population

Evolution of displacements and strains in sheared amorphous solids

The local deformation of two-dimensional Lennard-Jones glasses under imposed shear strain is studied via computer simulations. Both the mean squared displacement and mean squared strain rise linearly with the length of the strain interval $\Delta \gamma$ over which they are measured. However, the increase in displacement does not represent single-particle diffusion. There are long-range spatial correlations in displacement associated with slip lines with an amplitude of order the particle size. Strong dependence on system size is also observed. The probability distributions of displacement and strain are very different. For small $\Delta \gamma$ the distribution of displacement has a plateau followed by an exponential tail. The distribution becomes Gaussian as $\Delta \gamma$ increases to about .03. The strain distributions consist of sharp central peaks associated with elastic regions, and long exponential tails associated with plastic regions. The latter persist to the largest $\Delta \gamma$ studied.Comment: Submitted to J. Phys. Cond. Mat. special volume for PITP Conference on Mechanical Behavior of Glassy Materials. 16 Pages, 8 figure