Production of Millisecond Dips in Sco X-1 Count Rates by Dead Time Effects

Chang et al. (2006) reported millisecond duration dips in the X-ray intensity of Sco X-1 and attributed them to occultations of the source by small trans-Neptunian objects (TNOs). We have found multiple lines of evidence that these dips are not astronomical in origin, but rather the result of high-energy charged particle events in the RXTE PCA detectors. Our analysis of the RXTE data indicates that at most 10% of the observed dips in Sco X-1 could be due to occultations by TNOs, and, furthermore, we find no positive or supporting evidence for any of them being due to TNOs. We therefore believe that it is a mistake to conclude that any TNOs have been detected via occultation of Sco X-1.Comment: Submitted to ApJ; uses emulateapj.cls, 8 pages with 8 figure

Metastable Flux Configurations and de Sitter Spaces

We derive stability conditions for the critical points of the no-scale scalar potential governing the dynamics of the complex structure moduli and the axio-dilaton in compactifications of type IIB string theory on Calabi-Yau three-folds. We discuss a concrete example of a T^6 orientifold. We then consider the four-dimensional theory obtained from compactifications of type IIB string theory on non-geometric backgrounds which are mirror to rigid Calabi-Yau manifolds and show that the complex structure moduli fields can be stabilized in terms of H_{RR} only, i.e. with no need of orientifold projection. The stabilization of all the fields at weak coupling, including the axio-dilaton, may require to break supersymmetry in the presence of H_{NS} flux or corrections to the scalar potential.Comment: 24 page

Laser power conversion system analysis, volume 2

The orbit-to-ground laser power conversion system analysis investigated the feasibility and cost effectiveness of converting solar energy into laser energy in space, and transmitting the laser energy to earth for conversion to electrical energy. The analysis included space laser systems with electrical outputs on the ground ranging from 100 to 10,000 MW. The space laser power system was shown to be feasible and a viable alternate to the microwave solar power satellite. The narrow laser beam provides many options and alternatives not attainable with a microwave beam

A wide-spectrum language for verification of programs on weak memory models

Modern processors deploy a variety of weak memory models, which for efficiency reasons may (appear to) execute instructions in an order different to that specified by the program text. The consequences of instruction reordering can be complex and subtle, and can impact on ensuring correctness. Previous work on the semantics of weak memory models has focussed on the behaviour of assembler-level programs. In this paper we utilise that work to extract some general principles underlying instruction reordering, and apply those principles to a wide-spectrum language encompassing abstract data types as well as low-level assembler code. The goal is to support reasoning about implementations of data structures for modern processors with respect to an abstract specification. Specifically, we define an operational semantics, from which we derive some properties of program refinement, and encode the semantics in the rewriting engine Maude as a model-checking tool. The tool is used to validate the semantics against the behaviour of a set of litmus tests (small assembler programs) run on hardware, and also to model check implementations of data structures from the literature against their abstract specifications

Higgs Decay to Top Quarks at Hadron Colliders

Higgs bosons which decay principally to top quarks, such as in the minimal supersymmetric model, produce a peak-dip structure in the $gg\to t\bar t$ invariant-mass spectrum. This structure is potentially observable at the CERN Large Hadron Collider. (see BNL theory home page http://penguin.phy.bnl.gov/bnl.html for recent preprints)Comment: CPP-94-18, BNL-60339, ILL-(TH)-94-

Chandra X-ray Observations of the Quadruply Lensed Quasar RX J0911.4+0551

We present results from X-ray observations of the quadruply lensed quasar RX J0911.4+0551 using data obtained with the Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-ray Observatory. The 29 ks observation detects a total of ~404 X-ray photons (0.3 to 7.0 keV) from the four images of the lensed quasar. Deconvolution of the aspect corrected data resolves all four lensed images, with relative positions in good agreement with optical measurements. When compared to contemporaneous optical data, one of the lensed images (component A3) is dimmer by a factor of ~6 in X-rays with respect to the 2 brighter images (components A1 and A2). Spectral fitting for the combined images shows significant intrinsic absorption in the soft (0.2 to 2.4 keV) energy band, consistent with the mini-BAL nature of this quasar, while a comparison with ROSAT PSPC observations from 1990 shows a drop of ~6.5 in the total soft bandpass flux. The observations also detect ~157 X-ray photons arising from extended emission of the nearby cluster (peaked ~42" SW of RXJ0911.4+0551) responsible for the large external shear present in the system. The Chandra observation reveals the cluster emission to be complex and non-spherical, and yields a cluster temperature of kT = 2.3^{+1.8}_{-0.8} keV and a 2.0 to 10 keV cluster luminosity within a 1 Mpc radius of L_X = 7.6_{-0.2}^{+0.6} x 10^{43} ergs/s (error bars denote 90% confidence limits). Our mass estimate of the cluster within its virial radius is 2.3^{+1.8}_{-0.7} x 10^{14} solar, and is a factor of 2 smaller than, although consistent with, previous mass estimates based on the observed cluster velocity dispersion.Comment: 16 pages, 3 figures (figure 1 is color ps). Accepted by Ap

Frequent disturbances and chronic pressures constrain stony coral recovery on Florida’s Coral Reef

Acute disturbances and chronic pressures have an important and increasing influence on the structure of coral reef communities. For the viability of benthic taxa such as stony corals, a balance between loss following disturbance and recovery is vital. Coral populations on reefs with lower exposure to chronic pressures are often presumed to have increased resilience, enabling them to recover quickly following disturbance, but decades of anthropogenic stress and degradation may undermine the systematic recovery and reassembly of benthic communities. This study explored spatiotemporal changes in benthic community structure over a 15 yr period at three distinct coral reef regions with a gradient of chronic pressures in Florida, USA, (southeast Florida, the Florida Keys and the Dry Tortugas). We specifically assessed the spatial scale, potential drivers of change and resilience in stony coral, octocoral, sponge and macroalgae cover. Spatiotemporal changes were assessed at four different scales: among regions, habitats, sub-regions, and habitat types within regions. Cover of stony corals remained very low or declined in every region from 2004 to 2018, with corresponding increases in macroalgae cover. Stony coral recovery was limited regardless of regional differences in chronic pressure. Octocorals exhibited greater resilience due to increased recovery following disturbance and generally had higher cover than stony corals on Florida’s Coral Reef, while sponge cover was very stable over the study period. Acute disturbances, which affected sites on average once every 3 yr, negatively impacted stony coral and/or octocoral cover in every region and habitat, contributing to the regionwide proliferation of macroalgae. This study determined that high disturbance frequency and chronic anthropogenic pressures on Florida’s Coral Reef have led to sustained declines in stony corals and corresponding proliferation of macroalgae. Stony corals were expected to recover during inter-disturbance periods, but in Florida, even in locations with lower chronic pressure, recovery is severely limited