Spectral Models of Convection-Dominated Accretion Flows

For small values of the dimensionless viscosity parameter, namely $\alpha\lesssim 0.1$, the dynamics of non-radiating accretion flows is dominated by convection; convection strongly suppresses the accretion of matter onto the central object and transports a luminosity $\sim 10^{-3}-10^{-2} \dot M c^2$ from small to large radii in the flow. A fraction of this convective luminosity is likely to be radiated at large radii via thermal bremsstrahlung emission. We show that this leads to a correlation between the frequency of maximal bremsstrahlung emission and the luminosity of the source, $\nu_{\rm peak} \propto L^{2/3}$. Accreting black holes with X-ray luminosities $10^{-4} L_{Edd}\gtrsim L_X(0.5-10{\rm keV}) \gtrsim 10^{-7}L_{Edd}$ are expected to have hard X-ray spectra, with photon indices $\Gamma\sim2$, and sources with $L_X\lesssim 10^{-9}L_{Edd}$ are expected to have soft spectra, with $\Gamma\sim3.5$. This is testable with {\it Chandra} and {\it XMM}.Comment: final version accepted by ApJ; significant modifications from previous versio

Consciousness studies and evolutionary biology in Stoppard’s The Hard Problem

At the Royal National Theatre Platform Performance with Tom Stoppard on 6 February 2015, host Nicholas Hytner, then still director of the RNT, started the conversation by quoting from a letter Stoppard had sent Hytner in 2013. In the letter, Stoppard indicates that he wanted to combine writing about the banking crisis and the possibility of a person’s altruistic goodness in the context of evolutionary biology. The resulting play, › The Hard Problem‹ (2015) directed by Hytner at the Dorfman Theatre (World Premiere on 28 January 2015), was at the centre of that platform discussion, and is at the centre of this article. In the article we provide a detailed analysis of the play’s use of references to evolutionary biology and consciousness studies, and how these references relate to the characters, the plot, and the RNT production

Effects of Land Use Change on Juvenile Fishes, Blue Crab, and Brown Shrimp Abundance in the Estuarine Nursery Habitats of North Carolina

The U.S. coastal region is home to more than half of the American population. Also, the coastal counties' population is growing much faster than that of inland counties. With a high density and an increasing population, there are rapid changes in land-use and land-cover (LULC) types, characterized mostly by the increase in areal coverage of anthropogenic land uses (agriculture and developed), while "natural/unaltered" land uses (forest and wetland) are in decline. The human population growth and land-use and land-cover changes caused by humans generate nutrients and pollutants to coastal waters, which can affect water quality and aquatic life. Trends in the land-use and land-cover changes that might impact fish and invertebrate species abundance at 71 selected estuarine stations sampled by the North Carolina Division of Marine Fisheries (NCDMF) juvenile sampling program (Program 120) were analyzed. Land use categories of interest were forest, wetland, agriculture, and developed areas. The selected fish and invertebrate species were: Atlantic croaker (Micropogonias undulatus), Atlantic menhaden (Brevoortia tyrannus), pinfish (Lagodon rhomboides), southern flounder (Paralichthys lethostigma), spot (Leiostomus xanthurus), blue crab (Callinectes sapidus), and brown shrimp (Farfantepenaeus aztectus). Geographic information system (GIS) data, remotely sensed data and statistical techniques were used to quantify the LULC type changes between 1980 and 2000 within the immediate coastal watersheds of North Carolina. Forest has been the most affected, losing about 30.1% of its total area to the increase (~24.1%) of agriculture area. The wetland and developed land use varied depending on location, but their overall changes were small when the whole study area was considered. The long-term trends in abundance of juveniles of selected fish and invertebrate species indicated declines at certain sampling stations, and increases at others. In order to determine whether land use changes were correlated with changes in the selected species, and also to find which other factors might influence changes in their abundance, I analyzed seven predictor variables [(1) percent land use change within local catchments centered on the NCDMF sampling sites, (2) number of pollution point sources in large USDA Natural Resources Soil Conservation watersheds, (3) number of people in US Census tracts within watersheds, (4) water temperature, (5) water salinity, (6) station depth, and (7) distance to inlet (minimum distance by water to an ocean inlet) for each NCDMF juvenile fish and invertebrate trawl sampling program station] in a classification and regression tree statistical analysis to predict normalized change in trawl catch for the selected species in NCDMF Program 120 data between 1980 and 2004. Land use changes were found to be influential to the number of blue crab, southern founder and Atlantic croaker, and declines were observed at 47 stations when land use changes were greater than 13% (blue crab), at 30 stations when land use changes were greater than 21% (southern flounder), and 6 stations when land use changes were greater than 53% (Atlantic croaker). Water salinity was found to be more important than land use change for southern flounder catch, and increased catch was observed at stations with salinity 21 km and highest when salinity was > 14 ppt. Spot showed an increase in abundance when distance to inlet was greater than 42 km and the human population was > 883 people/census tract in the year 2000.  Land use change impacts were observed in the classification and regression tree analysis for blue crab at 66% of the NCDMF stations, 42% of stations for southern flounder, and 8% of stations for Atlantic croaker. These three species were ranked first, second, and fourth in commercial value in North Carolina fisheries, and were the only species of those selected for this study that were considered to be overfished in 2000 by the NCDMF. Thus, land use change had the greatest impact on species that were targets of intensive commercial fishing and had low adult spawning stock. This result suggests an interaction between commercial fisheries harvest and land development for agriculture along the coast. Recruitment of these species may have been low because of reduced spawning stock due to commercial harvests, and this reduced recruitment was most noticeable where land use changes were high. Species that were not intensively harvested (pinfish) or had stable adult stock sizes as determined by the NCDMF (brown shrimp, Atlantic menhaden, and spot) were able to produce many recruits, and this high recruitment may have allowed colonization of areas with marginal habitat due to land use changes. Few stations showed declines in abundance of the juvenile stages of these latter species, suggesting that land use change was not a significant factor between 1980 and 2004. However, post-recruitment mortality (survivorship after June throughout the summer and fall) was not monitored by NCDMF and should be studied in the future. The results of this study serve as an early warning to coastal managers regarding the potential impact of coastal land use changes. The abundance and growth of these valuable fishes and invertebrates in North Carolina estuarine nursery habitats could be reduced in the future, given the national trends in coastal development and fishery harvests.  Ph.D

A numerical study of the correspondence between paths in a causal set and geodesics in the continuum

This paper presents the results of a computational study related to the path-geodesic correspondence in causal sets. For intervals in flat spacetimes, and in selected curved spacetimes, we present evidence that the longest maximal chains (the longest paths) in the corresponding causal set intervals statistically approach the geodesic for that interval in the appropriate continuum limit.Comment: To the celebration of the 60th birthday of Rafael D. Sorki

Coherent Bayesian analysis of inspiral signals

We present in this paper a Bayesian parameter estimation method for the analysis of interferometric gravitational wave observations of an inspiral of binary compact objects using data recorded simultaneously by a network of several interferometers at different sites. We consider neutron star or black hole inspirals that are modeled to 3.5 post-Newtonian (PN) order in phase and 2.5 PN in amplitude. Inference is facilitated using Markov chain Monte Carlo methods that are adapted in order to efficiently explore the particular parameter space. Examples are shown to illustrate how and what information about the different parameters can be derived from the data. This study uses simulated signals and data with noise characteristics that are assumed to be defined by the LIGO and Virgo detectors operating at their design sensitivities. Nine parameters are estimated, including those associated with the binary system, plus its location on the sky. We explain how this technique will be part of a detection pipeline for binary systems of compact objects with masses up to 20 \sunmass, including cases where the ratio of the individual masses can be extreme.Comment: Accepted for publication in Classical and Quantum Gravity, Special issue for GWDAW-1

Coherent Bayesian inference on compact binary inspirals using a network of interferometric gravitational wave detectors

Presented in this paper is a Markov chain Monte Carlo (MCMC) routine for conducting coherent parameter estimation for interferometric gravitational wave observations of an inspiral of binary compact objects using data from multiple detectors. The MCMC technique uses data from several interferometers and infers all nine of the parameters (ignoring spin) associated with the binary system, including the distance to the source, the masses, and the location on the sky. The Metropolis-algorithm utilises advanced MCMC techniques, such as importance resampling and parallel tempering. The data is compared with time-domain inspiral templates that are 2.5 post-Newtonian (PN) in phase and 2.0 PN in amplitude. Our routine could be implemented as part of an inspiral detection pipeline for a world wide network of detectors. Examples are given for simulated signals and data as seen by the LIGO and Virgo detectors operating at their design sensitivity.Comment: 10 pages, 4 figure

Using Markov chain Monte Carlo methods for estimating parameters with gravitational radiation data

We present a Bayesian approach to the problem of determining parameters for coalescing binary systems observed with laser interferometric detectors. By applying a Markov Chain Monte Carlo (MCMC) algorithm, specifically the Gibbs sampler, we demonstrate the potential that MCMC techniques may hold for the computation of posterior distributions of parameters of the binary system that created the gravity radiation signal. We describe the use of the Gibbs sampler method, and present examples whereby signals are detected and analyzed from within noisy data.Comment: 21 pages, 10 figure

A Large, Uniform Sample of X-ray Emitting AGN from the ROSAT All-Sky and Sloan Digital Sky Surveys: the Data Release 5 Sample

We describe further results of a program aimed to yield ~10^4 fully characterized optical identifications of ROSAT X-ray sources. Our program employs X-ray data from the ROSAT All-Sky Survey (RASS), and both optical imaging and spectroscopic data from the Sloan Digital Sky Survey (SDSS). RASS/SDSS data from 5740 deg^2 of sky spectroscopically covered in SDSS Data Release 5 (DR5) provide an expanded catalog of 7000 confirmed quasars and other AGN that are probable RASS identifications. Again in our expanded catalog, the identifications as X-ray sources are statistically secure, with only a few percent of the SDSS AGN likely to be randomly superposed on unrelated RASS X-ray sources. Most identifications continue to be quasars and Seyfert 1s with 15<m<21 and 0.01<z<4; but the total sample size has grown to include very substantial numbers of even quite rare AGN, e.g., now including several hundreds of candidate X-ray emitting BL Lacs and narrow-line Seyfert 1 galaxies. In addition to exploring rare subpopulations, such a large total sample may be useful when considering correlations between the X-ray and the optical, and may also serve as a resource list from which to select the "best" object (e.g., X-ray brightest AGN of a certain subclass, at a preferred redshift or luminosity) for follow-on X-ray spectral or alternate detailed studies.Comment: Accepted for publication in AJ; 32 pages, including 11 figures, and 6 example table

A coupled optical-thermal-electrical model to predict the performance of hybrid PV/T-CCPC roof-top systems

A crossed compound parabolic concentrator (CCPC) is applied into a photovoltaic/thermal (PV/T) hybrid solar collector, i.e. concentrating PV/T (CPV/T) collector, to develop new hybrid roof-top CPV/T systems. However, to optimise the system configuration and operational parameters as well as to predict their performances, a coupled optical, thermal and electrical model is essential. We establish this model by integrating a number of submodels sourced from literature as well as from our recent work on incidence-dependent optical efficiency, six-parameter electrical model and scaling law for outdoor conditions. With the model, electrical performance and cell temperature are predicted on specific days for the roof-top systems installed in Glasgow, Penryn and Jaen. Results obtained by the proposed model reasonably agree with monitored data and it is also clarified that the systems operate under off-optimal operating condition. Long-term electric performance of the CPV/T systems is estimated as well. In addition, effects of transient terms in heat transfer and diffuse solar irradiance on electric energy are identified and discussed

Host-Associated and Free-Living Phage Communities Differ Profoundly in Phylogenetic Composition

Phylogenetic profiling has been widely used for comparing bacterial communities, but has so far been impossible to apply to viruses because of the lack of a single marker gene analogous to 16S rRNA. Here we developed a reference tree approach for matching viral sequences and applied it to the largest viral datasets available. The resulting technique, Shotgun UniFrac, was used to compare host-associated and non-host-associated phage communities (130 total metagenomes), and revealed a profound split similar to that found with bacterial communities. This new informatics approach complements analysis of bacterial communities and promises to provide new insights into viral community dynamics, such as top-down versus bottom-up control of bacterial communities by viruses in a range of systems