Surprises in the Evaporation of 2-Dimensional Black Holes

Quantum evaporation of Callan-Giddings-Harvey-Strominger (CGHS) black holes is analyzed in the mean field approximation. This semi-classical theory incorporates back reaction. Detailed analytical and numerical calculations show that, while some of the assumptions underlying the standard evaporation paradigm are borne out, several are not. Furthermore, if the black hole is initially macroscopic, the evaporation process exhibits remarkable universal properties (which are distinct from the features observed in the simplified, exactly soluble models). Although the literature on CGHS black holes is quite rich, these features had escaped previous analyses, in part because of lack of required numerical precision, and in part because certain properties and symmetries of the model were not fully recognized. Finally, our results provide support for the full quantum gravity scenario recently developed by Ashtekar, Taveras and Varadarajan.Comment: 4 pages, 3 figure

The BSSN formulation is a partially constrained evolution system

Relativistic simulations in 3+1 dimensions typically monitor the Hamiltonian and momentum constraints during evolution, with significant violations of these constraints indicating the presence of instabilities. In this paper we rewrite the momentum constraints as first-order evolution equations, and show that the popular BSSN formulation of the Einstein equations explicitly uses the momentum constraints as evolution equations. We conjecture that this feature is a key reason for the relative success of the BSSN formulation in numerical relativity.Comment: 8 pages, minor grammatical correction

Black Hole Superradiance in Dynamical Spacetime

We study the superradiant scattering of gravitational waves by a nearly extremal black hole (dimensionless spin $a=0.99$) by numerically solving the full Einstein field equations, thus including backreaction effects. This allows us to study the dynamics of the black hole as it loses energy and angular momentum during the scattering process. To explore the nonlinear phase of the interaction, we consider gravitational wave packets with initial energies up to $10$ of the mass of the black hole. We find that as the incident wave energy increases, the amplification of the scattered waves, as well as the energy extraction efficiency from the black hole, is reduced. During the interaction the apparent horizon geometry undergoes sizable nonaxisymmetric oscillations. The largest amplitude excitations occur when the peak frequency of the incident wave packet is above where superradiance occurs, but close to the dominant quasinormal mode frequency of the black hole.Comment: 5 pages, 4 figures; revised to match PRD versio

Tasks and activities of the business rescue practitioner: a strategy as practice approach

A business rescue practitioner’s (BRP) tasks are complex, vaguely stated and involve a wide range of competencies not accessible to the average business person. Details about what exactly BRPs do during a rescue need to be determined in order to guide licensing and build a qualifications framework for the education of BRPs. Through an adapted ‘interview to the double’ (ITTD) process, information that 47 BRPs gave as instructions to a ‘double’ was elicited. All these instructions were framed as practices and praxis, then categorised into activities associated with the tasks as identified by the practitioners. Fifteen activities were derived from the practices and praxis in support of five tasks, namely: taking control, investigating the affairs, compiling a rescue plan, implementing the plan and complying with the statutory process. Five activities, namely: analyse feasibility, meet with stakeholders, analyse viability, prepare the rescue plan and follow statutory process, contributed 55% of what BRPs do, thus guiding the findings to give structure and direction to establishing what the educational requirements for BRPs should be.Key words: business rescue, practitioner, tasks, activities, practices, praxis, turnaround, qualifi cation

Students’ Comprehension of the representation of African American Vernacular in Alice Walker’s The Color Purple

This article was prompted by observations in tutorial lectures on African American Literature and reports on a subsequent pilot study. It explores students’ responses to African American Vernacular (AAV) as used in the novel The Color Purple by Alice Walker. A questionnaire was used to explore students’ comprehension of AAV. The results indicate that although students were confident of their understanding of AAV, most could not correctly translate it into Standard English (SE). The findings have implications for the field of Applied Linguistics, in terms of the way linguistic features affect the reading and teaching of a literary text, and suggests that students will benefit from guidelines for interpreting Walker’s representation of AAV.Keywords: African American Literature, African American Vernacular, dialect, Standard English (SE), literary text, reading, teachin

On the use of variability time-scales as an early classifier of radio transients and variables

We have shown previously that a broad correlation between the peak radio luminosity and the variability time-scales, approximately L ~ t^5, exists for variable synchrotron emitting sources and that different classes of astrophysical source occupy different regions of luminosity and time-scale space. Based on those results, we investigate whether the most basic information available for a newly discovered radio variable or transient - their rise and/or decline rate - can be used to set initial constraints on the class of events from which they originate. We have analysed a sample of ~ 800 synchrotron flares, selected from light-curves of ~ 90 sources observed at 5-8 GHz, representing a wide range of astrophysical phenomena, from flare stars to supermassive black holes. Selection of outbursts from the noisy radio light-curves has been done automatically in order to ensure reproducibility of results. The distribution of rise/decline rates for the selected flares is modelled as a Gaussian probability distribution for each class of object, and further convolved with estimated areal density of that class in order to correct for the strong bias in our sample. We show in this way that comparing the measured variability time-scale of a radio transient/variable of unknown origin can provide an early, albeit approximate, classification of the object, and could form part of a suite of measurements used to provide early categorisation of such events. Finally, we also discuss the effect scintillating sources will have on our ability to classify events based on their variability time-scales.Comment: Accepted for publication in MNRA

Flood modelling using data available on the Internet

Includes bibliographical references (p. 95-102).The aim of this study was to determine if sufficient data at no charge is available on the Internet to use as input to a free and open source hydrological model for use in a flood monitoring system. As such, the monitoring system would be SensorWeb enabled. The study area is the C83A quaternary catchment (746 km2) in the Northern Free State, part of the Vaal primary catchment in South Africa

Radioactive silicon as a marker in thin-film silicide formation

A new technique using radioactive 31Si (half-life =2.62 h), formed in a nuclear reactor, as a marker for studying silicide formation is described. A few hundred angstroms of radioactive silicon is first deposited onto the silicon substrate, followed immediately by the deposition of a few thousand angstroms of the metal. When the sample is heated, a silicide is first formed with the radioactive silicon. Upon further silicide formation, this band of radioactive silicide can move to the surface of the sample if silicide formation takes place by diffusion of the metal or by silicon substitutional and/or vacancy diffusion. However, if the band of radioactive silicide stays at the silicon substrate interface it can be concluded that silicon diffuses by interstitial and/or grain-boundary diffusion. This technique was tested by studying the formation of Ni2Si on silicon at 330 °C. From a combination of ion-beam sputtering, radioactivity measurement, and Rutherford backscattering it is found that the band of radioactive silicide moves to the surface of the sample during silicide formation. From these results, implanted noble-gas marker studies and the rate dependence of Ni2Si growth on grain size, it is concluded that nickel is the dominant diffusing species during Ni2Si formation, and that it moves by grain-boundary diffusion

Dissociation mechanism for solid-phase epitaxy of silicon in the Si <100>/Pd2Si/Si (amorphous) system

Solid-phase epitaxial growth (SPEG) of silicon was investigated by a tracer technique using radioactive 31Si formed by neutron activation in a nuclear reactor. After depositing Pd and Si onto activated single-crystal silicon substrates, Pd2Si was formed with about equal amounts of radioactive and nonradioactive Si during heating at 400 °C for 5 min. After an 1-sec annealing stage (450-->500 °C in 1 h) this silicide layer, which moves to the top of the sample during SPEG, is etched off with aqua regia. From the absence of radioactive 31Si in the etch, it is concluded that SPEG takes place by a dissociation mechanism rather than by diffusion