Masses, Beaming and Eddington Ratios in Ultraluminous X-ray Sources

I suggest that the beaming factor in bright ULXs varies as $b \propto \dot m^{-2}$, where $\dot m$ is the Eddington ratio for accretion. This is required by the observed universal $L_{\rm soft} \propto T^{-4}$ relation between soft--excess luminosity and temperature, and is reasonable on general physical grounds. The beam scaling means that all observable properties of bright ULXs depend essentially only on the Eddington ratio $\dot m$, and that these systems vary mainly because the beaming is sensitive to the Eddington ratio. This suggests that bright ULXs are stellar--mass systems accreting at Eddington ratios of order 10 -- 30, with beaming factors b \ga 0.1. Lower--luminosity ULXs follow bolometric (not soft--excess) $L \sim T^4$ correlations and probably represent {\it sub}--Eddington accretion on to black holes with masses \sim 10\msun. High--mass X-ray binaries containing black holes or neutron stars and undergoing rapid thermal-- or nuclear--timescale mass transfer are excellent candidates for explaining both types. If the $b \propto \dot m^{-2}$ scaling for bright ULXs can be extrapolated to the Eddington ratios found in SS433, some objects currently identified as AGN at modest redshifts might actually be ULXs (`pseudoblazars'). This may explain cases where the active source does not coincide with the centre of the host galaxy.Comment: MNRAS Letters, in pres

The Brightest Black Holes

I suggest that there are two classes of ultraluminous X-ray sources (ULXs), corresponding to super-Eddington mass inflow in two situations: (a) thermal-timescale mass transfer in high-mass X-ray binaries, and (b) long-lasting transient outbursts in low-mass X-ray binaries. These two classes are exemplified by SS433 and microquasars like GRS 1915+105 respectively. The observed ULX population is a varying mixture of the two, depending on the star formation history of the host galaxy. ULXs in galaxies with vigorous star formation (such as the Antennae) are generally SS433--like, while ULXs in elliptical galaxies must be of the microquasar type. The latter probably have significantly anisotropic radiation patterns. They should also be variable, but demonstrating this may require observations over decades. The close analogy between models of X-ray binaries and active galactic nuclei (AGN) suggests that there should exist an apparently super-Eddington class of the latter, which may be the ultrasoft AGN, and a set of X-ray binaries with Doppler--boosted X-ray emission. These are presumably a subset of the ULXs, but remain as yet unidentified.Comment: 4 pages, no figures; accepted for MNRAS Letter

Minimal generating sets of non-modular invariant rings of finite groups

It is a classical problem to compute a minimal set of invariant polynomial generating the invariant ring of a finite group as an algebra. We present here an algorithm for the computation of minimal generating sets in the non-modular case. Apart from very few explicit computations of Groebner bases, the algorithm only involves very basic operations, and is thus rather fast. As a test bed for comparative benchmarks, we use transitive permutation groups on 7 and 8 variables. In most examples, our algorithm implemented in Singular works much faster than the one used in Magma, namely by factors between 50 and 1000. We also compute some further examples on more than 8 variables, including a minimal generating set for the natural action of the cyclic group of order 11 in characteristic 0 and of order 15 in characteristic 2. We also apply our algorithm to the computation of irreducible secondary invariants.Comment: 14 pages v3: Timings updated. One example adde

Low Scale Technicolour at LEP

We discuss the phenomenology of an $SU(2)_{TC}$ technicolour model with a low technicolour confinement scale ${\Lambda}_{TC} \sim 50-100 GeV$. Such a low technicolour scale may give rise to the first hints of technicolour being seen at LEPI and spectacular technicolour signals at LEPII.Comment: 12 pages+1 figure (available by post), Latex, SHEP92/93-2

Transient Optical Characterisation of Donor-Acceptor Block Copolymers for Use in Solar Cells

This thesis presents a study of photo-active, semiconducting block copolymers for use in molecular solar cells. Current state-of-the-art organic devices utilise blends of two (or more) materials that are co-deposited from a common solution; the resulting structures formed are determined by material properties and deposition conditions, but often result in configurations that are detrimental to device performance. An answer to this problem comes in the form of the block copolymer; using these materials, devices can be formed from a single material active layer. In addition, the counterbalance of forces within films of block copolymer can lead to nano-scale self-assembly that allows for a strong degree of control over layer equilibrium morphology. Such control will be an important step forward in the evolution of molecular solar cells. The main body of this work is concerned with the study of the photo-physics of photo-conductive block copolymers, especially the generation of free charge. First, an investigation is made into the inherent structure-function relationship in block copolymers. A varying chain length is seen to drastically affect the photoluminescence quenching and yield of long-lived charges. Photovoltaic devices made using these materials show a peak efficiency of 0.11% and correlate with the spectroscopic results, subject to a trade off between charge generation and transport/collection. In a second investigation, the effects of post-fabrication annealing on block copolymer films are considered; studies on annealed samples lead to the conclusion that domain crystallinity is a significant factor in determining the yields of long-lived charge carriers. It is found that these yields are comparable with those of a standard blend (that achieve 75% photon to electron conversion efficiency). Annealing leads to increases in photovoltaic device performance over unannealed samples, although additional control over active layer morphology is necessary for these materials to attain their potential. Following this, a comparative study is made between a block copolymer and a similarly composed blend formed from well studied polyfluorene copolymers. Further advantages of block copolymers are highlighted, including the stability of morphologies generated under different deposition conditions. Finally, a novel tool set is introduced using a block copolymer sample to emphasise the experiments potential with regard to studying interfacial photophysical effects

Thinking in Chinese vs. Thinking in English: Social Preference and Risk Attitudes of Multicultural Minds

This paper investigates whether language priming activates different cultural identities and norms associated with the language communicated with respect to social preference and risk attitudes. Our contribution is on identifying the conditions where there will be language priming effects. We conduct economic games with bilingual subjects using Chinese and English as instructions. It is found that language priming affects social preference, but only in context involving strategic interactions. In social preference games involving strategic interactions, e.g., the trust game, subjects in the Chinese treatment are more trusting and trustworthy. In individual choice games, such as the dictator game, there is no treatment difference. Further, we also find that language priming affects risk attitudes. Subjects in the Chinese treatment prefer to pick Chinese lucky numbers in Mark Six lottery. These findings suggest that the effect of language priming is context dependent.language, bilingual, biculture, social preference, risk attitudes

Sense of Control Affects Investment Behavior

Preference for control affects investment behavior. Participants of laboratory experiments invest different amount of money in a risky asset when face with two different methods of control which have identical payoff structure and probability distribution, but provide different sense of control. Preference for controlling and not controlling are both observed. Participants increase their investment when their preferred method of control is used. Participants who prefer to control more reduce their investment more strongly when face with less control. Preference for control has larger effect on investment behavior when participants are induced to have a comparative mindset rather than non-comparative mindset.Preference for control, sense of control, risk attitudes, illusion of control, source preference, portfolio choice, behavioral finance, comparative mindset, non-comparative mindset

Spectacular narratives: Twister, independence day, and frontier mythology

Big-screen spectacle has become increasingly important to Hollywood in recent decades. It formed a central part of a post-war strategy aimed at tempting lost audiences back to the cinema in the face of demographic changes and the development of television and other domestic leisure activities. More recently, in an age in which the big Hollywood studios have become parts of giant conglomerates, the prevalence of spectacle and special effects has been boosted by a demand to engineer products that can be further exploited in multimedia forms such as computer games and theme-park rides, secondary outlets that can sometimes generate more profits than the films on which they are based. These and other developments have led some commentators to announce, or predict, the imminent demise of narrative as a central component of Hollywood cinema. But the case has been considerably overstated. Narrative is far from being eclipsed, even in the most spectacular and effects-oriented of today’s blockbuster attractions. These films still tend to tell reasonably coherent stories, even if they may sometimes be looser and less well integrated than classical models. More important for my argument, contemporary spectaculars also continue to manifest the kinds of underlying thematic oppositions and reconciliations associated with a broadly ‘structuralist’ analysis of narrative. This very important dimension of narrative has been largely ignored by those who identify, celebrate or more often bemoan a weakening of plot or character development in many spectacular features

Lifestyles, identity and young people's experiences of mountain biking

It has been widely recognised that, for young people, experiencing the natural environment may hold multiple benefits for well-being and the future development of healthy lifestyles. The Active England programme awarded funding aimed at increasing participation in sport and physical activity at Bedgebury Forest in Kent, with a particular emphasis on young people as a key target group. Mountain biking, as a popular youth sport that often occurs in woodlands, was promoted under the scheme and provides the case study upon which this Note is based