Comptonization of an isotropic distribution in moving media: higher-order effects

We consider the Comptonization of an isotropic radiation field by a thermal distribution of electrons with non-vanishing bulk velocity. We include all relativistic effects, including induced scattering and electron recoil, in the derivation of a kinetic equation which is correct to O(theta^2, beta theta^2, beta^2 theta), where beta is the bulk velocity (in units of c) and theta is the ratio of the electron temperature to mass. The result given here manifestly conserves photon number, and easily yields the energy transfer rate between the radiation and electrons. We also confirm recent calculations of the relativistic corrections to the thermal and kinematic Sunyaev-Zel'dovich effect.Comment: Minor revisions. To appear in the Astrophysical Journa

Upscaling of Relative Permeability to Minimise Numerical Dispersion

Imperial Users onl

Diagram automorphisms of quiver varieties

We show that the fixed-point subvariety of a Nakajima quiver variety under a diagram automorphism is a disconnected union of quiver varieties for the `split-quotient quiver' introduced by Reiten and Riedtmann. As a special case, quiver varieties of type D arise as the connected components of fixed-point subvarieties of diagram involutions of quiver varieties of type A. In the case where the quiver varieties of type A correspond to small self-dual representations, we show that the diagram involutions coincide with classical involutions of two-row Slodowy varieties. It follows that certain quiver varieties of type D are isomorphic to Slodowy varieties for orthogonal or symplectic Lie algebras.Comment: 43 pages. In version 2, at the referee's suggestion, we slightly expand some statements (Theorem 1.2 and Proposition 3.19) to include the relevant affine varieties. This version is to appear in Advances in Mathematic

The Federal Housing Administration in the New Millennium

The first challenge in attempting to predict the future of the Federal Housing Administration (FHA) is to understand why it is still here. No other depression-era mortgage-market institution has survived without substantial modification. We conclude that its survival has depended on its ability to invent new purposes for itself. For example, it changed from a replacement for failed private mortgage insurance using economic soundness as an insurance criterion to an innovator in high-risk lending based on an acceptable risk criterion. FHA has developed special programs to serve the needs of specific groups. We believe this pattern of change in purposes also is the key to FHA survival in the new millennium., We review potential future purposes for FHA and find that severalparticularly, maintaining mortgage credit flows in declining regional housing marketswill require a substantial FHA presence in mortgage markets. This is important because it implies that a marginalized FHA cannot serve several of the important purposes that it is likely to be asked to serve in the new millennium. Accordingly, we believe that FHA market share will be maintained and perhaps expanded in the new millennium, even with increasing competition from conventional lending

Excavations at Tas-Silg, 1996

For the first time ever, the Department of Classics and Archaeology of the University of Malta conducted its own excavations at the site of Tas-Silg which is located at Marsaxlokk in the south-east of the island of Malta. These excavations were directed by the authors, who would like to thank Mr Simon Mason and Mr Nicholas Vella who were responsible for the field supervision. We would also like to thank the area supervisors, namely Mr Andrew Appleyard, Ms Carmen Michelle 'Buhagiar, Ms Aloisia de Trafford, Mr Joseph Magro Conti, Mr Paul C. Saliba as well as Mr Andre Corrado; the latter agreed to act as an area supervisor for a while when the need arose. Obviously, we do also appreciate very much all the hard work of the "diggers" who were in fact students from.the aforementioned Department of Classics and Archaeology, the foreign and local students who formed part of the first Summer School in Archaeology which was organized by the International Office of the University of Malta, and some very eager and hard-working volunteers.peer-reviewe

The concreteness effect in healthy ageing; An attenuation or preservation?

Previous research has shown that adults process concrete words faster when they share a taxonomic (similarity) relationship, and process abstract words faster when sharing a thematic (association) relationship (Crutch, Connell & Warrington, 2009). The current study tested if this dissociation could be replicated with older adults (65+) given conflicting evidence of the attenuation/preservation of the concreteness effect in healthy aging (Borghi & Setti, 2017; Peters & Daum, 2008). Healthy younger (N = 17) and older (N = 17) adults completed the odd-one-out task employed by Crutch et al. using four item sets in which the related words were either concrete or abstract, and related by similarity or association, e.g., Jeep-Taxi-Lorry-Mushroom (concrete-similarity), Crime-Punishment-Theft-Mimic (abstract-association). A significant interaction was found between concept type and semantic relation whereby reaction times were faster for concrete-similarity over concrete-association words, and faster for abstract-association over abstract-similarity words. No age effects were found in processing concrete or abstract concepts. The concreteness effect was found to be present for both younger and older adults suggesting that, contrary to expectation, older adults still show an advantage in processing concrete over abstract concepts with implications for Embodied Cognition.Non peer reviewedDownloa

Phylogenetic and phenotypic divergence of an insular radiation of birds

Evolutionary divergence of lineages is one of the key mechanisms underpinning large scale patterns in biogeography and biodiversity. Island systems have been highly influential in shaping theories of evolutionary diversification and here I use the insular Zosteropidae of the south west Pacific to investigate the roles of ecology and biogeography in promoting evolutionary divergence. Initially I build a phylogenetic tree of the study group and use it to reveal the pattern of colonisation and diversification. My results suggest a complex history of dispersal with the observed pattern most likely a result of repeated bouts of colonisation and extinction. I then use the new phylogeny to quantify the diversification rates of the Zosteropidae. I find a very high rate of lineage divergence and suggest the most likely explanation relates to extensive niche availability in the south west Pacific. I also find evidence for an overall slowdown in diversification combined with repeated bursts of accelerated speciation, consistent with a model of taxon cycles. I do not find evidence for sympatric speciation, however. Finally I combine morphological and phylogenetic data to investigate the mode of evolution, evidence for character displacement and influence of biogeography on trait evolution. I find little support for the traditional theory of character displacement in sympatric species. I do, however, find some support for biogeographic theories. Taken together my results do not support traditional theories on the ecological and biogeographical basis of divergence, even in those cases where Zosterops have been used as exemplars. This appears to be because those theories assume rather simple patterns of colonisation and a static ecological system. Instead, my results suggest that evolutionary diversification is dominated by recurrent waves of colonisation and extinction, which, viewed at any particular moment, tend to obscure any underlying ecological rules

Thermal and kinematic corrections to the microwave background polarization induced by galaxy clusters along the line of sight

We derive analytic expressions for the leading-order corrections to the polarization induced in the cosmic microwave background (CMB) due to scattering off hot electrons in galaxy clusters along the line of sight. For a thermal distribution of electrons with a kinetic temperature of 10 keV and a bulk peculiar velocity of 1000 km/s, the dominant corrections to the polarization induced by the primordial CMB quadrupole and the cluster peculiar velocity arise from electron thermal motion and are at the level of 10 per cent in each case, near the peak of the polarization signal. When more sensitive measurements become feasible, these effects will be significant for the determination of transverse peculiar velocities, and the value of the CMB quadrupole at the cluster redshift, via the cluster polarization route.Comment: 7 pages, 2 figures. Version accepted for MNRAS. Minor expansion of text in some section

Borrower Self-Selection, Underwriting Costs, and Subprime Mortgage Credit Supply

In the U.S., households participate in two very different types of credit markets. Personal lending is characterized by continuous risk-based pricing in which lenders offer households a continuous distribution of borrowing possibilities based on estimates of their creditworthiness. This contrasts sharply with mortgage markets where lenders specialize in specific risk categories of borrowers and mortgage supply is stepwise linear. The contrast between continuous lending for personal loans and discrete lending by specialized lenders for mortgage credit has led to concerns regarding the efficiency and equity of mortgage lending. This paper sheds both theoretical and empirical light on the differences in the two credit markets. The theory section demonstrates why, in a perfectly competitive credit market where all lenders have the same underwriting technology, mortgage credit supply curves are stepwise linear and lenders specialize in prime or subprime lending. The empirical section then provides evidence that borrowers are being effectively sorted based on risk characteristics by the market