Spectral Properties of Blast Wave Models of Gamma-Ray Burst Sources

We calculate the spectrum of blast wave models of gamma-ray burst sources, for various assumptions about the magnetic field density and the relativistic particle acceleration efficiency. For a range of physically plausible models we find that the radiation efficiency is high, and leads to nonthermal spectra with breaks at various energies comparable to those observed in the gamma-ray range. Radiation is also predicted at other wavebands, in particular at X-ray, optical/UV and GeV/TeV energies. We discuss the spectra as a function of duration for three basic types of models, and for cosmological, halo and galactic disk distances. We also evaluate the gamma-ray fluences and the spectral characteristics for a range of external densities. Impulsive burst models at cosmological distances can satisfy the conventional X-ray paucity constraint S_x/S_\gamma \siml few percent over a wide range of durations, but galactic models can do so only for bursts shorter than a few seconds, unless additional assumptions are made. The emissivity is generally larger for bursts in a denser external environment, with the efficiency increasing up to the point where all the energy input is radiated away.Comment: 24 pages of Tex, plus 17 figures uuencoded tar-compressed postscript file

Optimal solvers for PDE-Constrained Optimization

Optimization problems with constraints which require the solution of a partial differential equation arise widely in many areas of the sciences and engineering, in particular in problems of design. The solution of such PDE-constrained optimization problems is usually a major computational task. Here we consider simple problems of this type: distributed control problems in which the 2- and 3-dimensional Poisson problem is the PDE. The large dimensional linear systems which result from discretization and which need to be solved are of saddle-point type. We introduce two optimal preconditioners for these systems which lead to convergence of symmetric Krylov subspace iterative methods in a number of iterations which does not increase with the dimension of the discrete problem. These preconditioners are block structured and involve standard multigrid cycles. The optimality of the preconditioned iterative solver is proved theoretically and verified computationally in several test cases. The theoretical proof indicates that these approaches may have much broader applicability for other partial differential equations

The Lyman Break Galaxies: their Progenitors and Descendants

We study the evolution of Lyman Break Galaxies (LBGs) from z=5 to z=0 by tracing the merger trees of galaxies in a large-scale hydrodynamic simulation based on a Lambda cold dark matter model. In particular, we emphasize on the range of properties of the sample selected by the rest-frame V band luminosity, in accordance with recent near-IR observations. The predicted rest-frame V band luminosity function agrees well with the observed one when dust extinction is taken into account. The stellar content and the star formation histories of LBGs are also studied. We find that the LBGs intrinsically brighter than Mv=-21.0 at z=3 have stellar masses of at least 10^9\Msun, with a median of 10^{10}h^{-1}\Msun. The brightest LBGs (Mv<-23) at z=3 merge into clusters/groups of galaxies at z=0, as suggested from clustering studies of LBGs. Roughly one half of the galaxies with -23<Mv<-22 at z=3 fall into groups/clusters, and the other half become typical L* galaxies at z=0 with stellar mass of ~10^{11}\Msun. Descendants of LBGs at the present epoch have formed roughly 30% of their stellar mass by z=3, and the half of their current stellar population is 10 Gyr old, favoring the scenario that LBGs are the precursors of the present day spheroids. We find that the most luminous LBGs have experienced a starburst within 500 Myr prior to z=3, but also have formed stars continuously over a period of 1 Gyr prior to z=3 when all the star formation in progenitors is coadded. We also study the evolution of the mean stellar metallicity distribution of galaxies, and find that the entire distribution shifts to lower metallicity at higher redshift. The observed sub-solar metallicity of LBGs at z=3 is naturally predicted in our simulation.Comment: 29 pages, including 11 figures, ApJ in press. One reference adde

Importance of an Astrophysical Perspective for Textbook Relativity

The importance of a teaching a clear definition of the ``observer'' in special relativity is highlighted using a simple astrophysical example from the exciting current research area of ``Gamma-Ray Burst'' astrophysics. The example shows that a source moving relativistically toward a single observer at rest exhibits a time ``contraction'' rather than a ``dilation'' because the light travel time between the source and observer decreases with time. Astrophysical applications of special relativity complement idealized examples with real applications and very effectively exemplify the role of a finite light travel time.Comment: 5 pages TeX, European Journal of Physics, in pres

The Radiative Feedback of the First Cosmological Objects

In hierarchical models of structure formation, an early cosmic UV background (UVB) is produced by the small (T_vir < 10^4 K) halos that collapse before reionization. The UVB at energies below 13.6eV suppresses the formation of stars or black holes inside small halos, by photo-dissociating their only cooling agent, molecular H2. We self-consistently compute the buildup of the early UVB in Press-Schechter models, coupled with H2 photo-dissociation both in the intergalactic medium (IGM), and inside virialized halos. We find that the intergalactic H2 has a negligible effect on the UVB, both because its initial optical depth is small (tau<0.1), and because it is photo-dissociated at an early stage. If the UV sources in the first collapsed halos are stars, then their UV flux suppresses further star-formation inside small halos. This results in a pause in the buildup of the UVB, and reionization is delayed until larger halos (T_vir> 10^4 K) collapse. If the small halos host mini-quasars with hard spectra extending to approximately 1 keV, then their X-rays balance the effects of the UVB, the negative feedback does not occur, and reionization can be caused by the small halos.Comment: 16 pages, 16 figures included, uses emulateapj.sty. Submitted to Ap

Neutrinos from Early-Phase, Pulsar-Driven Supernovae

Neutron stars, just after their formation, are surrounded by expanding, dense, and very hot envelopes which radiate thermal photons. Iron nuclei can be accelerated in the wind zones of such energetic pulsars to very high energies. These nuclei photo-disintegrate and their products lose energy efficiently in collisions with thermal photons and with the matter of the envelope, mainly via pion production. When the temperature of the radiation inside the envelope of the supernova drops below $\sim 3\times 10^6$ K, these pions decay before losing energy and produce high energy neutrinos. We estimate the flux of muon neutrinos emitted during such an early phase of the pulsar - supernova envelope interaction. We find that a 1 km$^2$ neutrino detector should be able to detect neutrinos above 1 TeV within about one year after the explosion from a supernova in our Galaxy. This result holds if these pulsars are able to efficiently accelerate nuclei to energies $\sim 10^{20}$ eV, as postulated recently by some authors for models of Galactic acceleration of the extremely high energy cosmic rays (EHE CRs).Comment: 16 pages, 3 figures, revised version submitted to Ap

Internal Migration and Regional Population Dynamics in Europe: Portugal Case Study

Report prepared for the Council of Europe (Directorate of Social and Economic Affairs, Population and Migration Division) and for European Commission (Directorate General V, Employment, Industrial Relations and Social Affairs, Unit E1, Analysis and Research on the Social Situation). This paper reports on internal migration and regional population dynamics in Portugal. It examines internal migration patterns and trends in two years, 1981 and 1991, and compares them. Portugal is a country that has reached a population plateau and is currently experiencing very little overall population growth. In 1981 its population was 9.83 millions, only 9.87 millions in 1991 and 9.96 millions in 1997. During the 1980s natural increase steadily diminished and in the 1990s it has virtually ceased. International migration was predominantly negative in balance during the period between the mid 1980s and early 1990s, but has moved into small positive gains in the last few years. When population changes for the 1981-91 period are examined, the picture is broadly of gains in the major urban-centred regions of Norte and Lisboa Vale Tejo, and in the resort industry region of the Algarve. In the more peripheral regions, population decreases occurred, that is in the Centro, Alentejo and Açores regions with Madeira being roughly in balance. Natural increase was strong in the Norte and island regions and compensated for net out-migration, while elsewhere it was weak. All Portuguese NUTS 2 regions except the Algarve lost through migration, with emigration dominating any counter-flows due to internal migration. However, the strong influence of internal migration is apparent when the components of population change at concelhos scale are analysed. The largest cities, Lisboa and Porto, lose population in the 1981-91 period, particularly the capital, and part of the population losses represent migration to surrounding concelhos in the Outer parts of the city regions. Most of the rest of Grande Porto outside of the main city gains population through migration. Around Lisboa these suburban migration gains were experienced by the margins of Lisboa itself, the northern suburbs along the north bank of the Tagus and in a majority of the concelhos of Península Setúbal across the Tagus estuary from Lisboa, namely the southern suburbs linked to the city by two major road bridges. The Censuses of 1981 and 1991, which count migrants in the circa 15-month period before the census date (14½ and 15½ months in the two censuses), provide a picture of how internal migration affects people in different life stages. The all ages pattern of migration in 1979-81 is one of greatest losses from interior and eastern regions (NUTS 3 scale), lesser losses from remoter coastal regions and gains to the urban regions centred on Porto, Lisboa and the Algarve resorts. However, the centres of the Porto and Lisboa regions lose migrants, confirming the earlier interpretation of the components of population change. The situation in 1989-91 was similar but there were fewer regions with great migration losses, more regions with moderate losses and gains around the main urban nodes and the southern tourist coast. This general pattern of redistribution characterises the childhood and family ages (0-14, 25-44) and is intensified in terms of losses from the large cities and suburban gains in the 45-64 age group. Young persons, however, behave differently. In 1979-81 most of the regions of the country lost internal migrants in the 15-24 age group, while Lisboa, Porto and the Algarve experienced substantial migration gains. So, while the family ages showed a de-concentration pattern, the ages at which young adults start independent life careers showed centralising tendencies. The spatial pattern was broadly similar in 1989-91 but more interior regions posted migration gains. The retirement ages, 65 to 74, showed similar centralisation for the female population but not for the male. This age group was the only one that exhibited strong gender differences in the pattern of migration and considerable changes between 1979-81 and 1989-91. Whereas in the former period, the pattern was very much one of interior loss and coastal and urban region gain, in 1989-91, losses from the interior were relatively lower and moderate gains more widespread. At the oldest ages of 75 years or more the strong pattern of interior loss and coastal gain reasserted itself. Portuguese population dynamics thus exhibit some rural depopulation (mainly of the young and the very old), some urbanisation (migration to more densely settled regions around the biggest cities), some suburbanisation (de-concentration within the largest urban regions) and some regional flows to resource exploiting regions (sun and sea in the resort coast of the Algarve). Population gains in the 1981-91 decade also occurred in several coastal and interior small towns and medium-sized urban centres outside of Lisboa and Porto. This reveals that urbanisation was not just a metropolitan phenomenon but was a widespread process. Note that when considering whether the Portuguese population is centralising or decentralising, attention must be paid to the scale of migration observed. So, for example, population may be moving into the metropolitan regions of Lisboa and Porto and therefore centralising, but within those regions the population may be shifting from city to suburbs and to outer parts of the city region, and therefore decentralising. This nested system of flows may be obscured if attention is focused exclusively on total net migration

Delivering at the right price - the costs of primary maternity care at the Diepkloof Community Health Centre, Soweto

This paper reports on a study of the costs of primary maternity care services at the Diepkloof Conmmnity Health Centre (DK) in Soweto. DK, the Soweto connnunity health centre system as a whole and nmnerous other non-hospital settings provide a wide range of Illaternal health services to substantial numbers of women, and relieve hospitals of a major potential clinical burden. However, no research has been done in South Africa on the relative costs of the provision of these services in different settings and by different types of health worker. The cost structure of these services at DK is presented and the costs of antenatal care, deliveries in midwife-run labour wards, postnatal care (at the health centre and at home) and family planning services detailed. Some comparisons are made with existing data for another community health centre and with Baragwanath Hospital. These results are relevant to policy and planning of maternal health services. They are also shown to be of relevance to management and several areas of potential improvement of these services are noted

Is it time for integration of surgical skills simulation into the United Kingdom undergraduate medical curriculum? A perspective from King’s College London School of Medicine

PURPOSE: Changes in undergraduate medical curricula, combined with reforms in postgraduate education, have training implications for surgical skills acquisition in a climate of reduced clinical exposure. Confidence and prior experience influences the educational impact of learning. Currently there is no basic surgical skills (BSS) programme integrated into undergraduate curricula in the United Kingdom. We explored the role of a dedicated BSS programme for undergraduates in improving confidence and influencing careers in King's College London School of Medicine, and the programme was evaluated. METHODS: A programme was designed in-line with the established Royal College of Surgeons course. Undergraduates were taught four key skills over four weeks: knot-tying, basic-suturing, tying-at-depth and chest-drain insertion, using low-fidelity bench-top models. A Likert-style questionnaire was designed to determine educational value and influence on career choice. Qualitative data was collected. RESULTS: Only 29% and 42% of students had undertaken previous practice in knot-tying and basic suturing, respectively. 96% agreed that skills exposure prior to starting surgical rotations was essential and felt a dedicated course would augment undergraduate training. There was a significant increase in confidence in the practice and knowledge of all skills taught (p<0.01), with a greater motivation to be actively involved in the surgical firm and theatres. CONCLUSION: A simple, structured BSS programme can increase the confidence and motivation of students. Early surgical skills targeting is valuable for students entering surgical, related allied, and even traditionally non-surgical specialties such as general practice. Such experience can increase the confidence of future junior doctors and trainees. We advocate the introduction of a BSS programme into United Kingdom undergraduate curricula