Particle Acceleration at Relativistic Shocks in Extragalactic Systems

Diffusive shock acceleration (DSA) at relativistic shocks is expected to be an important acceleration mechanism in a variety of astrophysical objects including extragalactic jets in active galactic nuclei and gamma ray bursts. These sources remain strong and interesting candidate sites for the generation of ultra-high energy cosmic rays. In this paper, key predictions of DSA at relativistic shocks that are salient to the issue of cosmic ray ion and electron production are outlined. Results from a Monte Carlo simulation of such diffusive acceleration in test-particle, relativistic, oblique, MHD shocks are presented. Simulation output is described for both large angle and small angle scattering scenarios, and a variety of shock obliquities including superluminal regimes when the de Hoffman-Teller frame does not exist. The distribution function power-law indices compare favorably with results from other techniques. They are found to depend sensitively on the mean magnetic field orientation in the shock, and the nature of MHD turbulence that propagates along fields in shock environs. An interesting regime of flat spectrum generation is addressed, providing evidence for its origin being due to shock drift acceleration. The impact of these theoretical results on gamma-ray burst and blazar science is outlined. Specifically, Fermi gamma-ray observations of these cosmic sources are already providing significant constraints on important environmental quantities for relativistic shocks, namely the frequency of scattering and the level of field turbulence.Comment: 11 pages, 6 figures, to appear in Proc. of the 8th International Astrophysics Conference "Shock Waves in Space and Astrophysical Environments" (2010), eds. X. Ao, R. Burrows and G. P. Zank (AIP Conf. Proc., New York

Kicks and Induced Spins of Neutron Stars at Birth

Using simulations of non-rotating supernova progenitors, we explore the kicks imparted to and the spins induced in the compact objects birthed in core collapse. We find that the recoil due to neutrino emissions can be a factor affecting core recoil, comparable to and at times larger than the corresponding kick due to matter recoil. This result would necessitate a revision of the general model of the origin of pulsar proper motions. In addition, we find that the sign of the net neutrino momentum can be opposite to the sign of the corresponding matter recoil. As a result, at times the pulsar recoil and ejecta can be in the same direction. Moreover, our results suggest that the duration of the dipole in the neutrino emissions can be shorter than the duration of the radiation of the neutron-star binding energy. This allows a larger dipole asymmetry to arise, but for a shorter time, resulting in kicks in the observed pulsar range. Furthermore, we find that the spin induced by the aspherical accretion of matter can leave the residues of collapse with spin periods comparable to those inferred for radio pulsars and that there seems to be a slight anti-correlation between the direction of the induced spin and the net kick direction. This could explain such a correlation among observed radio pulsars. Finally, we find that the kicks imparted to black holes are due to the neutrino recoil alone, resulting in birth kicks $\le$100 km s$^{-1}$ most of the time.Comment: 29 pages, 24 figures, accepted for publication in MNRA

Impact of a modified progressive Copenhagen adduction exercise programme on hip adduction strength and postexercise muscle soreness in professional footballers

BackgroundReduced hip adduction strength has been identified as a key predisposing factor in developing hip and groin injuries. The Copenhagen adduction programme has been shown to increase hip adduction strength in semiprofessional footballers but can cause muscle soreness. Therefore, a modified progressive Copenhagen adduction (MPCA) programme has been designed to increase hip adduction strength while limiting muscle soreness.ObjectiveTo investigate the effect of an 8-week MPCA exercise on eccentric hip adduction and abduction strength in senior professional footballers.Methods25 senior professional footballers completed an 8-week MPCA strengthening programme. Eccentric hip adduction (EHAD) and eccentric hip abduction (EHAB) strengths were measured. Changes in preintervention and postintervention strengths and EHAD:EHAB ratios were calculated. The statistical significance between strength changes was assessed with dependent t-tests and Wilcoxon signed-rank tests due to the distribution of the data (p<0.05). Delayed onset of muscle soreness (DOMS) and rate of perceived exertion were measured throughout the programme.ResultsThere were statistically significant increases in EHAD strength (24% and 25%, left and right), EHAB strength (10% and 13%, left and right) and the EHAD:EHAB ratio (12% and 10%, left and right) (p<0.01).Professional footballers were able to complete the MPCA exercise with low levels of DOMS.ConclusionAn 8-week MPCA exercise elicited significant EHAD and EHAB strength increases with reduced levels of muscle soreness in senior professional footballers

The Physical Effects of Progenitor Rotation: Comparing Two Long-Duration 3D Core-Collapse Supernova Simulations

We analyse and determine the effects of modest progenitor rotation in the context of core-collapse supernovae by comparing two separate long-duration three-dimensional simulations of 9 M$_{\odot}$ progenitors, one rotating with an initial spin period of $\sim$60 seconds and the other non-rotating. We determine that both models explode early, though the rotating model explodes a bit earlier. Despite this difference, the asymptotic explosion energies ($\sim$10$^{50}$ ergs) and residual neutron star baryon masses ($\sim$1.3 M$_{\odot}$) are similar. We find that the proto-neutron star (PNS) core can deleptonize and cool significantly more quickly. Soon into the evolution of the rotating model, we witness more vigorous and extended PNS core convection that early in its evolution envelopes the entire inner sphere, not just a shell. Moreover, we see a corresponding excursion in both the $\nu_e$ luminosity and gravitational-wave strain that may be diagnostic of this observed dramatic phenomenon. In addition, after bounce the innermost region of the rotating model seems to execute meridional circulation. The rotationally-induced growth of the convective PNS region may facilitate the growth of core B-fields by the dynamo mechanism by facilitating the achievement of the critical Rossby number condition for substantial growth of a dipole field, obviating the need for rapid rotation rates to create dipole fields of significance. The next step is to explore the progenitor-mass and spin dependencies across the progenitor continuum of the supernova explosion, dynamics, and evolution of PNS convection and its potential role in the generation of magnetar and pulsar magnetic fields.Comment: Withdrawn pending further calculation

The Essential Character of the Neutrino Mechanism of Core-Collapse Supernova Explosions

Calibrating with detailed 2D core-collapse supernova simulations, we derive a simple core-collapse supernova explosion condition based solely upon the terminal density profiles of state-of-the-art stellar evolution calculations of the progenitor massive stars. This condition captures the vast majority of the behavior of the one hundred 2D state-of-the-art models we performed to gauge its usefulness. The goal is to predict, without resort to detailed simulation, the explodability of a given massive star. We find that the simple maximum fractional ram pressure jump discriminant we define works well ~90% of the time and we speculate on the origin of the few false positives and false negatives we witness. The maximum ram pressure jump generally occurs at the time of accretion of the silicon/oxygen interface, but not always. Our results depend upon the fidelity with which the current implementation of our code Fornax adheres to Nature and issues concerning the neutrino-matter interaction, the nuclear equation of state, the possible effects of neutrino oscillations, grid resolution, the possible role of rotation and magnetic fields, and the accuracy of the numerical algorithms employed remain to be resolved. Nevertheless, the explodability condition we obtain is simple to implement, shows promise that it might be further generalized while still employing data from only the unstable Chandrasekhar progenitors, and is a more credible and robust simple explosion predictor than can currently be found in the literature.Comment: 20 pages, 17 figures. Accepted by MNRA