6,002 research outputs found
Cosmic ray acceleration to ultrahigh energy in radio galaxies
The origin of ultrahigh energy cosmic rays (UHECRs) is an open question. In
this proceeding, we first review the general physical requirements that a
source must meet for acceleration to 10-100 EeV, including the consideration
that the shock is not highly relativistic. We show that shocks in the backflows
of radio galaxies can meet these requirements. We discuss a model in which
giant-lobed radio galaxies such as Centaurus A and Fornax A act as
slowly-leaking UHECR reservoirs, with the UHECRs being accelerated during a
more powerful past episode. We also show that Centaurus A, Fornax A and other
radio galaxies may explain the observed anisotropies in data from the Pierre
Auger Observatory, before examining some of the difficulties in associating
UHECR anisotropies with astrophysical sources.Comment: 6 pages, 4 figures. Proceedings of UHECR 2018, 8-12 October 2018,
Paris, Franc
Amplification of perpendicular and parallel magnetic fields by cosmic ray currents
Cosmic ray (CR) currents through magnetised plasma drive strong instabilities
producing amplification of the magnetic field. This amplification helps explain
the CR energy spectrum as well as observations of supernova remnants and radio
galaxy hot spots. Using magnetohydrodynamic (MHD) simulations, we study the
behaviour of the non-resonant hybrid (NRH) instability (also known as the Bell
instability) in the case of CR currents perpendicular and parallel to the
initial magnetic field. We demonstrate that extending simulations of the
perpendicular case to 3D reveals a different character to the turbulence from
that observed in 2D. Despite these differences, in 3D the perpendicular NRH
instability still grows exponentially far into the non-linear regime with a
similar growth rate to both the 2D perpendicular and 3D parallel situations. We
introduce some simple analytical models to elucidate the physical behaviour,
using them to demonstrate that the transition to the non-linear regime is
governed by the growth of thermal pressure inside dense filaments at the edges
of the expanding loops. We discuss our results in the context of supernova
remnants and jets in radio galaxies. Our work shows that the NRH instability
can amplify magnetic fields to many times their initial value in parallel and
perpendicular shocks.Comment: Published in MNRAS. 14 pages, 12 figures, 2 tables. Replacement
corrects some typesetting error
Probing the Gravitational Wave Signature from Cosmic Phase Transitions at Different Scales
We present a new signature by which to one could potentially discriminate
between a spectrum of gravitational radiation generated by a self-ordering
scalar field vs that of inflation, specifically a comparison of the magnitude
of a flat spectrum at frequencies probed by future direct detection experiments
to the magnitude of a possible polarization signal in the Cosmic Microwave
Background (CMB) radiation. In the process we clarify several issues related to
the proper calculation of such modes, focusing on the effect of
post-horizon-crossing evolution.Comment: 4 pages, Phys. Rev D version (revised in response to referee's
comments
Distributed computer system enhances productivity for SRB joint optimization
Initial calculations of a redesign of the solid rocket booster joint that failed during the shuttle tragedy showed that the design had a weight penalty associated with it. Optimization techniques were to be applied to determine if there was any way to reduce the weight while keeping the joint opening closed and limiting the stresses. To allow engineers to examine as many alternatives as possible, a system was developed consisting of existing software that coupled structural analysis with optimization which would execute on a network of computer workstations. To increase turnaround, this system took advantage of the parallelism offered by the finite difference technique of computing gradients to allow several workstations to contribute to the solution of the problem simultaneously. The resulting system reduced the amount of time to complete one optimization cycle from two hours to one-half hour with a potential of reducing it to 15 minutes. The current distributed system, which contains numerous extensions, requires one hour turnaround per optimization cycle. This would take four hours for the sequential system
The symbolic dimensions of whale bone use in Thule winter dwellings
La maison d'os de baleine thuléenne est une des caractéristiques les plus impressionnantes du paysage arctique. Les éléments d'os de baleine boréale à l'intérieur de 18 maisons d'hiver thuléennes le long de la côte sud-est de l’île Somerset (Nunavut) ont été cartographiés et leur mode de distribution a été étudié par l'application d’un index d’utilité architecturale de la baleine boréale et par l’analyse des composants principaux. Les résultats suggèrent que même si l'os de baleine était initialement choisi pour la construction d’habitations sur la base de sa valeur architecturale, le statut socio-économique peut être reflété par le positionnement de certains éléments en os. En utilisant des analogies historiques provenant des Inupiat du Nord de l'Alaska, on apprend que le positionnement de divers éléments particulièrement dans le tunnel d'entrée semble avoir été lié au symbolisme de la baleine.The Thule whale bone house is one of the most impressive features of the arctic landscape. Bowhead whale bone elements within 18 Thule winter houses along the southeastern coast of Somerset Island (Nunavut) were mapped and the patterning of these elements was investigated through the application of a bowhead architectural utility index and through principal components analysis. The results suggest that while whale bone was initially selected for dwelling construction on the basis of architectural value, socio-economic status may be reflected in the positioning of certain bone elements. Informed by historic North Alaskan Inupiat analogies, the positioning of various elements within the entrance tunnel in particular appears to have been related to whale symbolism
Effects of riparian zone buffer widths on vegetation diversity in southern Appalachian headwater catchments
In mountainous areas such as the southern Appalachians USA, riparian zones are difficult to define. Vegetation is a commonly used riparian indicator and plays a key role in protecting water resources, but adequate knowledge of floristic responses to riparian disturbances is lacking. Our objective was to quantify changes in stand-level floristic diversity of riparian plant communities before (2004) and two, three, and seven years after shelterwood harvest using highlead cable-yarding and with differing no cut buffer widths of 0 m, 10 m, and 30 m distance from the stream edge. An unharvested reference stand was also studied for comparison. We examined: (1) differences among treatment sites using a mixed linear model with repeated measures; (2) multivariate relationships between ground-layer species composition and environmental variables (soil water content, light transmittance, tree basal area, shrub density, and distance from stream) using nonmetric multidimensional scaling; and (3) changes in species composition over time using a multi-response permutation procedure. We hypothesized that vegetation responses (i.e., changes in density, species composition, and diversity across the hillslope) will be greatest on harvest sites with an intermediate buffer width (10-m buffer) compared to more extreme (0-m buffer) and less extreme (30-m buffer and no-harvest reference) disturbance intensities. Harvesting initially reduced overstory density and basal area by 83% and 65%, respectively, in the 0-m buffer site; reduced by 50% and 74% in the 10-m buffer site; and reduced by 45% and 29% in the 30-m buffer site. Both the 0-m and 10-m buffer sites showed increased incident light variability across the hillslope after harvesting; whereas, there was no change in the 30-m and reference sites over time. We found significant changes in midstory and ground-layer vegetation in response to harvesting with the greatest responses on the 10-m buffer site, supporting our hypotheses that responses will be greatest on sites with intermediate disturbance. Ground-layer species composition differed significantly over time in the 0-m buffer and 10-m buffer sites (both P \u3c 0.0001), but did not change in the 30-m buffer and reference sites (both P \u3e 0.100). Average compositional dissimilarity increased after seven years, indicating greater within stand heterogeneity (species diversity) after harvesting. These vegetation recovery patterns provide useful information for evaluating management options in riparian zones in the southern Appalachians
Radio-loud flares from microquasars and radio-loudness of quasars
The low-frequency power spectra of the X-ray and radio emission from four
microquasars suggest that two distinct modes of energy output are at work: (i)
the `coupled' mode in which the X-ray and radio luminosities are closely
coupled and vary only weakly, and (ii) the `flaring' mode, which dramatically
boosts the radio luminosity but makes no impact on the X-ray luminosity. The
systems are in the flaring mode only a few percent of the time. However, flares
completely dominate the power spectrum of radio emission, with the consequence
that sources in which the flaring mode occurs, such as GRS 1915+105 and Cyg
X-3, have radio power spectra that lie more than an order of magnitude above
the corresponding X-ray power spectra. Of the four microquasars for which we
have examined data, in only one, Cyg X-1, is the flaring mode seemingly
inactive. While Cyg X-1 is a black-hole candidate, one of the three flaring
sources, Sco X-1, is a neutron star. Consequently, it is likely that both modes
are driven by the accretion disk rather than black-hole spin. Radio imaging
strongly suggests that the flaring mode involves relativistic jets. A typical
microquasar is in the flaring mode a few percent of the time, which is similar
to the fraction of quasars that are radio loud. Thus there may be no essential
difference between radio-loud and radio-quiet quasars; radio loudness may
simply be a function of the epoch at which the source is observed.Comment: 5 pages, 3 figures. Matching version to appear in MNRA
Inflating with the QCD Axion
We show that the QCD axion can drive inflation via a series of tunneling
events. For axion models with a softly broken symmetry, the axion
potential has a series of local minima and may be modeled by a tilted
cosine. Chain inflation results along this tilted cosine: the field tunnels
from an initial minimum near the top of the potential through a series of ever
lower minima to the bottom. This results in sufficient inflation and reheating.
QCD axions, potentially detectable in current searches, may thus simultaneously
solve problems in particle physics and provide inflation.Comment: 5 pages, 1 figure, revised for submission to PR
What are the most practical primary care screens for post-traumatic stress disorder?
The 4-item Primary Care Post-Traumatic Stress Disorder screen (PC-PTSD) is a simple and effective tool to identify symptoms of post-traumatic stress disorder (PTSD) in primary care patients (strength of recommendation [SOR ]: B, 1 good-quality prospective cohort study and 1 good-quality retrospective cohort study). The 7-item Breslau screen also predictably identifies patients with PTSD symptoms (SOR : B, 1 good-quality prospective cohort study)
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