28,390 research outputs found
Gravitational Lensing by Cold Dark Matter Catastrophes
Intrinsically cold particle dark matter inevitably creates halos with sharp
discontinuities in projected surface density caused by the projection of fold
catastrophes onto the sky. In principle, these imperfections can be detected
and measured with gravitational lensing through discontinuities in image
magnification and image structure. Lens solutions are discussed for the most
common universal classes of discontinuities. Edges caused by cold particles
such as condensed axions and thermal WIMPs are very sharp, respectively about
and of the halo scale. Their structure can be resolved by
stellar and quasi-stellar sources which show sudden changes in brightness or
even sudden disappearances (sometimes within hours) as edges are crossed.
Images of extended objects such as edge-on galaxies or jets can show sudden
bends at an edge, or stretched, parity-inverted reflection symmetry about a
sharp line. Observational strategies and prospects are briefly discussed.Comment: 9 pages, AASTeX. Final version, with explanatory figure added, to be
published in the Astrophysical Journa
Hall current effects in dynamic magnetic reconnection solutions
The impact of Hall current contributions on flow driven planar magnetic merging solutions is discussed. The Hall current is important if the dimensionless Hall parameter (or normalized ion skin depth) satisfies cH>Ī· where Ī· is the inverse Lundquist number for the plasma. A dynamic analysis of the problem shows, however, that the Hall current initially manifests itself, not by modifying the planar reconnection field, but by inducing a non-reconnecting perpendicular "separator" component in the magnetic field. Only if the stronger condition c2/H > Ī· is satisfied can Hall currents be expected to affect the planar merging. These analytic predictions are then tested by performing a series of numerical experiments in periodic geometry, using the full system of planar magnetohydrodynamic (MHD) equations. The numerical results confirm that the nature of the merging changes dramatically when the Hall coupling satisfies c2/H > Ī·. In line with the analytic treatment of sheared reconnection, the coupling provided by the Hall term leads to the emergence of multiple current layers that can enhance the global Ohmic dissipation at the expense of the reconnection rate. However, the details of the dissipation depend critically on the symmetries of the simulation, and when the merging is "head-on" (i.e., comprises fourfold symmetry) the reconnection rate can be enhanced
Hydrogen-Poor Circumstellar Shells from Pulsational Pair-Instability Supernovae with Rapidly Rotating Progenitors
In certain mass ranges, massive stars can undergo a violent pulsation
triggered by the electron/positron pair instability that ejects matter, but
does not totally disrupt the star. After one or more of these pulsations, such
stars are expected to undergo core-collapse to trigger a supernova explosion.
The mass range susceptible to this pulsational phenomena may be as low as 50-70
Msun if the progenitor is of very low metallicity and rotating sufficiently
rapidly to undergo nearly homogeneous evolution. The mass, dynamics, and
composition of the matter ejected in the pulsation are important aspects to
determine the subsequent observational characteristics of the explosion. We
examine the dynamics of a sample of stellar models and rotation rates and
discuss the implications for the first stars, for LBV-like phenomena, and for
superluminous supernovae. We find that the shells ejected by pulsational
pair-instability events with rapidly rotating progenitors (>30% the critical
value) are hydrogen-poor and helium and oxygen-rich.Comment: 14 pages, 2 figure
Recommended from our members
Addressing the needs of international postgraduate students: the role of social capital
Self-evidently, international students are likely to have considerable informational and social needs when coming to study in the UK. This paper reports the findings of a small study of such needs among international postgraduate students in the School of Health Sciences. It was an interview study, with twelve students participating. Though the study was designed to support the creation of an online resource, the findings suggest that meeting those needs satisfactorily requires a multi-faceted approach. Both face-to-face and online processes are needed to help students to develop the social capital necessary to success in undertaking the course, getting to know the university, and adjusting to life in England
Parts and materials application review for space systems
Parts and materials application review for project management of space systems engineerin
EXIST: Mission Design Concept and Technology Program
The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed very large
area coded aperture telescope array, incorporating 8m^2 of pixellated Cd-Zn-Te
(CZT) detectors, to conduct a full-sky imaging and temporal hard x-ray (10-600
keV) survey each 95min orbit. With a sensitivity (5sigma, 1yr) of ~0.05mCrab
(10-150 keV), it will extend the ROSAT soft x-ray (0.5-2.5keV) and proposed
ROSITA medium x-ray (2-10 keV) surveys into the hard x-ray band and enable
identification and study of sources ~10-20X fainter than with the ~15-100keV
survey planned for the upcoming Swift mission. At ~100-600 keV, the ~1mCrab
sensitivity is 300X that achieved in the only previous (HEAO-A4, non-imaging)
all-sky survey. EXIST will address a broad range of key science objectives:
from obscured AGN and surveys for black holes on all scales, which constrain
the accretion history of the universe, to the highest sensitivity and
resolution studies of gamma-ray bursts it will conduct as the Next Generation
Gamma-Ray Burst mission. We summarize the science objectives and mission
drivers, and the results of a mission design study for implementation as a free
flyer mission, with Delta IV launch. Key issues affecting the telescope and
detector design are discussed, and a summary of some of the current design
concepts being studied in support of EXIST is presented for the wide-field but
high resolution coded aperture imaging and very large area array of imaging CZT
detectors. Overall mission design is summarized, and technology development
needs and a development program are outlined which would enable the launch of
EXIST by the end of the decade, as recommended by the NAS/NRC Decadal Survey.Comment: 14 pages, 8 figures, 2 tables. PDF file only. Presented at SPIE (Aug.
2002) and to appear in Proc. SPIE, vol. 485
Electric-field Manipulation of the Lande' g Tensor of Holes in In0.5Ga0.5As/GaAs Self-assembled Quantum Dots
The effect of an electric field on spin precession in In0.5Ga0.5As/GaAs
self-assembled quantum dots is calculated using multiband real-space
envelope-function theory. The dependence of the Lande' g tensor on electric
fields should permit high-frequency g tensor modulation resonance, as well as
direct, nonresonant electric-field control of the hole spin. Subharmonic
resonances have also been found in g tensor modulation resonance of the holes,
due to the strong quadratic dependence of components of the hole g tensor on
the electric field.Comment: 4 pages, 2 figure
- ā¦