164 research outputs found
Toward stable 3D numerical evolutions of black-hole spacetimes
Three dimensional (3D) numerical evolutions of static black holes with
excision are presented. These evolutions extend to about 8000M, where M is the
mass of the black hole. This degree of stability is achieved by using
growth-rate estimates to guide the fine tuning of the parameters in a
multi-parameter family of symmetric hyperbolic representations of the Einstein
evolution equations. These evolutions were performed using a fixed gauge in
order to separate the intrinsic stability of the evolution equations from the
effects of stability-enhancing gauge choices.Comment: 4 pages, 5 figures. To appear in Phys. Rev. D. Minor additions to
text for clarification. Added short paragraph about inner boundary dependenc
Some functional equations related to the characterizations of information measures and their stability
The main purpose of this paper is to investigate the stability problem of
some functional equations that appear in the characterization problem of
information measures.Comment: 36 pages. arXiv admin note: text overlap with arXiv:1307.0657,
arXiv:1307.0631, arXiv:1307.0664, arXiv:1307.065
Twisting K3 x T^2 Orbifolds
We construct a class of geometric twists of Calabi-Yau manifolds of
Voisin-Borcea type (K3 x T^2)/Z_2 and study the superpotential in a type IIA
orientifold based on this geometry. The twists modify the direct product by
fibering the K3 over T^2 while preserving the Z_2 involution. As an important
application, the Voisin-Borcea class contains T^6/(Z_2 x Z_2), the usual
setting for intersecting D6 brane model building. Past work in this context
considered only those twists inherited from T^6, but our work extends these
twists to a subset of the blow-up modes. Our work naturally generalizes to
arbitrary K3 fibered Calabi-Yau manifolds and to nongeometric constructions.Comment: 57 pages, 4 figures; uses harvmac.tex, amssym.tex; v3: minor
corrections, references adde
Fractional vortices on grain boundaries --- the case for broken time reversal symmetry in high temperature superconductors
We discuss the problem of broken time reversal symmetry near grain boundaries
in a d-wave superconductor based on a Ginzburg-Landau theory. It is shown that
such a state can lead to fractional vortices on the grain boundary. Both
analytical and numerical results show the structure of this type of state.Comment: 9 pages, RevTeX, 5 postscript figures include
Extending the lifetime of 3D black hole computations with a new hyperbolic system of evolution equations
We present a new many-parameter family of hyperbolic representations of
Einstein's equations, which we obtain by a straightforward generalization of
previously known systems. We solve the resulting evolution equations
numerically for a Schwarzschild black hole in three spatial dimensions, and
find that the stability of the simulation is strongly dependent on the form of
the equations (i.e. the choice of parameters of the hyperbolic system),
independent of the numerics. For an appropriate range of parameters we can
evolve a single 3D black hole to -- , and are
apparently limited by constraint-violating solutions of the evolution
equations. We expect that our method should result in comparable times for
evolutions of a binary black hole system.Comment: 11 pages, 2 figures, submitted to PR
Recommended from our members
Infrared thermography system on DIII-D
Six infrared cameras measure temperature changes on the protective graphite armor inside the DIII-D vacuum vessel. Simultaneous time dependent temperature measurements are made on armor tiles located on the centerpost and divertor regions, and on both outboard limiters. The nearly-complete poloidal coverage is useful in measuring both the plasma heat flux distributions inside the vessel and the plasma power balance. Spatial resolution of each camera system is {approx lt}1 cm, while the minimum resolvable time is 125 {mu}sec. Data from the IR TV systems is recorded on video tape, and is post-processed serially, using an image processor with an AT-compatible microcomputer. The processing system controls all VCRs, interprets DIII-D timing pulses, digitizes video data in the pre-determined regions of interest, averages digitized signals to reduce noise, and constructs data files which are then stored as part of the permanent shot record
Current-carrying cosmic string loops 3D simulation: towards a reduction of the vorton excess problem
The dynamical evolution of superconducting cosmic string loops with specific
equations of state describing timelike and spacelike currents is studied
numerically. This analysis extends previous work in two directions: first it
shows results coming from a fully three dimensional simulation (as opposed to
the two dimensional case already studied), and it now includes fermionic as
well as bosonic currents. We confirm that in the case of bosonic currents,
shocks are formed in the magnetic regime and kinks in the electric regime. For
a loop endowed with a fermionic current with zero-mode carriers, we show that
only kinks form along the string worldsheet, therefore making these loops
slightly more stable against charge carrier radiation, the likely outcome of
either shocks or kinks. All these combined effects tend to reduce the number
density of stable loops and contribute to ease the vorton excess problem. As a
bonus, these effects also may provide new ways of producing high energy cosmic
rays.Comment: 11 pages, RevTeX 4 format, 8 figures, submitted to PR
Slow-blue nuclear hypervariables in PanSTARRS-1
We discuss 76 large amplitude transients (Îm > 1.5) occurring in the nuclei of galaxies, nearly all with no previously known active galactic nucleus (AGN). They have been discovered as part of the Pan-STARRS1 (PS1) 3Ï survey, by comparison with Sloan Digital Sky Survey (SDSS) photometry a decade earlier, and then monitored with the Liverpool Telescope, and studied spectroscopically with the William Herschel Telescope (WHT). Based on colours, light-curve shape, and spectra, these transients fall into four groups. A few are misclassified stars or objects of unknown type. Some are red/fast transients and are known or likely nuclear supernovae. A few are either radio sources or erratic variables and so likely blazars. However the majority (âŒ66âperâcent) are blue and evolve slowly, on a time-scale of years. Spectroscopy shows them to be AGN at z ⌠0.3 â 1.4, which must have brightened since the SDSS photometry by around an order of magnitude. It is likely that these objects were in fact AGN a decade ago, but too weak to be recognized by SDSS; they could then be classed as âhypervariableâ AGN. By searching the SDSS Stripe 82 quasar database, we find 15 similar objects. We discuss several possible explanations for these slow-blue hypervariables â (i) unusually luminous tidal disruption events; (ii) extinction events; (iii) changes in accretion state; and (iv) large amplitude microlensing by stars in foreground galaxies. A mixture of explanations (iii) and (iv) seems most likely. Both hold promise of considerable new insight into the AGN phenomenon
Embodied Action, Enacted Bodies. The Example of Hypoglycaemia.
We all know that we have and are our bodies. But might it be possible to leave this common place? In the present article we try to do this by attending to the way we do our bodies. The site where we look for such action is that of handling the hypoglycaemias that sometimes happen to people with diabetes. In this site it appears that the body, active in measuring, feeling and countering hypoglycaemias is not a bounded whole: its boundaries leak. Bits and pieces of the outside get incorporated within the active body; while the centre of some bodily activities is beyond the skin. The body thus enacted is not self-evidently coherent either. There are tensions between the bodyÂżs organs; between the control under which we put our bodies and the erratic character of their behaviour; and between the various needs and desires single bodies somehow try to combine. Thus to say that a body is a whole, or so we conclude, skips over a lot of work. One does not hang together as a matter of course: keeping oneself together is something the embodied person needs to do. The person who fails to do so dies
Planck 2015 results. XXVII. The Second Planck Catalogue of Sunyaev-Zeldovich Sources
We present the all-sky Planck catalogue of Sunyaev-Zeldovich (SZ) sources detected from the 29 month full-mission data. The catalogue (PSZ2) is the largest SZ-selected sample of galaxy clusters yet produced and the deepest all-sky catalogue of galaxy clusters. It contains 1653 detections, of which 1203 are confirmed clusters with identified counterparts in external data-sets, and is the first SZ-selected cluster survey containing > confirmed clusters. We present a detailed analysis of the survey selection function in terms of its completeness and statistical reliability, placing a lower limit of 83% on the purity. Using simulations, we find that the Y5R500 estimates are robust to pressure-profile variation and beam systematics, but accurate conversion to Y500 requires. the use of prior information on the cluster extent. We describe the multi-wavelength search for counterparts in ancillary data, which makes use of radio, microwave, infra-red, optical and X-ray data-sets, and which places emphasis on the robustness of the counterpart match. We discuss the physical properties of the new sample and identify a population of low-redshift X-ray under- luminous clusters revealed by SZ selection. These objects appear in optical and SZ surveys with consistent properties for their mass, but are almost absent from ROSAT X-ray selected samples
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