Non-radial strong curvature naked singularities in five dimensional perfect fluid self-similar space-time

We study five dimensional(5D) spherically symmetric self-similar perfect fluid space-time with adiabatic equation of state, considering all the families of future directed non-spacelike geodesics. The space-time admits globally strong curvature naked singularities in the sense of Tipler and thus violates the cosmic censorship conjecture provided a certain algebraic equation has real positive roots. We further show that it is the weak energy condition (WEC) that is necessary for visibility of singularities for a finite period of time and for singularities to be gravitationally strong. We, also, match the solution to 5D Schwarzschild solution using the junction conditions.Comment: 10 pages, no figure. To appear in Gravitation & Cosmology, Vol 10(2004), No 4(40),pp1-10, Russi

Effect of a Radiation Cooling and Heating Function on Standing Longitudinal Oscillations in Coronal Loops

Standing long-period (with the periods longer than several minutes) oscillations in large hot (with the temperature higher than 3 MK) coronal loops have been observed as the quasi-periodic modulation of the EUV and microwave intensity emission and the Doppler shift of coronal emission lines, and have been interpreted as standing slow magnetoacoustic (longitudinal) oscillations. Quasi-periodic pulsations of shorter periods, detected in thermal and non-thermal emissions in solar flares could be produced by a similar mechanism. We present theoretical modelling of the standing slow magnetoacoustic mode, showing that this mode of oscillation is highly sensitive to peculiarities of the radiative cooling and heating function. We generalised the theoretical model of standing slow magnetoacoustic oscillations in a hot plasma, including the effects of the radiative losses, and accounting for plasma heating. The heating mechanism is not specified and taken empirically to compensate the cooling by radiation and thermal-conduction. It is shown that the evolution of the oscillations is described by a generalised Burgers equation. Numerical solution of an initial value problem for the evolutionary equation demonstrates that different dependences of the radiative cooling and plasma heating on the temperature lead to different regimes of the oscillations, including growing, quasi-stationary and rapidly decaying. Our findings provide a theoretical foundation for probing the coronal heating function, and may explain the observations of decayless long-period quasi-periodic pulsations in flares. The hydrodynamic approach employed in this study should be considered with caution in the modelling of non-thermal emission associated with flares, as it misses potentially important non-hydrodynamic effects.Comment: Accepted by The Astrophysical Journal (March 2016), 17 pages with 12 figure

Gravitational collapse with equation of state

We investigate here gravitational collapse of a perfect fluid with a linear isentropic equation of state $p = k \rho$. A class of collapse models is given which is a family of solutions to Einstein equations and the final fate of collapse is analyzed in terms of the formation of black holes and naked singularities. The collapse evolves from a regular initial data and the positivity of energy conditions and other physical regularity conditions are satisfied. As we provided here an explicit class, this gives useful insights into the endstates of collapse with a physically reasonable and relevant equation of state and for the cosmic censorship hypothesis.Comment: 8 pages, 4 figures, added compatibility conditio

FPGA based Agile Algorithm-On-Demand Co-Processor

With growing computational needs of many real-world applications, frequently changing specifications of standards, and the high design and NRE costs of ASICs, an algorithm-agile FPGA based co-processor has become a viable alternative. In this article, we report about the general design of an algorith-agile co-processor and the proof-of-concept implementation.Comment: Submitted on behalf of EDAA (http://www.edaa.com/

Constraints on Axion-like Particles and Nucleon Pairing in Dense Matter from the Hot Neutron Star in HESS J1731-347

If the thermal evolution of the hot young neutron star in the supernova remnant HESS J1731-347 is driven by neutrino emission, it provides a stringent constraint on the coupling of light (mass $\ll 10$ keV) axion-like particles to neutrons. Using Markov-Chain Monte Carlo we find that for the values of axion-neutron coupling $g_{ann}^2 > 7.7 \times 10^{-20}$ (90% c.l.) the axion cooling from the bremsstrahlung reaction $n+n\rightarrow n+n +a$ is too rapid to account for the high observed surface temperature. This implies that the Pecci-Quinn scale or axion decay constant $f_a > 6.7 \times 10^7$ GeV for KSVZ axions and $f_a > 1.7 \times 10^9$ GeV for DFSZ axions. The high temperature of this neutron star also allows us to tighten constraints on the size of the nucleon pairing gaps.Comment: 17 pages, 12 figure

Anisotropic re-entrant spin-glass features in a metallic kagome lattice, Tb3Ru4Al12

We report the results of ac and dc magnetic susceptibility and isothermal magnetization measurements (T= 2-300 K) on the single crystals of a metallic kagome lattice, Tb3Ru4Al12, reported recently to undergo reentrant magnetism with the onset of long range antiferromagnetic order below (TN=) 22K. The magnetization data obtained on the crystal with the c-axis orientation along magnetic-field reveal spin-glass-like characteristics near 17 K (below TN). However, for the orientation along basal plane, such glassy anomalies are not observable above 2 K. In this respect, this compound behaves like an anisotropic reentrant spin-glass. Possible implications of this finding to the field of geometrically frustrated magnetism is considered.Comment: 9 pages, 4 figure

What is the spacetime of {\em physically realizable} spherical collapse?

We argue that a particular spacetime, a spherically symmetric spacetime with hyper-surface orthogonal, radial, homothetic Killing vector, is a physically meaningful spacetime that describes the problem of spherical gravitational collapse in its full "physical" generality.Comment: Latex4, No Figures, Comments and Criticisms most welcom

Extraordinarily large intrinsic magnetodielectric coupling of Tb member within the Haldane spin-chain family, R2BaNiO5

The Haldane spin-chain compound, Tb2BaNiO5, has been known to order antiferromagnetically below (T_N= ) 63 K. The present magnetic studies on the polycrystals bring out that there is another magnetic transition at a lower temperature (T_2= ) 25 K, with a pronounced magnetic-field induced metamagnetic and metaelectric behavior. Multiferroic features are found below T_2 only, and not at T_N. The most intriguing observation is that the observed change of dielectric constant is intrinsic and largest (e.g., about 18% at 15 K) within this Haldane spin-chain family, R2BaNiO5. Taking into account that this trend (the largest change for Tb case within this family) correlates with a similar trend in T_N (with the values of T_N being about 55, 58, 53 and 32 K for Gd, Dy, Ho and Er cases), we believe that an explanation usually offered for this T_N behavior in rare-earth systems is applicable for this behavior as well . That is, single-ion anisotropy following crystal-field splitting is responsible for this extraordinary magnetodielectric effect in this Tb case. To our knowledge, such an observation was not made in the past literature of multiferroics.Comment: 4 figures; Physical Review B (in press

Dielectric and multiferroic behavior in a Haldane spin-chain compound Sm2BaNiO5 due to an interplay between crystal-field effect and exchange interaction

The Haldane spin-chain (S=1) insulating compound, Sm2BaNiO5, which has been proposed to order antiferromagnetically around (T_N=) 55 K, was investigated for its complex dielectric permittivity, magnetodielectric and pyrocurrent behavior as a function of temperature (T). In order to enable meaningful discussions, the results of ac and dc magnetizatioin and heat-capacity studies are also reported. We emphasize on the following findings: (i) There is a pyrocurrent peak near T_N, but it is shown not to arise from ferroelectricity, but possibly due to 'thermally stimulated depolarization current', unlike in many other members of this rare-earth series, in which case ferroelectric features were reported at or above T_N; (ii) however, the pyrocurrent measured in the presence of a bias electric field (after cooling in zero electric field) as well as dielectric constant reveal a weak peak with increasing T around 22 K - the temperature around which population of the exchange-split excited state of Kramers doublet has been known to occur. This finding suggests that this compound presents a novel situation in which multiferroicity is induced by an interplay between crystal-field effects and exchange interaction. No multiglass features could be observed down to 2 K unlike in many members of this family.Comment: 6 figure

Design of plasmonic photonic crystal resonant cavities for polarization sensitive infrared photodetectors

We design a polarization-sensitive resonator for use in midinfrared photodetectors, utilizing a photonic crystal cavity and a single or double-metal plasmonic waveguide to achieve enhanced detector efficiency due to superior optical confinement within the active region. As the cavity is highly frequency and polarization-sensitive, this resonator structure could be used in chip-based infrared spectrometers and cameras that can distinguish among different materials and temperatures to a high degree of precision.Comment: 14 pages, 12 figure