An Extended Galactic Population of Low-Luminosity X-Ray Sources (CVs?) and the Diffuse X-Ray Background

Summary of abstract: The existence of a new population of yet unrecognized x-ray sources has been often suggested to resolve some pecularities in the properties of the x-ray background (XRB), and has recently been indicated by an analysis of resolved sources in deep ROSAT observations (Hasinger \etal 1993). We investigate the possibility that the indicated new population of x-ray sources is Galactic in origin, and derive its properties which would resolve the discrepancy found in the number counts of faint sources, and be consistent with observational constraints on the total background intensity, the XRB anisotropy, the number of unidentified bright sources, the Galaxy's total x-ray luminosity, and with the results of fluctuation analyses of the unresolved XRB. We find that a flattened Galactic halo (or a thick disk) distribution with a scale height of a few Kpc satisfies all the above requirements. The typical x-ray luminosity of the sources is $\approx 10^{30-31}$ erg/s in the 0.5-2 KeV band, the number density of sources in the Solar vicinity is $\sim 10^{-4.5} pc^{-3}$, their total number in the Galaxy is $\sim 10^{8.5}$, and they contribute $\sim 10^{39}$ erg/s to the Galaxy's total x-ray luminosity. We discuss the possible nature of these sources (subdwarfs, LMXBs, old neutron stars), and argue that the inferred x-ray and optical luminosities of the sources, their $\sim 2-4$ KeV spectrum, and the derived local number density and spatial distribution are all consistent with them being intrinsicly faint cataclysmic variables with low accretion rates. We discuss the possible origin of such population, including an origin from disrupted globular clusters or dark clusters, make predictions, and suggest observational tests.Comment: 20 pages, PostScript file (330K including figures), submitted to Ap

Efficient Coupling between Dielectric-Loaded Plasmonic and Silicon Photonic Waveguides

The realization of practical on-chip plasmonic devices will require efficient coupling of light into and out of surface plasmon waveguides over short length scales. In this letter, we report on low insertion loss for polymer-on-gold dielectric-loaded plasmonic waveguides end-coupled to silicon-on-insulator waveguides with a coupling efficiency of 79 ± 2% per transition at telecommunication wavelengths. Propagation loss is determined independently of insertion loss by measuring the transmission through plasmonic waveguides of varying length, and we find a characteristic surface-plasmon propagation length of 51 ± 4 μm at a free-space wavelength of λ = 1550 nm. We also demonstrate efficient coupling to whispering-gallery modes in plasmonic ring resonators with an average bending-loss-limited quality factor of 180 ± 8

Bootstrap Learning Via Modular Concept Discovery

Suppose a learner is faced with a domain of problems about which it knows nearly nothing. It does not know the distribution of problems, the space of solutions is not smooth, and the reward signal is uninformative, providing perhaps a few bits of information but not enough to steer the learner effectively. How can such a learner ever get off the ground? A common intuition is that if the solutions to these problems share a common structure, and the learner can solve some simple problems by brute force, it should be able to extract useful components from these solutions and, by composing them, explore the solution space more efficiently. Here, we formalize this intuition, where the solution space is that of typed functional programs and the gained information is stored as a stochastic grammar over programs. We propose an iterative procedure for exploring such spaces: in the first step of each iteration, the learner explores a finite subset of the domain, guided by a stochastic grammar; in the second step, the learner compresses the successful solutions from the first step to estimate a new stochastic grammar. We test this procedure on symbolic regression and Boolean circuit learning and show that the learner discovers modular concepts for these domains. Whereas the learner is able to solve almost none of the posed problems in the procedure’s first iteration, it rapidly becomes able to solve a large number by gaining abstract knowledge of the structure of the solution space.Engineering and Applied Science

A Useful Formula for Periodic Jacobi Matrices on Trees

We introduce a function of the density of states for periodic Jacobi matrices on trees and prove a useful formula for it. This allows new, streamlined proofs of the gap labeling and Aomoto index theorems. We prove a version of this new formula for the Anderson model on trees

The statistics of local motion signals in naturalistic movies

Extraction of motion from visual input plays an important role in many visual tasks, such as separation of figure from ground and navigation through space. Several kinds of local motion signals have been distinguished based on mathematical and computational considerations (e.g., motion based on spatiotemporal correlation of luminance, and motion based on spatiotemporal correlation of flicker), but little is known about the prevalence of these different kinds of signals in the real world. To address this question, we first note that different kinds of local motion signals (e.g., Fourier, non-Fourier, and glider) are characterized by second-and higher-order correlations in slanted spatiotemporal regions. The prevalence of local motion signals in natural scenes can thus be estimated by measuring the extent to which each of these correlations are present in space-time patches and whether they are coherent across spatiotemporal scales. We apply this technique to several popular movies. The results show that all three kinds of local motion signals are present in natural movies. While the balance of the different kinds of motion signals varies from segment to segment during the course of each movie, the overall pattern of prevalence of the different kinds of motion and their subtypes, and the correlations between them, is strikingly similar across movies (but is absent from white noise movies). In sum, naturalistic movies contain a diversity of local motion signals that occur with a consistent prevalence and pattern of covariation, indicating a substantial regularity of their high-order spatiotemporal image statistics

Efficient 3-Party Distributed ORAM

Distributed Oblivious RAM (DORAM) protocols---in which parties obliviously access a shared location in a shared array---are a fundamental component of secure-computation protocols in the RAM model. We show here an efficient, 3-party DORAM protocol with semi-honest security for a single corrupted party. To the best of our knowledge, ours is the first protocol for this setting that runs in constant rounds, requires sublinear communication and linear work, and makes only black-box use of cryptographic primitives. We believe our protocol is also concretely more efficient than existing solutions. As a building block of independent interest, we construct a 3-server distributed point function with security against two colluding servers that is simpler and has better concrete efficiency than prior work