Time-dependent, compositionally driven convection in the oceans of accreting neutron stars

We discuss the effect of chemical separation as matter freezes at the base of the ocean of an accreting neutron star, and the subsequent enrichment of the ocean in light elements and inward transport of heat through convective mixing. We extend the steady-state results of Medin & Cumming 2011 to transiently accreting neutron stars, by considering the time-dependent cases of heating during accretion outbursts and cooling during quiescence. Convective mixing is extremely efficient, flattening the composition profile in about one convective turnover time (weeks to months at the base of the ocean). During accretion outbursts, inward heat transport has only a small effect on the temperature profile in the outer layers until the ocean is strongly enriched in light elements, a process that takes hundreds of years to complete. During quiescence, however, inward heat transport rapidly cools the outer layers of the ocean while keeping the inner layers hot. We find that this leads to a sharp drop in surface emission at around a week followed by a gradual recovery as cooling becomes dominated by the crust. Such a dip should be observable in the light curves of these neutron star transients, if enough data is taken at a few days to a month after the end of accretion. If such a dip is definitively observed, it will provide strong constraints on the chemical composition of the ocean and outer crust.Comment: 22 pages, 11 figures, submitted to Ap

Condensed Surfaces of Magnetic Neutron Stars, Thermal Surface Emission, and Particle Acceleration Above Pulsar Polar Caps

For sufficiently strong magnetic fields and/or low temperatures, the neutron star surface may be in a condensed state with little gas or plasma above it. Such surface condensation can significantly affect the thermal emission from isolated neutron stars, and may lead to the formation of a charge-depleted acceleration zone ("vacuum gap") in the magnetosphere above the stellar polar cap. Using the latest results on the cohesive property of magnetic condensed matter, we quantitatively determine the conditions for surface condensation and vacuum gap formation in magnetic neutron stars. We find that condensation can occur if the thermal energy kT of the neutron star surface is less than about 8% of its cohesive energy Q_s, and that a vacuum gap can form if the neutron star's rotation axis and magnetic moment point in opposite directions and kT is less than about 4% of Q_s. Thus, vacuum gap accelerators may exist for some neutron stars. Motivated by this result, we also study the physics of pair cascades in the vacuum gap model for photon emission by accelerating electrons and positrons due to both curvature radiation and resonant/nonresonant inverse Compton scattering. Our calculations of the condition of cascade-induced vacuum breakdown and the related pulsar death line/boundary generalize previous works to the superstrong field regime. We find that inverse Compton scatterings do not produce a sufficient number of high energy photons in the gap and thus do not lead to pair cascades for most neutron star parameters. We discuss the implications of our results for the recent observations of neutron star thermal radiation as well as for the detection/non-detection of radio emission from high-B pulsars and magnetars.Comment: 25 pages, 11 figures. Minor changes. MNRAS in pres

Código alimentario argentino : buenas nuevas para el rotulado de alimentos

Fil: Medin, Roxana. Universidad de Buenos Aires. Facultad de Medicina. Cátedra de Bromatología y Tecnología Aimentaria; Argentina.Fil: Medin, Silvia. Universidad de Buenos Aires. Facultad de Medicina. Cátedra de Bromatología y Tecnología Aimentaria; Argentina.Si bien desde hace décadas existen disposiciones legales en cuanto a la comercialización de\nalimentos en nuestro país, el consumidor final se ha visto frecuentemente perjudicado por falta de\ninformación o confusión en el rotulado de los mismos. Hasta el año pasado se utilizaban palabras tales\ncomo "diet" o "light" para identificar productos que no cumplían con los requisitos legales para ser\nconsiderados bajos en calorías. Además, mediante la propaganda masiva se intensificó el consumo de\neste tipo de alimentos como referentes de buena salud, cuando en realidad sólo están destinados a\nsatisfacer necesidades particulares de nutrición y alimentación de determinados grupos poblacionales.\nCon el objetivo de aclarar estas y otras cuestiones, a partir de 2004 se puso en práctica la\nactualización del Código Alimentario Argentino, que apunta a proteger la salud de la población y la\nbuena fe en las transacciones comerciales

Cohesive property of magnetized neutron star surfaces: Computations and implications

The cohesive energy of condensed matter in strong magnetic fields is a fundamental quantity characterizing magnetized neutron star surfaces. The cohesive energy refers to the energy required to pull an atom out of the bulk condensed matter at zero pressure. Theoretical models of pulsar and magnetar magnetospheres depend on the cohesive properties of the surface matter in strong magnetic fields. For example, depending on the cohesive energy of the surface matter, an acceleration zone ("polar gap") above the polar cap of a pulsar may or may not form. Also, condensation of the neutron star surface, if it occurs, can significantly affect thermal emission from isolated neutron stars. We describe our calculations of the cohesive property of matter in strong magnetic fields, and discuss the implications of our results to the recent observations of neutron star surface emission as well as to the detection/non-detection of radio emission from magnetars.Comment: 12 pages, 4 figures. Minor changes to Author/Comments fields. To appear in Advances in Space Researc

Evolution and Devolution of Knowledge: A Tale of Two Biologies

Anthropological inquiry suggests that all societies classify animals and plants in similar ways. Paradoxically, in the same cultures that have seen large advances in biological science, citizenry's practical knowledge of nature has dramatically diminished. Here we describe historical, cross-cultural and developmental research on how people ordinarily conceptualize organic nature (folkbiology), concentrating on cognitive consequences associated with knowledge devolution. We show that results on psychological studies of categorization and reasoning from “standard populations” fail to generalize to humanity at large. Usual populations (Euro-American college students) have impoverished experience with nature, which yields misleading results about knowledge acquisition and the ontogenetic relationship between folkbiology and folkpsychology. We also show that groups living in the same habitat can manifest strikingly distinct behaviors, cognitions and social relations relative to it. This has novel implications for environmental decision making and management, including commons problems.

Beyond binomial and negative binomial: adaptation in Bernoulli parameter estimation

Estimating the parameter of a Bernoulli process arises in many applications, including photon-efficient active imaging where each illumination period is regarded as a single Bernoulli trial. Motivated by acquisition efficiency when multiple Bernoulli processes (e.g., multiple pixels) are of interest, we formulate the allocation of trials under a constraint on the mean as an optimal resource allocation problem. An oracle-aided trial allocation demonstrates that there can be a significant advantage from varying the allocation for different processes and inspires the introduction of a simple trial allocation gain quantity. Motivated by achieving this gain without an oracle, we present a trellis-based framework for representing and optimizing stopping rules. Considering the convenient case of Beta priors, three implementable stopping rules with similar performances are explored, and the simplest of these is shown to asymptotically achieve the oracle-aided trial allocation. These approaches are further extended to estimating functions of a Bernoulli parameter. In simulations inspired by realistic active imaging scenarios, we demonstrate significant mean-squared error improvements up to 4.36 dB for the estimation of p and up to 1.86 dB for the estimation of log p.https://arxiv.org/abs/1809.08801https://arxiv.org/abs/1809.08801First author draf

Optimal stopping times for estimating Bernoulli parameters with applications to active imaging

We address the problem of estimating the parameter of a Bernoulli process. This arises in many applications, including photon-efficient active imaging where each illumination period is regarded as a single Bernoulli trial. We introduce a framework within which to minimize the mean-squared error (MSE) subject to an upper bound on the mean number of trials. This optimization has several simple and intuitive properties when the Bernoulli parameter has a beta prior. In addition, by exploiting typical spatial correlation using total variation regularization, we extend the developed framework to a rectangular array of Bernoulli processes representing the pixels in a natural scene. In simulations inspired by realistic active imaging scenarios, we demonstrate a 4.26 dB reduction in MSE due to the adaptive acquisition, as an average over many independent experiments and invariant to a factor of 3.4 variation in trial budget.Accepted manuscrip