Spark Model for Pulsar Radiation Modulation Patterns

A non-stationary polar gap model first proposed by Ruderman & Sutherland (1975) is modified and applied to spark-associated pulsar emission at radio wave-lengths. It is argued that under physical and geometrical conditions prevailing above pulsar polar cap, highly non-stationary spark discharges do not occur at random positions. Instead, sparks should tend to operate in well determined preferred regions. At any instant the polar cap is populated as densely as possible with a number of two-dimensional sparks with a characteristic dimension as well as a typical distance between adjacent sparks being about the polar gap height. Our model differs, however, markedly from its original 'hollow cone' version. The key feature is the quasi-central spark driven by pair production process and anchored to the local pole of a sunspot-like surface magnetic field. This fixed spark prevents the motion of other sparks towards the pole, restricting it to slow circumferential drift across the planes of field lines converging at the local pole. We argue that the polar spark constitutes the core pulsar emission, and that the annular rings of drifting sparks contribute to conal components of the pulsar beam. We found that the number of nested cones in the beam of typical pulsar should not excced three; a number also found by Mitra & Deshpande (1999) using a completely different analysis.Comment: 31 pages, 8 figures, accepted by Ap

Spot-like Structures of Neutron Star Surface Magnetic Fields

There is growing evidence, based on both X-ray and radio observations of isolated neutron stars, that besides the large--scale (dipolar) magnetic field, which determines the pulsar spin--down behaviour, small--scale poloidal field components are present, which have surface strengths one to two orders of magnitude larger than the dipolar component. We argue in this paper that the Hall--effect can be an efficient process in producing such small--scale field structures just above the neutron star surface. It is shown that due to a Hall--drift induced instability, poloidal magnetic field structures can be generated from strong subsurface toroidal fields, which are the result of either a dynamo or a thermoelectric instability acting at early times of a neutron star's life. The geometrical structure of these small--scale surface anomalies of the magnetic field resembles that of some types of ``star--spots''. The magnetic field strength and the length--scales are comparable with values that can be derived from various observations.Comment: 4 pages, 2 figures, accepted by Astronomy & Astrophysics Letters; language improved, 2nd para of Sect. 3 change

Exclusão e abandono em uma liga de futebol infantil e juvenil na Argentina

Resumen: Introducción: El fútbol infantil y juvenil es un fenómeno cultural y social de gran importancia en Argentina. Objetivo: Este trabajo tiene como objetivo dar cuenta de aspectos fundamentales de la estructura organizativa del fútbol infantil y juvenil en la liga de Mar del Plata, una de las ciudades más importantes del país. Métodos: El análisis de datos cuantitativos básicos permite dar cuenta de los aspectos fundamentales de la estructura organizativa del fútbol infantil y juvenil en esta liga. Este análisis elemental de cuantitativo se combina con una interpretación de informes basada tanto en un análisis discursivo como etnográfico. Resultados y discusión: Existe un severo proceso de exclusión y abandono en la liga de fútbol de Mar del Plata (LMF). No sólo ocurre que el número de jugadoras mujeres es extremadamente bajo en comparación con el número de jugadores varones, sino que también ocurre que más de mil varones son abandonados año tras año por la LMF. El complejo proceso de abandono y exclusión también está fuertemente ligado a una serie de fenómenos diversos, como el sexismo, la ausencia de personas con discapacidad, la imitación de prácticas de fútbol profesional y el uso de etiquetas y estereotipos. Conclusiones: Dado que promueve sistemáticamente la exclusión y el abandono, el LMF no puede cumplir una función educativa o inclusiva, y ni siquiera puede ayudar a formar jugadores profesionales de alto nivel. En su estado actual, la estructura organizativa de la LMF debería implementar algunas transformaciones radicales pero plausibles.Abstract: Introduction: Children’s and youth football is a cultural and social phenomenon of huge importance in Argentina. Aim: This work aims at accounting for fundamental aspects of the organizational structure of children’s and youth football in the league of Mar del Plata, one of the most important cities in the country. Methods: The analysis of basic quantitative data enables us to account for the fundamental aspects of the organizational structure of children’s and youth football in the league of Mar del Plata. This elementary quantitative analysis is combined with an interpretation of reports based both on a discursive and an ethnographic analysis. Results & discussion: There is a severe process of exclusion and abandonment in the Mar del Plata football league (MFL). Not only it is the case that the number of female players is extremely low compared to the number of male players, but it is also the case that more than a thousand male players are abandoned year after year by the MFL. The complex process of abandonment and exclusion is also strongly connected with a series of diverse phenomena, like sexism, the absence of people with disabilities, the imitation of professional football practices, and the use of labels and stereotypes. Conclusions: Since it systematically promotes exclusion and abandonment, the MFL cannot fulfill an educational or inclusive function, and cannot even help to form high-level professional players. In its current state, the organizational structure of the MFL should apply some radical but plausible transformations.Resumo: Introdução: O futebol infantil e juvenil é um fenômeno cultural e social de grande importância na Argentina. Objetivo: Este trabalho visa dar conta dos aspectos fundamentais da estrutura organizacional do futebol infanto-juvenil da liga de Mar del Plata, uma das cidades mais importantes do país. Métodos: A análise de dados quantitativos básicos permite dar conta dos aspectos fundamentais da estrutura organizacional do futebol infanto-juvenil desta liga. Esta análise quantitativa elementar é combinada com uma interpretação de relatórios com base na análise discursiva e etnográfica. Resultados e discussão: Há um severo processo de exclusão e abandono no campeonato de futebol de Mar del Plata (LMF). Não só o número de jogadoras femininas é extremamente baixo em comparação com o número de jogadores masculinos, mas também acontece que mais de mil jogadores masculinos são abandonados ano após ano pela LMF. O complexo processo de abandono e exclusão também está fortemente ligado a uma série de fenômenos diversos, como o sexismo, a ausência de pessoas com deficiência, a imitação de práticas profissionais do futebol e o uso de rótulos e estereótipos. Conclusões: Por promover sistematicamente a exclusão e o abandono, o LMF não pode cumprir uma função educativa ou inclusiva e nem mesmo pode ajudar na formação de jogadores profissionais de alto nível. Em seu estado atual, a estrutura organizacional do MFL deve aplicar algumas transformações radicais, mas plausíveis

Frequency dependence of pulsar radiation patterns

We report on new results from simultaneous, dual frequency, single pulse observation of PSR B0329+54 using the Giant Metrewave Radio Telescope. We find that the longitude separation of subpulses at two different frequencies (238 and 612 MHz) is less than that for the corresponding components in the average profile. A similar behaviour has been noticed before in a number of pulsars. We argue that subpulses are emitted within narrow flux tubes of the dipolar field lines and that the mean pulsar beam has a conal structure. In such a model the longitudes of profile components are determined by the intersection of the line of sight trajectory with subpulse-associated emission beams. Thus, we show that the difference in the frequency dependence of subpulse and profile component longitudes is a natural property of the conal model of pulsar emission beam. We support our conclusions by numerical modelling of pulsar emission, using the known parameters for this pulsar, which produce results that agree very well with our dual frequency observations.Comment: 24 pages, 8 figures. Accepted for publication in Ap

Time-scales of Radio Emission in PSR J0437-4715 at 327 MHz

Time-scales of radio emission are studied in PSR J0437-4715 at 327 MHz using almost half a million periods of high quality data from Ooty Radio Telescope. The radio emission in this milli second pulsar occurs on a short (s) time-scale of approximately 0.026 +- 0.001 periods, and on a (l) time-scale that is much longer than the widths of the components of the integrated profile (approximately 0.05 periods). The width of the s emission increases with its increasing relative contribution to the total radio emission. This may provide constraints for the details of discharge of vacuum gaps above pulsar polar caps. The s emission occasionally takes place in the form of intense spikes, which are confined to the main component of the integrated profile for 90 per cent of the time. The positions of spikes within a component of the integrated profile have no simple relation to the shape of that component. This may have impact on the interpretation of the integrated profile components in terms of independent regions of emission on the polar cap.Comment: Accepted for publication in Vol 543 (1 Nov 2000) of The Astrophysical Journa

Unraveling the drift behaviour of the remarkable pulsar PSR B0826-34

We present new results from high sensitivity GMRT observations of PSR B0826-34. We provide a model to explain the observed subpulse drift properties of this pulsar, including the apparent reversals of the drift direction. In this model, PSR B0826-34 is close to being an aligned rotator. We solve for the emission geometry of this pulsar and show that the angle between the rotation and the magnetic axes is less than 5 deg. We see evidence for as many as 6 to 7 drifting bands in the main pulse at 318 MHz, which are part of a circulating system of about 15 spark-associated subpulse emission beams. We provide quantitative treatments of the aliasing problem and various effects of geometry. The observed drift rate is an aliased version of the true drift rate, such that a subpulse drifts to the location of the adjacent subpulse (or a multiple thereof) in about one pulsar period. We show that small variations, of the order of 3-8%, in the mean drift rate are then enough to explain the apparent reversals of drift direction. We find the mean circulation time of the drift pattern to be significantly longer than the predictions of the original RS75 model and propose an explanation for this, based on modified models with temperature regulated partial ion flow in the polar vacuum gap. From the variation of the mean subpulse separation across the main pulse window, we show that the spark pattern is not centred around the dipole axis, but around a point much closer (within a degree or so) to the rotation axis -- we discuss the implication of this.Comment: 23 pages (including 9 figure). Submitted to Astronomy and Astrophysics on November 11, 200

Asymptotic approximations to the nodes and weights of Gauss-Hermite and Gauss-Laguerre quadratures

Asymptotic approximations to the zeros of Hermite and Laguerre polynomials are given, together with methods for obtaining the coefficients in the expansions. These approximations can be used as a standalone method of computation of Gaussian quadratures for high enough degrees, with Gaussian weights computed from asymptotic approximations for the orthogonal polynomials. We provide numerical evidence showing that for degrees greater than $100$ the asymptotic methods are enough for a double precision accuracy computation ($15$-$16$ digits) of the nodes and weights of the Gauss--Hermite and Gauss--Laguerre quadratures.Comment: Submitted to Studies in Applied Mathematic

Computation of the Marcum Q-function

Methods and an algorithm for computing the generalized Marcum $Q-$function ($Q_{\mu}(x,y)$) and the complementary function ($P_{\mu}(x,y)$) are described. These functions appear in problems of different technical and scientific areas such as, for example, radar detection and communications, statistics and probability theory, where they are called the non-central chi-square or the non central gamma cumulative distribution functions. The algorithm for computing the Marcum functions combines different methods of evaluation in different regions: series expansions, integral representations, asymptotic expansions, and use of three-term homogeneous recurrence relations. A relative accuracy close to $10^{-12}$ can be obtained in the parameter region $(x,y,\mu) \in [0,\,A]\times [0,\,A]\times [1,\,A]$, $A=200$, while for larger parameters the accuracy decreases (close to $10^{-11}$ for $A=1000$ and close to $5\times 10^{-11}$ for $A=10000$).Comment: Accepted for publication in ACM Trans. Math. Soft