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Search Results: 1 - 10 of 138 matches for " Turolla "
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X--ray Emission from Old, Isolated, Accreting Neutron Stars
R. Turolla
Physics , 1996,
Abstract: Accretion onto old, isolated neutron stars is reviewed. The detection of their X--ray emission with ROSAT is discussed and a summary of the current status of observations is presented.
Política econ?mica do segundo governo FHC: mudan?a em condi??es adversas
Oliveira, Gesner;Turolla, Frederico;
Tempo Social , 2003, DOI: 10.1590/S0103-20702003000200008
Abstract: this paper analyses the macroeconomic policy of fhc's second government (1999-2002), that was marked by fiscal, foreign exchange and monetary changes. in the fiscal area, a primary superavit was generated and the efforts towards a structural reform were increased. regarding foreign exchange policies, the move from administered exchange to one of dirty floating allowed for an adjustment of the high deficit in the current account that occurred in the first term of office. in the monetary field, the regime of inflation goals substituted the subordination of the monetary policy to the defense of the exchange regime. these alterations brought a more sustainable and transparent regime to the country, but its efficiency was limited due to the inheritance of the old regime, that generated unbalances and also to the international and domestic financial instabilities of the period. the new regime is adequate for the creation of conditions for the recovery of investments and for economic growth, but the main present chalenges have more to do with the institutional and regulatory aspects than with the macroeconomic policy regime.
Vacuum Breakdown near a Black Hole Charged by Hypercritical Accretion
A. Treves,R. Turolla
Physics , 1998, DOI: 10.1086/307159
Abstract: We consider a black hole accreting spherically from the surrounding medium. If accretion produces a luminosity close to the Eddington limit the hole acquires a net charge so that electrons and ions can fall with the same velocity. The condition for the electrostatic field to be large enough to break the vacuum near the hole horizon translates into an upper limit for the hole mass, $M\sim 6.6\times 10^{20} {\rm g}.$ The astrophysical conditions under which this phaenomenon can take place are rather extreme, but in principle they could be met by a mini black hole residing at the center of a star.
Low-magnetic-field magnetars
R. Turolla,P. Esposito
Physics , 2013, DOI: 10.1142/S0218271813300243
Abstract: It is now widely accepted that soft gamma repeaters and anomalous X-ray pulsars are the observational manifestations of magnetars, i.e. sources powered by their own magnetic energy. This view was supported by the fact that these `magnetar candidates' exhibited, without exception, a surface dipole magnetic field (as inferred from the spin-down rate) in excess of the electron critical field (~4.4E+13 G). The recent discovery of fully-qualified magnetars, SGR 0418+5729 and Swift J1822.3-1606, with dipole magnetic field well in the range of ordinary radio pulsars posed a challenge to the standard picture, showing that a very strong field is not necessary for the onset of magnetar activity (chiefly bursts and outbursts). Here we summarize the observational status of the low-magnetic-field magnetars and discuss their properties in the context of the mainstream magnetar model and its main alternatives.
Pulse profiles from thermally emitting neutron stars
R. Turolla,L. Nobili
Physics , 2013, DOI: 10.1088/0004-637X/768/2/147
Abstract: The problem of computing the pulse profiles from thermally emitting spots on the surface of a neutron star in general relativity is reconsidered. We show that it is possible to extend Beloborodov (2002) approach to include (multiple) spots of finite size in different positions on the star surface. Results for the pulse profiles are expressed by comparatively simple analytical formulas which involve only elementary functions.
Risk in the Uppsala Internationalization Model Risco no Modelo de Internacionaliza o de Uppsala
Debora Chiavegatti,Frederico Araujo Turolla
Revista Organiza??es em Contexto , 2011,
Abstract: This paper revisits the evolution of the Uppsala behavioral model of internationalization since the 1970s and inserts Lemos, Johanson and Vahlne’s graphic proposal in this context, where it plays a central role. The lack of knowledge about the market is the most critical restriction to the difficulties involved in the internationalization process (LEMOS; JOHANSON; VAHLNE, 2010) and is in the origin of the behavioral model that explains internationalization in sequential stages of knowledge acquisition. Liability of foreignness is relevant in the original version. There is a review of the Uppsala model, in which this perspective of the internationalization process was revisited, especially for the market complexity, suggesting that such movement is not a sequence of steps and stages planned and deliberated by a rational analysis. In the new version, the idea of liability of outsidership, referring to the existing networks, is relevant. The Uppsala school’s new proposal considers relationship networks as a central element to enter foreign markets. Este ensaio revisita a evolu o do modelo comportamental de internacionaliza o de Uppsala desde os anos setenta e insere neste contexto a proposi o gráfica de Lemos, Johanson e Vahlne (2010), onde o risco desempenha um papel central. A falta de conhecimento sobre o mercado representa a restri o mais crítica para as dificuldades envolvidas no processo de internacionaliza o (Lemos, Johanson e Vahlne, 2010) e está na origem do modelo comportamental que explica a internacionaliza o em estágios seqüenciais de aquisi o de conhecimento. Na vers o original, é relevante a liability of foreignness. Verifica-se uma revis o do modelo de Uppsala, em que esta perspectiva do processo de internacionaliza o foi revisitada em especial pela complexidade do mercado, propondo que tal movimenta o n o seria uma sequência de passos e etapas planejados e deliberados por uma análise racional. Na nova vers o, é relevante a idéia de liability of outsidership, que se refere às redes existentes. A nova proposta da Escola de Uppsala passa a considerar a rede de relacionamentos (networks) como elemento central da entrada nos mercados estrangeiros.
The Application of the Newman-Janis Algorithm in Obtaining Interior Solutions of the Kerr Metric
S. P. Drake,R. Turolla
Physics , 1997, DOI: 10.1088/0264-9381/14/7/021
Abstract: In this paper we present a class of metrics to be considered as new possible sources for the Kerr metric. These new solutions are generated by applying the Newman-Janis algorithm (NJA) to any static spherically symmetric (SSS) ``seed'' metric. The continuity conditions for joining any two of these new metrics is presented. A specific analysis of the joining of interior solutions to the Kerr exterior is made. The boundary conditions used are those first developed by Dormois and Israel. We find that the NJA can be used to generate new physically allowable interior solutions. These new solutions can be matched smoothly to the Kerr metric. We present a general method for finding such solutions with oblate spheroidal boundary surfaces. Finally a trial solution is found and presented.
Initial spin periods of neutron stars in supernova remnants
S. B. Popov,R. Turolla
Physics , 2012, DOI: 10.1007/s10509-012-1100-z
Abstract: We present estimates of initial spin periods, $P_0$, for radio pulsars associated with supernova remnants. By using the published data on 30 objects, we were able to derive a reliable estimate for the initial spin period, assuming standard magneto-dipole spin-down (braking index n=3), in many cases. Our set of estimates is still not sufficient to infer the exact shape of the initial period distribution. However, we show that a gaussian distribution with mean and deviation $\sim 0.1$ s is consistent with our results, while flat, wide distributions and very narrow ones are disfavored.
An efficient algorithm for two-dimensional radiative transfer in axisymmetric circumstellar envelopes and disks
C. P. Dullemond,R. Turolla
Physics , 2000,
Abstract: We present an algorithm for two-dimensional radiative transfer in axisymmetric, circumstellar media. The formal integration of the transfer equation is performed by a generalization of the short characteristics (SC) method to spherical coordinates. Accelerated Lambda Iteration (ALI) and Ng's algorithm are used to converge towards a solution. By taking a logarithmically spaced radial coordinate grid, the method has the natural capability of treating problems that span several decades in radius, in the most extreme case from the stellar radius up to parsec scale. Flux conservation is guaranteed in spherical coordinates by a particular choice of discrete photon directions and a special treatment of nearly-radially outward propagating radiation. The algorithm works well from zero up to very high optical depth, and can be used for a wide variety of transfer problems, including non-LTE line formation, dust continuum transfer and high temperature processes such as compton scattering. In this paper we focus on multiple scattering off dust grains and on non-LTE transfer in molecular and atomic lines. Line transfer is treated according to an ALI scheme for multi-level atoms/molecules, and includes both random and systematic velocity fields. The algorithms are implemented in a multi-purpose user-friendly radiative transfer program named RADICAL. We present two example computations: one of dust scattering in the Egg Nebula, and one of non-LTE line formation in rotational transitions of HCO$^{+}$ in a flattened protostellar collapsing cloud.
A Note on Bimodal Accretion Disks
C. P. Dullemond,R. Turolla
Physics , 1998, DOI: 10.1086/305973
Abstract: The existence of bimodal disks is investigated. Following a simple argument based on energetic considerations we show that stationary, bimodal accretion disk models in which a Shakura--Sunyaev disk (SSD) at large radii matches an advection dominated accretion flow (ADAF) at smaller radii are never possible using the standard slim disk approach, unless some extra energy flux is present. The same argument, however, predicts the possibility of a transition from an outer Shapiro--Lightman--Eardley (SLE) disk to an ADAF, and from a SLE disk to a SSD. Both types of solutions have been found.
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