Thermal structures of accreting neutron stars with neutrino losses due to strong pion condensations
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Quiescent X-ray luminosities are presented in low mass X-ray binaries with use of evolutionary calculations. The calculated luminosities are compared with observed ones in terms of timeaveraged mass accretion rate. It is shown that neutrino emission by strong pion condensation can explain quiescent X-ray luminosity of SAX J1808.4-3658 and we do not need direct Urca processes concerning nucleons and/or hyperons.The conventional picture of disk accretion onto magnetized neutron stars has been challenged by the spin changes observed in a few X-ray pulsars, and by theoretical results from numerical simulations of disk-magnetized star interactions. Here we present a model for the torque exerted by accretion disks on magnetized neutron stars, assuming accretion continues even for fast rotators.
Accretion disc
Intermediate polar
Spin-up
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The conventional picture of disk accretion onto magnetized neutron stars has been challenged by the spin changes observed in a few X-ray pulsars, and by theoretical results from numerical simulations of disk-magnetized star interactions. These indicate possible accretion during the propeller regime and the spin-down torque increasing with the accretion rate. Here we present a model for the accretion torque exerted by the disk on a magnetized neutron star, assuming accretion continues even for rapid rotators. The accretion torque is shown to have some different characteristics from that in the conventional model, but in accord with observations and numerical calculations of accretion-powered magnetized neutron stars. We also discuss its possible applications to the spin evolution in X-ray pulsars.
Intermediate polar
Accretion disc
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An extended pion source, which can be temporarily created by a high energy nuclear collision, will also absorb and distort the outgoing pions. We discuss how this effect alters the interferometric pattern of the two-pion momentum correlation function. In particular, we show that the two-pion correlation function decreases rapidly when the opening angle between the pions increases. The opening-angle dependence should serve as a new means of obtaining information about the pion source in the analysis of experimental data.
Momentum (technical analysis)
Source function
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The production of pions by nucleons is studied in an attempt to take approximate account of the strong pionnucleon interaction. The calculation is based on the assumption that the probability of the process N + N yields pi + N + N' is determined by the energy of the created pion relative to one of the nucleons. Using experimental values for the matrix elements of the pion-nucleon interaction, we calculate the spectrum of pions and nucleons im the reaction N + N yields pi r N + N', and also the intensity of pion emission as a functiom of the angle between pion and nucleon. The results are compared with experiment. (auth)
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The nature of the 5-12 s "anomalous" X-ray pulsars (AXPs) remains a mystery. Among the models that have been proposed to explain the properties of AXPs, the most likely are (1) isolated accreting neutron stars evolved from the Thorne-Żytkow objects (TŻOs) due to complete spiral-in during the common envelope (CE) evolution of high-mass X-ray binaries (HMXBs), and (2) magnetars, which are neutron stars with ultrahigh (~1014-1015 G) surface magnetic fields. We have critically examined the predicted change of a neutron star's spin in the accretion model, and found that it is unable to account for the steady spin-down observed in AXPs. A simple analysis also shows that any accretion disk around an isolated neutron star has an extremely limited lifetime. A more promising explanation for such objects is the magnetar model.
Magnetar
Accretion disc
Envelope (radar)
X-ray pulsar
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This is a review of our understanding of the evolution of the spin periods of neutron stars in accretion‐powered binary x‐ray pulsars during the course of evolution of these binary systems. I discuss how neutron stars born in binaries as rotation‐powered pulsars are initially spun down by electromagnetic and plasma torques, until accretion of matter from the companion star begins. I describe how accretion torques spin up and spin down the star during the main x‐ray emission phase of neutron stars accreting from disks, winds, or equatorial rings produced by mass loss from their companions. I outline the final phase of spinup of the neutron stars into recycled rotation‐powered pulsars.
X-ray pulsar
X-ray binary
X-ray burster
Compact star
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Field theory (psychology)
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We present a model for the anomalous X-ray pulsars (AXPs) in which the emission is powered by accretion from a fossil disk, established from matter falling back onto the neutron star following its birth. The time-dependent accretion drives the neutron star toward a "tracking" solution in which the rotation period of the star increases slowly, in tandem with the declining accretion rate. For appropriate choices of disk mass, neutron star magnetic field strength, and initial spin period, we demonstrate that a rapidly rotating neutron star can be spun down to periods characteristic of AXPs on timescales comparable to the estimated ages of these sources. In other cases, accretion onto the neutron star switches off after a short time and the star becomes an ordinary radio pulsar. Thus, in our picture, radio pulsars and AXPs are drawn from the same underlying population, in contrast to the situation in models involving neutron stars with ultrastrong magnetic fields, which require a new population of stars with very different properties.
X-ray pulsar
Intermediate polar
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The response was studied of a rotating neutron star to fluctuating torques and it was found that the observed variations in the pulsation periods of the compact X-ray sources Cen X-3 and Her X-1 could be caused by short time scale fluctuations in the accretion torques acting on the neutron stars. The sizes and rates of the required fluctuations are consistent with current accretion models. Such fluctuations can cause period variations either (a) directly, by causing a random walk of the star's angular velocity or (b) indirectly, by exciting a long-period mode of the neutron star, such as the Tkachenko mode of the rotating neutron superfluid. Phenomena in compact X-ray sources and cataclysmic variables which may be caused by fluctuating mass flow rates are also discussed.
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