Reviews of topical problems

Quasi-spherical subsonic accretion in X-ray pulsars

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Lomonosov Moscow State University, Shternberg State Astronomical Institute, Universitetskii prosp. 13, Moscow, 119889, Russian Federation

A theoretical model is considered for quasi-spherical subsonic accretion onto slowly rotating magnetized neutron stars. In this regime the accreting matter settles down subsonically onto the rotating magnetosphere, forming an extended quasi-static shell. Angular momentum transfer in the shell occurs via large-scale convective motions resulting, for observed pulsars, in an almost iso-angular-momentum $\omega \sim 1/R^2$ rotation law inside the shell. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh—Taylor instabilities, with allowance for cooling. A settling accretion regime is possible for moderate accretion rates $\dot M \lesssim \dot M_* \simeq 4\times 10^{16}$ g s−1. At higher accretion rates a free-fall gap above the neutron star magnetosphere appears due to rapid Compton cooling, and the accretion becomes highly non-stationary. Observations of spin-up/spin-down rates of quasi-spherically wind accreting equilibrium X-ray pulsars with known orbital periods (like, e.g., GX~301-2 and Vela~X-1) enable us to determine the main dimensionless parameters of the model as well as to estimate surface magnetic field of the neutron star. For equilibrium pulsars, the independent measurements of the neutron star magnetic field allows for an estimate of the stellar wind velocity of the optical companion without using complicated spectroscopic measurements. For non-equilibrium pulsars, a maximum value is shown to exist for the spin-down rate of the accreting neutron star. From observations of the spin-down rate and the X-ray luminosity in such pulsars (e.g., GX 1+4, SXP 1062 and 4U 2206+54), a lower limit can be put on the neutron star magnetic field, which in all cases turns out to be close to the standard value and which agrees with cyclotron line measurements. The model further explains both the spin-up/spin-down of the pulsar frequency on large time-scales and also accounts for the irregular short-term frequency fluctuations, which may correlate or anti-correlate with the observed X-ray luminosity fluctuations.

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Fulltext is also available at DOI: 10.3367/UFNe.0183.201304a.0337
PACS: 95.30.Lz, 97.10.Gz, 97.80.Jp, 98.70.Qy (all)
DOI: 10.3367/UFNe.0183.201304a.0337
Citation: Shakura N I, Postnov K A, Kochetkova A Yu, Hjalmarsdotter L "Quasi-spherical subsonic accretion in X-ray pulsars" Phys. Usp. 56 321–346 (2013)
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Received: 16th, August 2012, revised: 23rd, November 2012, 27th, November 2012

Оригинал: Шакура Н И, Постнов К А, Кочеткова А Ю, Ялмарсдоттер Л «Квазисферическая дозвуковая аккреция на рентгеновские пульсары» УФН 183 337–364 (2013); DOI: 10.3367/UFNr.0183.201304a.0337

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