Settling accretion onto slowly rotating X-ray pulsars

Abstract

Quasi-spherical subsonic accretion onto slowly rotating magnetized NS is considered, when the accreting matter settles down subsonically onto the rotating magnetosphere, forming an extended quasi-static shell. The shell mediates the angular momentum transfer to/from the rotating NS magnetosphere by large-scale convective motions, which lead to an almost iso-angular-momentum 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 instability while taking cooling into account. The settling regime of accretion is possible for moderate X-ray luminosities L <4 10^36 erg/s. At higher luminosities a free-fall gap above the NS magnetosphere appears due to rapid Compton cooling, and accretion becomes highly non-stationary. From observations of spin-up/spin-down rates of wind accreting equilibrium XPSRs with known orbital periods (GX 301-2, Vela X-1), the main dimensionless parameters of the model and be determined and the NS magnetic field can be estimated. For equilibrium pulsars with independent measurements of the magnetic field the velocity of the stellar wind can be estimated without the use of complicated spectroscopic measurements. For non-equilibrium pulsars, a maximum possible value of the spin-down rate of the accreting neutron star exists. 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 on the neutron star magnetic field is obtained, which in all cases turns out to be close to the standard 10^12-10^13 G value, in agreement with cyclotron line measurements. The model explains both the spin-up/spin-down of the pulsar frequency on large time-scales and the irregular short-term frequency fluctuations, which may correlate or anti-correlate with the X-ray luminosity fluctuations.

LaTeX, 41 pages, 4 figures, accepted for publication in Physics-Uspekhi (translation from the Russian paper to appear in Uspekhi Fizicheskikh Nauk, 2013); revised and extended version of paper published in MNRAS 420 216 (2012). arXiv admin note: substantial text overlap with arXiv:1110.3701

References in corpus (7)

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• VLT/UVES spectroscopy of Wray 977, the hypergiant companion to the X-ray pulsar GX301-2
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• SXP 1062: an Evolved Magnetar in a BeXB ?
• Further evidence for the presence of a neutron star in 4U 2206+54. INTEGRAL and VLA observations
• Could SXP 1062 be an Accreting Magnetar?
• Signs of magnetic accretion in X-ray pulsars

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• Spin-up/spin-down of neutron star in Be-X-ray binary system GX 304-1
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