On the magnetic fields of Be/X-ray pulsars in the Small Magellanic Cloud: Figure 1.

Abstract

We explore the possibility of explaining the properties of the Be/X-ray pulsars observed in the Small Magellanic Cloud (SMC) within the magnetic levitation accretion scenario. This implies that their X-ray emission is powered by a wind-fed accretion on to a neutron star (NS) which captures matter from a magnetized stellar wind. The NS in this case is accreting matter from a non-Keplerian magnetically levitating disc which is surrounding its magnetosphere. This allows us to explain the observed periods of the pulsars in terms of spin equilibrium without the need of invoking dipole magnetic fields outside the usual range ∼ 10 11 − 10 13 G inferred from cyclotron features of Galactic high-mass X-ray binaries. We find that the equilibrium period of a NS, under certain conditions, depends strongly on the magnetization of the stellar wind of its massive companion and, correspondingly, on the magnetic field of the massive companion itself. This may help to explain why similar NSs in binaries with similar properties rotate with different periods yielding a large scatter of periods of the accretionpowered pulsar observed in SMC and our galaxy.

The spin-down mechanism of the X-ray pulsar 4U 2206+54

“…Otherwise, a Keplerian accretion disk forms in the system. The condition of disk formation (r circ > r a ) can be written as v rel < v cr , where [12] v cr ≃ 210 ξ…”

A new look at anomalous X-ray Pulsars

“…Under these conditions, the star begins its evolution in the ejector state for all excepted values of the parameterṀ, given in the paper [18]. Its spin period in this case gradually increases according to the canonical radio-pulsar model and reaches the critical value at which the star switches to the propeller state on the time-scale [32] τ ej ≃ 10 6 f −1/2 m…”

“…Magnetic accretion onto a neutron star have been recently discussed in [49,48,32]. These studies have shown that transformation of the quasi-spherical flow into a magnetic slab takes place if the Shvartsman radius exceeds a canonical Alfvén radius.…”

Spin evolution of long-period X-ray pulsars

“…Recently, however, doubts have been pronounced on a feasibility of an assumption about a presence of super-strong magnetic fields on the surface of long-period X-ray pulsars [11,12]. Investigation of the long-period X-ray pulsar GX 301-2 has revealed that a controversy between theoretical predictions and observational data arises only when one attempts to explain the episodes of its rapid breaking.…”

“…The magnetic field of the neutron star estimated from observations of its rapid spin-up episodes agrees with the value obtained through observations of the cyclotron feature in the X-ray spectra of this source (see [11] and references therein). An attempt to describe the episode of rapid spin-down of the neutron star SPX 1062 within the conventional model leads to a more paradoxical result: The Alfven radius of the neutron star turns out to exceed its corotation radius, r cor = GM ns /ω 2 s 1/3 , that excludes a possibility of stationary matter accretion onto its surface and obscures the nature of this X-ray source [12]. These results indicate that the spin-down torque exerted on a neutron star by the accretion flow is likely to exceed |K (0) sd |, and underestimation of this parameter may be connected with undue simplification of commonly used accretion scenarios.…”