On the peculiar torque reversals and the X-ray luminosity history of the accretion-powered X-ray pulsar 4U 1626--67

Abstract

We have investigated the rotational and the X-ray luminosity ($L_{\mathrm{x}}$) observations of 4U 1626--67 accumulated during the last four decades. It has been recorded that the source underwent a torque reversal twice. We have tried to understand whether this eccentrical sign-switch of the period derivative ($\dot{P}$) of the source could be accounted for with the existing torque models. We have found that the observed source properties are better estimated with the distances close to the lower limit of the previously predicted distance range ($5-13$ kpc). Furthermore, assuming an inclined rotator, we have considered the partial accretion/ejection from the co-rotation radius that leads different $L_{\mathrm{x}}$-$\dot{P}$ profiles than the aligned rotator cases. We have concluded that the oblique rotator assumption brings at least equally best fitting to the observed X-ray luminosity and the rotational properties of 4U 1626--67. More importantly, the estimated change of the mass accretion rate which causes the change in observed $L_{\mathrm{x}}$ of 4U 1626--67 is much less than that is found in an aligned rotator case. In other words, without the need for a substantial modification of mass accretion rate from the companion star, the range of the observed X-ray luminosity could be explained naturally with an inclined magnetic axis and rotation axis of the neutron star.