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

ABSTRACT The X-ray luminosity (Lx) and the rotational properties of 4U 1626–67 have been measured at regular intervals during the last four decades. It has been recorded that the source underwent torque reversals twice. We have tried to understand whether these eccentrical sign-switches of the spin period derivative ($\dot{P}$) of 4U 1626–67 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 inner disc radius that leads to different Lx–$\dot{P}$ profiles than the aligned rotator cases. We have concluded that the oblique rotator assumption with the inclination angle χ ∼ (10°−30°) brings at least equally best fitting to the observed Lx and $\dot{P}$ of 4U 1626–67. More importantly, the estimated change of the mass accretion rate, which causes the change in observed Lx 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 Lx could be explained naturally with an inclined magnetic axis and rotation axis of the neutron star.