The superslow pulsation X-ray pulsars in high mass X-ray binaries

Abstract

There exists a special class of X-ray pulsars that exhibit very slow pulsation of $P_{\rm spin}>1000$ s in the high mass X-ray binaries (HMXBs). We have studied the temporal and spectral properties of these superslow pulsation neutron star binaries in hard X-ray bands with INTEGRAL observations. Long-term monitoring observations find spin period evolution of two sources: spin-down trend for 4U 2206+54 ($P_{\rm spin}\sim 5560$ s with $\dot{P}_{\rm spin}\sim 4.9\times 10^{-7}$ s s$^{-1}$) and long-term spin-up trend for 2S 0114+65 ($P_{\rm spin}\sim 9600$ s with $\dot{P}_{\rm spin}\sim -1\times 10^{-6}$ s s$^{-1}$) in the last 20 years. A Be X-ray transient, SXP 1062 ($P_{\rm spin}\sim 1062$ s), also showed a fast spin-down rate of $\dot{P}_{\rm spin}\sim 3\times 10^{-6}$ s s$^{-1}$ during an outburst. These superslow pulsation neutron stars cannot be produced in the standard X-ray binary evolution model unless the neutron star has a much stronger surface magnetic field ($B>10^{14}$ G). The physical origin of the superslow spin period is still unclear. The possible origin and evolution channels of the superslow pulsation X-ray pulsars are discussed. Superslow pulsation X-ray pulsars could be younger X-ray binary systems, still in the fast evolution phase preceding the final equilibrium state. Alternatively, they could be a new class of neutron star system $-$ accreting magnetars.