Understanding the coexistence of spin-up and spin-down behaviors in long period X-ray pulsars

TL;DR

This study models the rotational evolution of wind-fed accretion magnetars in long period X-ray pulsars, highlighting how magnetic field decay influences their spin periods and behaviors, and explaining observed spin trends in specific sources.

Contribution

It introduces detailed calculations of neutron star spin evolution considering magnetic decay, explaining the coexistence of different spin behaviors in long period X-ray pulsars.

Findings

Wind-fed accretion magnetars can reach spin periods over 1000 seconds.

Magnetic field decay decreases the equilibrium period at constant accretion rates.

Observed spin-up and spin-down rates are explained by accretion dynamics and magnetic decay.

Abstract

Assuming the wind-fed accretion magnetars in long period X-ray pulsars, we calculated the rotational evolution of the neutron stars. Our calculations considered the effects of the magnetic field decay in magnetars. The results show that wind-fed accretion magnetars can evolve to the long period X-ray pulsars with a spin period much longer than 1000 s. The spin-down trend observed in 4U 2206+54 like sources is expected when the young X-ray binary systems are on the way to their equilibrium period. Detailed calculations showed that its spin-down may be affected by accretion with outflows or accretion while spin-down. Due to the magnetic field decay in magnetars, wind-fed accretion magnetars will have a decreasing equilibrium period for a constant mass accretion rate. For 2S 0114+65, the spin-up rate due to magnetic field decay is one order of magnitude smaller than the observations. The spin-up rate of 2S 0114+65 may be attributed to the formation of a transient disk during wind accretion. The slowest X-ray pulsar AX J1910.7+0917 would be a link source between 4U 2206+54 and 2S 0114+65.