# On the Bimodal Spin Period Distribution of Be/X-ray Pulsars

## TL;DR

This paper explains the bimodal distribution of Be/X-ray pulsar spin periods by modeling their spin evolution with a magnetically threaded accretion disk, accounting for transient accretion and disk changes.

## Contribution

It introduces a comprehensive model including transient accretion and disk structure changes to explain the bimodal spin period distribution.

## Key findings

- Current pulsars are near spin equilibrium.
- Bimodal distribution is reproduced by equilibrium spin periods.
- Model aligns with observed period peaks.

## Abstract

It has been reported that there are two populations of Be/X-ray pulsars, with the pulse period distribution peaked at $\sim$ 10 s and $\sim$ 200 s, respectively. A possible explanation of this bimodal distribution is related to different accretion modes in Be/X-ray binaries. In this work, we investigate the spin evolution of Be/X-ray pulsars based on the magnetically threaded accretion disk model. Compared with previous works, we take into account several distinct and important factors of Be/X-ray binaries, including the transient accretion behavior and possible change of the accretion disk structure during quiescence. We demonstrate that current Be/X-ray pulsars are close to the spin equilibrium determined by the balance of spin-up during outbursts and spin-down during quiescence, and that the observed bimodal distribution can be well reproduced by the equilibrium spin periods with reasonable input parameters.