Radius of the neutron star magnetosphere during disk accretion

TL;DR

This study investigates how the spin-up rate of accreting neutron stars correlates with luminosity, confirming the disk accretion model and revealing variability in magnetosphere radius across different systems.

Contribution

It provides observational evidence supporting the disk accretion model for neutron stars and estimates the magnetosphere radius, highlighting its variability among pulsars.

Findings

The spin frequency derivative follows the $ u̇ \,\sim\, L_{bol}^{6/7}$ relation in most systems.

Hysteresis in the $ u̇(L_{bol})$ dependence is confirmed and observed in new systems.

The magnetosphere radius varies significantly between pulsars and depends on system parameters.

Abstract

The dependence of the spin frequency derivative $\dot{\nu}$ of accreting neutron stars with a strong magnetic field (X-ray pulsars) on the mass accretion rate (bolometric luminosity, $L_{bol}$) has been investigated for eight transient pulsars in binary systems with Be stars. Using data from the Fermi/GBM and Swift/BAT telescopes, we have shown that for seven of the eight systems the dependence $\dot{\nu}$ can be fitted by the model of angular momentum transfer through an accretion disk, which predicts the relation $\dot{\nu}\sim L^{6/7}_{bol}$. Hysteresis in the dependence $\dot{\nu}(L_{bol})$ has been confirmed in the system V 0332+53 and has been detected for the first time in the systems KS 1947+300, GRO J1008-57, and 1A 0535+26. The radius of the neutron star magnetosphere in all of the investigated systems have been estimated. We show that this quantity varies from pulsar to pulsar and depends strongly on the analytical model and the estimates for the neutron star and binary system parameters.