Torque reversals and wind variations of X-ray pulsar Vela X-1

TL;DR

This study investigates the connection between wind variations and torque reversals in Vela X-1, revealing that changes in stellar wind density influence the neutron star's spin behavior without significantly affecting luminosity.

Contribution

It provides new insights into how stellar wind fluctuations cause torque reversals and wind variations in Vela X-1, linking wind properties to spin dynamics.

Findings

Higher fluxes during spin-up intervals suggest increased stellar wind activity.

Increased column density correlates with spin-up periods, indicating more material on the orbital scale.

Wind variations may cause alternating accretion flow directions, affecting angular momentum transfer.

Abstract

The erratic spin history of Vela X-1 shows some continuous spin-up/spin-down trend over tens of days. We study the orbital profile and spectral property of Vela X-1 in these spin-up/spin-down intervals, using the spin history monitored by Fermi/GBM and light curve from Swift/BAT and MAXI/GSC. The BAT fluxes in the spin-up intervals are about 1.6 times those of the spin-down intervals for out-of-eclipse orbital phases. The spin-up intervals also show a higher column density than the spin-down intervals, indicating there are more material on the orbital scale for the spin-up intervals. It could be due to the variation of the stellar wind of the optical star (HD 77581) on tens of days. The varying wind could lead to alternating prograde/retrograde accreting flow to the neutron star, which dominates the transfer of the angular momentum to Vela X-1, but not the total observed luminosity.