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

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.

5 pages, 5 figs, to appear on MNRAS Letters

References in corpus (6)

• The ephemeris, orbital decay, and masses of 10 eclipsing HMXBs
• Towards a unified view of inhomogeneous stellar winds in isolated supergiant stars and supergiant high mass X-ray binaries
• High variability in Vela X-1: giant flares and off states
• Stellar wind accretion and accretion disk formation: applications to neutron star high mass X-ray binaries
• Spectral evidence of an accretion disk in wind-fed X-ray pulsar Vela X-1 during an unusual spin-up period
• Torque reversal and orbital profile of X-ray pulsar OAO 1657-415