Description
In classical supergiant X-ray binaries (SgXBs), the Bondi-Hoyle-Lyttleton wind accretion was usually assumed which caused negligible changes in angular momentum of accretors. The observed spin-up of the neutron star, however, is still a hanging problem. In this paper, we report an extended low state of Vela X-1 (at about orbital phases 0.16—0.2), lasting for at least 30 ks, observed with Chandra during the onset of an unusual spin-up period. During this low state, the continuum fluxes dropped by a factor of 10 compared to the preceding flare period, and the pulsation of the continua almost disappeared. Meanwhile, the Fe K$\alpha$ fluxes of the low state were similar to the preceding flare, leading to an equivalent width (EW) of 0.6 keV, as high as the EW during the eclipse phase of Vela X-1. The pulsation cessation and the high Fe K$\alpha$ EW indicate an axisymmetric structure with a column density larger than $10^{24}$ cm$^{-2}$ on the spatial scale of the accretion radius of Vela X-1. These phenomena are consistent with the existence of an accretion disk that leads to the following spin-up of Vela X-1. It indicates that the disk accretion, although not always, does occur in classical wind-fed SgXBs.
