Spectral evidence of an accretion disc in wind-fed X-ray pulsar Vela X-1 during an unusual spin-up period

In classical supergiant X-ray binaries (SgXBs), the Bondi-lloyle-Lyttleton wind accretion was usually assumed, and the angular momentum transport to the accretors is inefficient. The observed spin-up/spin-down behaviour of the neutron star in SgXBs is not well understood. In this paper, we report an extended low state of Vela X-1 (at 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 continuum pulsation almost disappeared. Meanwhile, the Fe Ku fluxes of the low state were similar to the preceding flare period, leading to an Fe K or. equivalent width (EW) of 0.6 keV, as high as the Fe K a EW during the eclipse phase of Vela X-1. Both the pulsation cessation and the high Fe K ce EW indicate an axisymmetric structure with a column density larger than 1024 cm 2 on a spatial scale of the accretion radius of Vela X-1. These phenomena are consistent with the existence of an accretion disc that leads to the following spin -up of Vela X-1. It indicates Mat disc accretion, although not always, does occur in classical wind -fed SgXBs.