The post-quiescence properties of Cir X-1 at orbital phase around periastron observed by NuSTAR and NICER

Abstract

ABSTRACT Cir X-1 is an unusual X-ray binary, which has resumed periodic outbursts since August 2019. We investigate the post-quiescence spectral properties of Cir X-1 at orbital phase around the periastron using NICER and NuSTAR X-ray observatories after the resumption of outbursts. We identify two distinct classes of spectral properties. The first class exhibits highly ionized absorption, whereas the second class does not. We suggest that the ionized absorption originates from the shelter of the disc wind, while the low-ionized/neutral absorption arises from the companion’s stellar wind. We also suggest that the disc wind may be always present in both classes, and a warped precession disc could alter the visibility of the disc wind.