X-ray iron absorption line in Swift J1858.6−0814

Abstract

We present the spectral analysis of bright steady states in an outburst of the transient neutron star low-mass X-ray binary Swift J1858.6−0814 observed with the Neutron star Interior Composition Explorer (NICER). We detected an ionized iron K absorption line (H-like Fe) at 6.97 keV in the spectrum. We estimate the photoionization parameter using the ratio of the equivalent widths of the Fe xxvi (H-like) (17 ± 5 eV) and Fe xxv (He-like) (<3 eV) and discuss the origin of the iron absorption line. The irradiated gas producing the absorption line would locate within (3–6) × 109 cm of the X-ray source. We suggest that the observed H-like Fe absorption line originates from the highly ionized gas in the inner accretion disk in Swift J1858.6−0814.