A Redshifted Inner Disk Atmosphere and Transient Absorbers in the Ultracompact Neutron Star X-Ray Binary 4U 1916–053

Abstract

The very small accretion disks in ultracompact X-ray binaries are special laboratories in which to study disk accretion and outflows. We report on three sets of new (250 ks total) and archival (50 ks) Chandra/HETG observations of the “dipping” neutron star X-ray binary 4U 1916–053, which has an orbital period of P  ≃ 50 minutes. We find that the bulk of the absorption in all three spectra originates in a disk atmosphere that is redshifted by v  ≃ 220–290 km s −1 , corresponding to the gravitational redshift at a radius of R  ∼ 1200 GM / c 2 . This shift is present in the strongest, most highly ionized lines (Si xiv and Fe xxvi ), with a significance of 5 σ . Absorption lines observed during dipping events (typically associated with the outermost disk) instead display no velocity shifts and serve as a local standard of rest, suggesting that the redshift is intrinsic to an inner disk atmosphere and not due to radial motion in the galaxy or a kick. In two spectra, there is also evidence of a more strongly redshifted component that would correspond to a disk atmosphere at R  ∼ 70 GM / c 2 ; this component is significant at the 3 σ level. Finally, in one spectrum, we find evidence of a disk wind with a blueshift of . If real, this wind would require magnetic driving.