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

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/c2. This shift is present in the strongest, most highly ionized lines (Si xiv and Fe xxvi), with a significance of 5σ. 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/c2; this component is significant at the 3σ level. Finally, in one spectrum, we find evidence of a disk wind with a blueshift of $v=-{1700}_{-1200}^{+1700}\,\mathrm{km}\,{{\rm{s}}}^{-1}$. If real, this wind would require magnetic driving.