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 similar or equal to 220-290 km s(-1), corresponding to the gravitational redshift at a radius of R similar to 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 sigma. 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 similar to 70 GM/c(2); this component is significant at the 3 sigma level. Finally, in one spectrum, we find evidence of a disk wind with a blueshift of v = - 1700(-1200)(+1200) km s(-1). If real, this wind would require magnetic driving.