A Redshifted Inner Disk Atmosphere and Transient Absorbers in the Ultra-Compact Neutron Star X-ray Binary 4U 1916-053

Abstract

The very small accretion disks in ultra-compact X-ray binaries (UCXBs) 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 $P\simeq 50$~minutes. We find that the bulk of the absorption in all three spectra originates in a disk atmosphere that is redshifted by $v\simeq 220-290$ $\text{km}$ ${\text{s}}^{-1}$, corresponding to the gravitational redshift at radius of $R\sim 1200$ $GM/c^2$. This shift is present in the strongest, most highly ionized lines (Si XIV and Fe XXVI), with a significance of 5$\ddot{I}ƒ$. 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 $R\sim 70$ $GM/c^2$; this component is significant at the 3$\ddot{I}ƒ$ level. Finally, in one spectrum, we find evidence of disk wind with a blue shift of $v={-1700}_{-1200}^{+1700}$ $\text{km}$ ${\text{s}}^{-1}$. If real, this wind would require magnetic driving.

15 Pages, 6 Figures, Accepted to ApJ Letters