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Description
Ultra-Compact X-ray Binaries (UCXBs) are a subclass of neutron star low-mass X-ray binaries (NS-LMXBs) characterized by very short orbital periods ($P<80$ min) and compact system sizes, making them ideal systems for probing the properties of accretion disks, in particular the structure and kinematics of winds and outflows. The detection of wind features in X-ray spectra is not always straightforward, but the advent of XRISM has significantly expanded our capabilities in this regard.
We present the first results from XRISM observations of the UCXB 4U 1916-05, a high-inclination system with an orbital period of $P = 50\ \rm min$. The light curve shows periodic dips caused by obscuration from a vertically extended, ionized absorber in the outer accretion disk. Spectral analysis reveals, for the first time, the presence of four distinct absorbing components in the atmosphere of the system. These include a remarkably high redshifted $ Fe{XXVI}$ absorption line at $zv \sim 1500\ \rm km/s$
, alongside a blueshifted feature at $zv\sim 70\ \rm km/s $ associated with a lower ionization state. We find that the source exhibits a complex behaviour both as a function of time and orbital phase, with the dipping intervals playing a key role in modulating the blueshifted component.
These results provide new insights into the structure and dynamics of ionized absorbers in high-inclination UCXBs, and suggest a physical connection between the dipping activity and the kinematics of the accretion disk wind.