Evidence of a non-conservative mass transfer in the ultra-compact X-ray source XB 1916-053

Abstract

The dipping source XB 1916-053 is a compact binary system with an orbital period of 50 min harboring a neutron star. Using ten new {\it Chandra} observations and one {\it Swift/XRT} observation, we are able to extend the baseline of the orbital ephemeris; this allows us to exclude some models that explain the dip arrival times. The Chandra observations provide a good plasma diagnostic of the ionized absorber and allow us to determine whether it is placed at the outer rim of the accretion disk or closer to the compact object. From the available observations we are able to obtain three new dip arrival times extending the baseline of the orbital ephemeris from 37 to 40 years. From the analysis of the dip arrival times we confirm an orbital period derivative of $\dot{P}=1.46(3) \times 10^{-11}$ s s$^{-1}$. We show that the $\dot{P}$ value and the luminosity values are compatible with a mass accretion rate lower than 10\% of the mass transfer rate. We show that the mass ratio $q=m_2/m_1$ of 0.048 explains the apsidal precession period and the nodal precession period. The observed absorption lines are associated with the presence of \ion{Ne}{x}, \ion{Mg}{xii}, \ion{Si}{xiv}, \ion{S}{xvi,} and \ion{Fe}{xxvi} ions. We observe a redshift in the absorption lines between $1.1 \times 10^{-3}$ and $1.3 \times 10^{-3}$. By interpreting it as gravitational redshift, as recently discussed in the literature, we find that the ionized absorber is placed at a distance of $10^8$ cm from the neutron star with a mass of 1.4 M$_{\odot}$ and has a hydrogen atom density greater than $10^{15}$ cm$^{-3}$. (Abstract abridged)