Evidence of a non-conservative mass transfer in the ultra-compact X-ray source XB 1916-053
Authors: Iaria R. , Sanna A. , Di Salvo T. , Gambino A. F. , Mazzola S. M. , Riggio A. , Marino A. , Burderi L. 
Published: December 2020

Context. The dipping source XB 1916-053 is a compact binary system with an orbital period of 50 min harboring a neutron star. It shows a positive and a negative superhump, which suggests the presence of a precessing elliptic accretion disk tilted with respect to the equatorial plane of the system. The orbital ephemeris indicates a large orbital period derivative, Ṗ/P = 1.53 × 10−7 yr−1, that can be explained assuming a high non-conservative mass transfer rate. Finally, the spectrum shows prominent absorption lines indicating the presence of an ionized absorber along the line of sight.
Aims. Using ten new Chandra observations and one 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.
Methods. 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. The Chandra spectra are fitted adopting a Comptonized continuum. To fit the absorption lines we adopt the ZXIPCF component obtaining information on the ionization parameter and the equivalent hydrogen column density of the ionized absorber.
Results. From the analysis of the dip arrival times we confirm an orbital period derivative of Ṗ = 1.46(3) × 10−11 s s−1. Furthermore, the unabsorbed 0.1–100 keV luminosity observed from the Chandra spectra show a variation between 3 × 1036 and 1.4 × 1037 erg s−1. We show that the Ṗ value and the luminosity values are compatible with neutron star masses higher than 1.4 M⊙ with a mass accretion rate lower than 10% of the mass transfer rate. We show that the mass ratio q = m2/m1 of 0.048 explains the apsidal precession period of 3.9 d and the nodal precession period of 4.86 d deduced from the superhump and infrahump detected period. The observed absorption lines are associated with the presence of Ne X, Mg XII, Si XIV, SXVI, and Fe XXVI ions. We observe a redshift in the absorption lines between 1.1 × 10−3 and 1.3 × 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 108 cm from the neutron star with a mass of 1.4 M⊙ and has a hydrogen atom density greater than 1015 cm−3. Instead, the absorber is more distant and could be placed at the outer rim of the accretion disk (1.7 × 1010 cm) during the dip activity.
Conclusions. We show that the mass ratio of the source is 0.048; this value is obtained from the nodal precession period of the disk and from the apsidal precession period taking into account the pressure term due to the spiral wave present in the disk. From our analysis we estimate a pitch angle of the spiral wave smaller than 30°, in agreement with the values observed in several cataclysmic variables. We show that the outer radius of the disk is truncated at the radius in which a 3:1 resonance occurs, which is 1.7 × 1010 cm for a neutron star mass of 1.4 M⊙. The large orbital period derivative is likely due to a high non-conservative mass transfer with a mass transfer rate of 10−8 M⊙ yr−1. The variation in observed luminosity could be explained assuming that the ejection point from which the matter leaves the system moves close to the inner Lagrangian point.