Signature of the presence of a third body orbiting around XB 1916-053

Context. The ultra-compact dipping source XB 1916-053 has an orbital period of close to 50 min and a companion star with a very low mass (less than 0.1 M_⊙). The orbital period derivative of the source was estimated to be 1.5(3) × 10^-11 s/s through analysing the delays associated with the dip arrival times obtained from observations spanning 25 years, from 1978 to 2002.
Aims: The known orbital period derivative is extremely large and can be explained by invoking an extreme, non-conservative mass transfer rate that is not easily justifiable. We extended the analysed data from 1978 to 2014, by spanning 37 years, to verify whether a larger sample of data can be fitted with a quadratic term or a different scenario has to be considered.
Methods: We obtained 27 delays associated with the dip arrival times from data covering 37 years and used different models to fit the time delays with respect to a constant period model.
Results: We find that the quadratic form alone does not fit the data. The data are well fitted using a sinusoidal term plus a quadratic function or, alternatively, with a series of sinusoidal terms that can be associated with a modulation of the dip arrival times due to the presence of a third body that has an elliptical orbit. We infer that for a conservative mass transfer scenario the modulation of the delays can be explained by invoking the presence of a third body with mass between 0.10-0.14 M_⊙, orbital period around the X-ray binary system of close to 51 yr and an eccentricity of 0.28 ± 0.15. In a non-conservative mass transfer scenario we estimate that the fraction of matter yielded by the degenerate companion star and accreted onto the neutron star is β = 0.08, the neutron star mass is ≥2.2 M_⊙, and the companion star mass is 0.028 M_⊙. In this case, we explain the sinusoidal modulation of the delays by invoking the presence of a third body with orbital period of 26 yr and mass of 0.055 M_⊙.
Conclusions: From the analysis of the delays associated with the dip arrival times, we find that both in a conservative and non-conservative mass transfer scenario we have to invoke the presence of a third body to explain the observed sinusoidal modulation. We propose that XB 1916-053 forms a hierarchical triple system.

Table 1 is available in electronic form at http://www.aanda.org