Updated orbital ephemeris of the ADC source X 1822-371: a stable orbital expansion over 40 years

Aims: Source X 1822-371 is an eclipsing compact binary system with a period close to 5.57 h and an orbital period derivative Ṗ_orb of 1.51(7)×10^-10 s s^-1. The very high value of Ṗ_orb is compatible with a super-Eddington mass transfer rate from the companion star, as suggested by X-ray and optical data. The XMM-Newton observation taken in 2017 allows us to update the orbital ephemeris and verify whether the orbital period derivative has been stable over the past 40 yr.
Methods: We added two new values obtained from the Rossi-XTE (RXTE) and XMM-Newton observations performed in 2011 and 2017, respectively, to the X-ray eclipse arrival times from 1977 to 2008. We estimated the number of orbital cycles and the delays of our eclipse arrival times spanning 40 yr, using as reference time the eclipse arrival time obtained from the RXTE observation taken in 1996.
Results: Fitting the delays with a quadratic model, we found an orbital period P_orb = 5.57062957(20) h and a Ṗ_orb value of 1.475(54)×10^-10 s s^-1. The addition of a cubic term to the model does not significantly improve the fit quality. We also determined a spin-period value of P_spin = 0.5915669(4) s and its first derivative Ṗ_spin = - 2.595(11) × 10^-12 s s^-1.
Conclusions: Our results confirm the scenario of a super-Eddington mass transfer rate; we also exclude a gravitational coupling between the orbit and the change in the oblateness of the companion star triggered by the nuclear luminosity of the companion star.