Detailed study of the X-ray and optical/UV orbital ephemeris of X1822-371

Aims: Recent studies of the optical/UV and X-ray ephemerides of X1822-371 have found some discrepancies in the value of the orbital period derivative. Because of the importance of this value in constraining the system evolution, we comprehensively analyse all the available optical/UV/X eclipse times of this source to investigate the origin of these discrepancies. Methods: We collected all previously published X-ray eclipse times from 1977 to 2008, to which we added the eclipse time observed by Suzaku in 2006. This point is very important to cover the time gap between the last RXTE eclipse time (taken in 2003) and the most recent Chandra eclipse time (taken in 2008). Similarly we collected the optical/UV eclipse arrival times covering the period from 1979 to 2006, adding a further eclipse time taken on 1978 and updating previous optical/UV ephemeris. We compared the X-ray and the optical/UV ephemeris, and finally derived a new ephemeris of the source by combining the eclipse arrival times in the X-ray and optical/UV bands. Results: The X-ray eclipse time delays calculated with respect to a constant orbital period model display a clear parabolic trend, confirming that the orbital period of this source constantly increases at a rate of Ṗorb = 1.51(7) × 10-10 s/s. Combining the X-ray and the optical/UV data sets, we find that Ṗorb = 1.59(9) × 10-10 s/s, which is compatible with the X-ray orbital solution. We also investigate the possible presence of a delay of the optical/UV eclipse with respect to the X-ray eclipse, finding that this delay may not be constant in time. In particular, this variation is compatible with a sinusoidal modulation of the optical/UV eclipse arrival times with respect to the long-term parabolic trend. In this case, the optical/UV eclipse should lag the X-ray eclipse and the time-lag oscillate about an average value. Conclusions: We confirm that the orbital period derivative is three orders of magnitude larger than expected from conservative mass transfer driven by magnetic braking and gravitational radiation