ORBITAL and SPIN PARAMETER VARIATIONS of PARTIAL ECLIPSING LOW MASS X-RAY BINARY X 1822-371

We report our measurements for the orbital and spin parameters of X 1822-371 using its X-ray partial eclipsing profile and pulsar timing from data collected by the Rossi X-ray Timing Explorer (RXTE). Four more X-ray eclipse times obtained by the RXTE 2011 observations were combined with historical records to trace the evolution of the orbital period. We found that a cubic ephemeris likely better describes the evolution of the X-ray eclipse times during a time span of about 34 years with a marginal second order derivative of P_orb = (-1.05 ± 0.59) × 10^-19s^-1. Using the pulse arrival time delay technique, the orbital and spin parameters were obtained from RXTE observations from 1998 to 2011. The detected pulse periods show that the neutron star in X 1822-371 is continuously spun-up with a rate of P_s = (-2.6288 ± 0.0095) × 10^-12 s s^-1. Although the evolution of the epoch of the mean longitude l = π/2 (i.e., T_π/2) gives an orbital period derivative value consistent with that obtained from the quadratic ephemeris evaluated from the X-ray eclipse, the detected T_π/2 values are significantly and systematically earlier than the corresponding expected X-ray eclipse times by 90 ± 11 s. This deviation is probably caused by asymmetric X-ray emissions. We also attempted to constrain the mass and radius of the neutron star using the spin period change rate and concluded that the intrinsic luminosity of X 1822-371 is likely more than 10³⁸ erg s^-1.