Orbital and Spin Parameter Variations of Partial Eclipsing Low Mass X-ray Binary X 1822-371

Abstract

We report our measurements for 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 evolution of orbital period. We found that a cubic ephemeris likely better describes evolution of the X-ray eclipse times during a time span of about 34 years with a marginal second order derivative of $\ddot{P}_{orb}=(-1.05 \pm 0.59) \times 10^{-19}$ s$^{-1}$. 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 $\dot{P}_{s}=(-2.6288 \pm 0.0095) \times 10^{-12}$ s s$^{-1}$. Evolution of the epoch of the mean longitude $l=\pi /2$ (i.e. $T_{\pi / 2}$) gives an orbital period derivative value consistent with that obtained from the quadratic ephemeris evaluated by the X-ray eclipse but the detected $T_{\pi / 2}$ values are significantly and systematically earlier than the corresponding expected X-ray eclipse times by $90 \pm 11$ 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 $10^{38}$ ergs s$^{-1}$.