Parameterization studies of the properties of the X-ray dips for Low Mass X-ray binary X1916-053

Abstract

The ultra-compact Low Mass X-ray Binary (LMXB) X1916-053, composed of a neutron star and a semi-degenerated white dwarf, exhibits periodic X-ray dips with variable width and depth. We have developed new methods to parameterize the dip to systematically study its variations. This helps to further understand binary and accretion disk behaviors. The RXTE 1998 observations clearly show a 4.87d periodic variation of the dip width. This is probably due to the nodal precession of the accretion disk, although there are no significant sidebands in the spectrum from the epoch folding search. From the negative superhump model (Larwood et. al. 1996), the mass ratio can be estimated as q = 0.045. Combined with more than 24 years of historical data, we found an orbital period derivative of $\dot{P}_{orb}/P_{orb}=(1.62 \pm 0.48)\times 10^{-7} yr^{-1}$ and established a quadratic ephemeris for the X-ray dips. The period derivative seems inconsistent with the prediction of the standard model of binary orbital evolution proposed by Rappaport et. al. (1987). On the other hand, the radiation-driven model (Tavani et. al. 1991) may properly interpret the period derivative even though the large mass outflow predicted by this model has never been observed in this system. With the best ephemeris, we obtained that the standard deviation of primary dips are smaller than that of secondary dips. This means that the primary dips are more stable than the secondary dips. Thus, we conclude that the primary dips of X1916-053 occur from the bulge at the rim instead of the ring of the disk proposed by Frank et. al. (1987).