2008ApJ...680.1405H

The ultracompact low-mass X-ray binary X1916-053, which is composed of a neutron star and a semidegenerate white dwarf, exhibits periodic X-ray dips of variable width and depth. We have developed a new method to parameterize this dipping behavior to systematically study its variations. This can help in further understanding binary and accretion disk behaviors. Observations by the Rossi X-Ray Timing Explorer from 1998 clearly show a 4.87 day periodic variation in the dip width. This is probably due to nodal precession of the accretion disk, although an epoch-folding search finds no significant sidebands in the spectrum. From the negative-superhump model of Larwood et al., the mass ratio can be estimated as q=0.045. Combining more than 24 years of historical data, we find an orbital period derivative P{dot}_orb/P_orb=(1.62±0.34)x10^–7/yr and establish a quadratic ephemeris for the X-ray dips. This period derivative seems inconsistent with the prediction from the standard model of binary orbital evolution proposed by Rappaport et al. On the other hand, Tavani's radiation-driven model can properly account for the period derivative, although the large mass outflow predicted by this model has never been observed in this system. With the best ephemeris, we find that the standard deviation of the primary dips is smaller than that of the 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 where the accretion stream encounters the rim of the disk, rather than the inner disk ring as previously proposed by Frank et al.