The high-mass X-ray binary SMC X-1 is an eclipsing binary with an orbital period of 3.89 d. This system exhibits a superorbital modulation with a period varying between \sim 40 d and \sim 65 d. The instantaneous frequency and the corresponding phase of the superorbital modulation can be obtained by a recently developed time-frequency analysis technique , the Hilbert-Huang transform ( HHT ) .
We present a phase-resolved analysis of both the spectra and the orbital profiles with the superorbital phase derived from the HHT .
The X-ray spectra observed by the Proportional Counter Array onboard the Rossi X-ray Timing Explorer are fitted well by a blackbody plus a Comptonized component .
The plasma optical depth , which is a good indicator of the distribution of material along the line of sight , is significantly anti-correlated with the flux detected at 2.5 - 25 keV .
However , the relationship between the plasma optical depth and the equivalent width of the iron line is not monotonic : there is no significant correlation for fluxes higher than \sim 35 mCrab but clear positive correlation when the intensity is lower than \sim 20 mCrab .
This indicates that the iron line production is dominated by different regions of this binary system in different superorbital phases .
To study the dependence of the orbital profile on the superorbital phase , we obtained the eclipse profiles by folding the All Sky Monitor light curve with the orbital period for different superorbital states .
A dip feature , similar to the pre-eclipse dip in Her X-1 , lying at orbital phase \sim 0.6 - 0.85 , was discovered during the superorbital transition state .
This indicates that the accretion disk has a bulge that absorbs considerable X-ray emission in the stream-disk interaction region .
The dip width is anti-correlated with the flux , and this relation can be interpreted by the precessing tilted accretion disk scenario .