Superorbital Phase-Resolved Analysis of SMC X-1

Abstract

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 ~40 d and ~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 ~35 mCrab but clear positive correlation when the intensity is lower than ~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 ~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.

18 pages, 6 figures, Accepted for publication in ApJ

References in corpus (7)

• Determination of the mass of the neutron star in SMC X-1, LMC X-4 and Cen X-3 with VLT/UVES
• An Extended and More Sensitive Search for Periodicities in RXTE/ASM X-ray Light Curves
• Tracking the Orbital and Super-orbital Periods of SMC X-1
• BeppoSAX observations of the accretion-powered X-ray pulsar SMC X-1
• Analysis of RXTE-PCA Observations of SMC X-1
• Time-Frequency Analysis of Superorbital Modulation of X-ray Binary SMC X-1 by Hilbert-Huang Transform
• Constraints on the Velocity and Spatial Distribution of Helium-like Ions in the Wind of SMC X-1 from Observations with XMM-Newton/RGS

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• CG X-1: an eclipsing Wolf-Rayet ULX in the Circinus galaxy
• Modeling the precession of the warped inner accretion disk in the pulsars LMC X-4 and SMC X-1 with NuSTAR and XMM-Newton
• Monitoring the Superorbital Period Variation and the Spin Period Evolution of SMC X-1
• Probing the polarized emission from SMC X-1: the brightest X-ray pulsar observed by IXPE
• Possible Periodic Dips in the Pulsating Ultraluminous X-ray Source M51 ULX-7
• Extending the baseline for SMC X-1's spin and orbital behavior with NuSTAR stray light
• A New Possible Accretion Scenario for Ultra-Luminous X-ray Sources
• Discovery of Red-shifted He-like Iron Absorption Line from Luminous Accreting Neutron Star SMC X-1
• Monitoring Observations of SMC X-1's Excursions (MOOSE) I: Program Description and Initial High-State Spectral Results