Photoionized Lines in the X-ray Spectra of SMC X-1

Abstract

We present a detailed spectral analysis of Chandra/ACIS-S CC mode observations of the massive X-ray binary system SMC X-1. The system was observed during both the high and low X-ray states of the roughly 60-day superorbital period. The continuum spectra during both states are well represented by a power law with photon index $α$=0.9 and a blackbody of kT = 0.15keV. The high state spectra are dominated by the continuum and independent of orbital phase whereas the low state spectra show a strong orbital dependence as well as line emission from O, Ne, Mg, Fe, and Si. This is consistent with the states attributed to disk precession: during the high state X-ray emission is dominated by the compact source which is abrubtly eclipsed and during the low state the compact object is hidden by the disk and a larger, less luminous scattering region is responsible for the X-ray emission. A prominent Ne IX feature places a stringent limit (Log $ξ$ = 2.0-2.5) on the ionization parameter which constrains the wind dynamics of the system. The Fe line fluxes are related linearly to the blackbody fluxes indicating that both originate in the same region or are excited by the same mechanism. There is evidence for structure in the Fe-line that cannot be fully resolved by the current observations. The pulse period measured during our observations, 0.7057147$\pm$0.00000027s shows that the uninterrupted spin-up trend of SMC X-1 continues. We discuss the implications of our results for models of SMC X-1.