Orbital modulations of X-ray light curves of Cygnus X-1 in its low/hard and high/soft states

Abstract

Abstract The black hole binary Cygnus X-1 has a 5.6-d orbital period. We first detected a clear intensity modulation with the orbital period in its high/soft state with six-year MAXI data, as well as in its low/hard state. In the low/hard state, the folded light curves showed an intensity drop at the superior conjunction of the black hole by a modulation factor (MF), which is the amplitude divided by the average, of 8 ± 1%, 4 ± 1%, and 3 ± 2% for 2–4, 4–10, and 10–20 keV bands, respectively, showing a spectral hardening at the superior conjunction of the black hole. Spectral analysis in the low/hard state, with a model consisting of a power law and a photoelectric absorption, showed that the hydrogen column density, NH, increased from (2.9 ± 0.4) × 1021 cm−2 to (4.7 ± 1.1) × 1021 cm−2 around the superior conjunction. The flux of the power-law component decreased by 6 ± 1%. On the other hand, MFs for the folded light curves in the high/soft state, were 4 ± 1% and 4 ± 2% for the 2–4 keV and 4–10 keV bands, respectively. We applied a model consisting of a power law and a disk blackbody with a photoelectric absorption. A modulation of the flux of the power-law component was found to be 7 ± 5% in MF, while the modulation of NH was less than 1 × 1021 cm−2. These results can be interpreted as follows: the modulation of both states can be mainly explained by scattering of X-rays by an ionized stellar wind, but, only at the superior conjunction in the low/hard state, a large photoelectric absorption appears because of the low ionization state of the wind in the line of sight at phase 0. Such a condition can be established by reasonable parameters of an inhomogeneous wind and the observed luminosities.