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

Abstract

The black hole binary Cygnus X-1 has a 5.6 day orbital period. We first detected a clear intensity modulation with the orbital period in its high/soft state with 6 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, with 8+-1%, 4+-1% and 3+-2% for 2-4 keV, 4-10 and 10-20 keV bands, showing a spectral hardening at the superior conjunction of the black hole. Spectral analysis with a model consisting of a power law and a photoelectric absorption, showed that the hydrogen column density increased from (2.9+-0.4)E+21 to (4.7+-1.1)E+21 cm^-2 around the superior conjunction, although more complex spectral variation, such as a partial absorption, was suggested, and the flux of the power law component decreased with 6+-1%. On the other hand, the MFs of the folded light curves in the high/soft state, were 4+-1% and 4+-2% for 2-4 keV and 4-10 keV bands, respectively. We applied a model consisting of a power law and a diskblackbody with a photoelectric absorption and found a modulation of the flux of the power law component with 7+-5% in MF, while the modulation of the hydrogen column density was less than 1E+21 cm^-2. These results can be interpreted as follows; the modulation of both states can be mainly explained by scattering of the 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 a low ionization state of the wind in the line of sight at the phase. Such a condition can be established by reasonable parameters of an in-homogeneous wind and the observed luminosities.