X-ray Variations at the Orbital Period from Cygnus X-1 in the High/Soft State

Abstract

Orbital variability has been found in the X-ray hardness of the black hole candidate Cygnus X-1 during the soft/high X-ray state using light curves provided by the Rossi X-ray Timing Explorer's All Sky Monitor. We are able to set broad limits on how the mass-loss rate and X-ray luminosity vary between the hard and soft states. The folded light curve shows diminished flux in the soft X-ray band at phase 0 (defined as the time of of the superior conjunction of the X-ray source). Models of the orbital variability provide slightly superior fits when the absorbing gas is concentrated in neutral clumps and better explain the strong variability in hardness. In combination with the previously established hard/low state dips, our observations give a lower limit to the mass loss rate in the soft state (Mdot<2x10^{-6} Msun/yr) than the limit in the hard state (Mdot<4x10^{-6} Msun/yr). Without a change in the wind structure between X-ray states, the greater mass-loss rate during the low/hard state would be inconsistent with the increased flaring seen during the high-soft state.Comment: Accepted for publication in The Astrophysical Journa