Secrets behind the RXTE/ASM light curve of Cyg X-3

Abstract

Context. In wind-fed X-ray binaries, the radiatively driven wind of the primary star can be suppressed by the X-ray irradiation of the compact secondary star, leading to an increased accretion rate. This causes feedback between the released accretion power and the luminosity of the compact star (X-ray source). Aims. We investigate the feedback process between the released accretion power and the X-ray luminosity of the compact star (a low-mass black hole) in the unique high-mass X-ray binary Cygnus X-3. We study whether the seemingly erratic behavior of the observed X-ray light curve and accompanying spectral state transitions could be explained by this scenario. Methods. The wind-fed accretion power is positively correlated with the extreme-ultraviolet (EUV) irradiation of the X-ray source. It is also larger than the bolometric luminosity of the X-ray source derived by spectral modeling and assumed to be an intrinsic property of the source. We assume that a part of the wind-fed power experiences a small amplitude variability around the source luminosity. The largest luminosity (lowest wind velocity) is constrained by the Roche-lobe radius, and the lowest one is constrained by the accretion without EUV irradiation. There is a delay between the EUV flux fixing the wind-fed power and that from the source. We modeled this feedback assuming different time profiles for the small amplitude variability. Results. We propose a simple heuristic model to couple the influence of EUV irradiation on the stellar wind (from the Wolf-Rayet companion star) with the X-ray source itself. The resulting time profile of luminosity mimics that of the input variability, albeit with a larger amplitude. The most important property of the input variability are turnover times when it changes its sign and starts to have either positive or negative feedback. The bolometric luminosity derived by spectral modeling is the time average of the resulting feedback luminosity. Conclusions. We demonstrate that the erratic behavior of the X-ray light curve of Cygnus X-3 may have its origin in the small amplitude variability of the X-ray source and feedback with the companion wind. This variability could arise in the accretion flow and/or due to the loss of kinetic energy in a jet or an accretion disk wind. In order to produce similar properties of the simulated light curve as observed, we have to restrict the largest accretion radius to a changing level, and assume variable timescales for the rise and decline phases of the light curve.