Eclipsing light curves for accretion flows around a rotating black hole and atmospheric effects of the companion star

Abstract

We calculate eclipsing light curves for accretion flows around a rotating black hole taking into account the atmospheric effects of the companion star. In the cases of no atmospheric effects, the light curves contain the information of the black hole spin because most of the X-ray photons around 1 keV usually come from the blueshifted part of the accretion flow near the black hole shadow, and the size and the position of the black hole shadow depend on the spin. In these cases, when most of the emission comes from the vicinity of the event horizon, the light curves become asymmetric at ingress and egress. We next investigate the atmospheric absorption and scattering effects of the companion stars. By using the solar-type atmospheric model, we have taken into account the atmospheric effects of the companion star, such as the photoionization by HI and HeI. We found that the eclipsing light curves observed at 1 keV possibly contain the information of the black hole spin. However, in our atmospheric model, the effects of the atmosphere are much larger than the effects of the black hole spin. Therefore, even in the case that the light curves contain the information of the black hole spin, it may be difficult to extract the information of the black hole spin if we do not have the realistic atmospheric profiles, such as the temperature, and the number densities for several elements. Even in such cases, the light-curve asymmetries due to the rotation of the accretion disc exist. Only when we have the reliable atmospheric model, in principle, the information of the strong-gravity regions, such as the black hole spin, can be obtained from the eclipsing light curves.