Schematic illustration of (a) a black hole-disk-corona model including thermal, Compton, and reflection components, and (b) a black hole disk model with reflection component from self-irradiation of the disk (thermal returning) and iterative reflection (reflection returning). The color
gradient in the disk indicates that the temperature increases towards the black hole.
As shown in Fig. 1(a), the observed spectrum of black hole
XRBs typically consists of three primary components: (i) a thermal component originating from a geometrically thin, optically thick accretion disk (Shakura & Sunyaev 1973; Page & Thorne 1974) ; (ii) a Comptonized component produced by a hot corona that inverse-Compton scatters soft disk photons (Thorne & Price 1975; Shapiro et al. 1976) ; and (iii) a reflection component resulting from the illumination of the disk by the corona (George & Fabian 1991). The disk is thought to be cold as it can efficiently emit radiation and its thermal spectrum peaks in the soft X-rays (∼ 1 keV). The coronal spectrum takes a power-law like form that can extend to above 100 keV.
A particularly important effect in this context is returning radiation (or self-irradiation) (purple and brown arrows in
Fig. 1(b)), i.e., disk emission that is bent back onto the accretion disk by the strong gravitational field of the black hole. The
first calculations of this effect were performed by Cunningham (1976) in the context of thermal disk spectra, while Li et al.(2005) later showed that it can mimic the effect of a higher accretion rate.
credit: Modeling X-ray reflection spectra from returning radiation:application to 4U 1630–47