2021MNRAS.501.5659T

We detected a so-called high-temperature blackbody (HBB) component, found in the 15-40 keV range, in the broad-band X-ray energy spectra of black hole (BH) candidate sources. A detailed study of this spectral feature is presented using data from five of the Galactic BH binaries, Cyg X-1, GX 339-4, GRS 1915+105, SS 433, and V4641 Sgr in the low/hard, intermediate, high/soft, and very soft spectral states (LHS, IS, HSS, and VSS, respectively) and spectral transitions between them using RXTE, INTEGRAL, and BeppoSAX data. In order to fit the broad-band energy spectra of these sources, we used an additive XSPEC model, composed of the Comptonization component and the Gaussian line component. In particular, we reveal that the IS spectra have the HBB component which colour temperature, kT_HBB, is in the range of 4.5-5.9 keV. This HBB feature has been detected in some spectra of these five sources only in the IS (for the photon index Γ > 1.9) using different X-ray telescopes. We also demonstrate that a time-scale of the HBB feature is of orders of magnitude shorter than the time-scale of the iron line and its edge. That leads us to conclude that these spectral features are formed in geometrically different parts of the source and which are not connected to each other. Recently, Laurent & Titarchuk demonstrated a presence of a gravitational redshifted annihilation line emission in a BH using the Monte Carlo simulations and, therefore, the observed HBB hump leads us to suggest this feature is a gravitational redshifted annihilation line observed in these BHs.