ИСТИНА

We present a review of main X-ray spectral and timing properties of compact objects [black hole (BH) and neutron star (NS) sources]. I emphasize how one can distinguish NS vs BH based on the behavior of X-ray spectral (photon) index vs mass accretion rate. Now it is well established that in BHs the photon index increases and then it saturates with mass accretion rate while in NSs the index remains almost constant around 2. I present the strong theoretical arguments to explain this effect and also to demonstrate the constancy of the index is a signature of photon reflection from NS surface while the index behavior in BH and paricularly its saturation is a signature of the converging flow into BH, namely it is the intrinsic signature of BH horizon.