An important diagnostic , strongly confirming the low pair dominance independently of the pair equilibrium calculations , is the absence of an annihilation feature in the observed spectrum . Purely thermal plasmas do not show distinct pair annihilation lines even if dominated by pairs ( Zdziarski 1986;Stern et al . 1995 ; . However , the data for this source rule out purely thermal plasmas , see Figure 1 . On the other hand , hybrid plasmas can emit X - ray spectra well reproduced by Comptonization on mostly thermal electrons , but still exhibit relatively narrow annihilation features . The absence of such a feature in the average spectrum therefore implies the pair abundance to be very low .