2017MNRAS.468.3689M

In recent years, fundamental relationships for the black hole X-ray binaries have been established between their X-ray luminosity L_X and the photon index Γ of their X-ray spectrum. For the moderate-luminosity regime, an anticorrelation between Γ and L_X has been observed. In this paper, aimed to verify if the moderate luminous X-ray binary system LS I +61°303 is a black hole, we analyse Swift observations of LS I +61°303. We compare the derived L_X versus Γ distribution, first with the statistical trend for black hole X-ray binaries, then with the trend of the pulsar PSR B1259-63 and finally with the individual trends of the black hole X-ray binaries Swift J1357.2-0933 and V404 Cygni. We find that the system PSR B1259-63 shows a positive correlation between Γ and L_X, whereas in contrast LS I +61°303 shows the same anticorrelation as for black hole X-ray binaries. Moreover, the trend of LS I +61°303 in the L_X/L_Eddington-Γ plane overlaps with that of the two black holes Swift J1357.2-0933 and V404 Cygni. All three systems, Swift J1357.2-0933, V404 Cygni and LS I +61°303 well trace the last part of the evolution of accreting black holes at moderate luminosity until their drop to quiescence.