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We present a discovery of the correlation between the X-ray spectral (photon)index and mass accretion rate observed in AGN NGC 4051. We analyzed spectraltransition episodes observed in NGC 4051 using XMM/Newton, Suzaku and RXTE. Weapplied a scaling technique for a black hole (BH) mass evaluation which uses acorrelation between the photon index and normalization of the seed (disk)component, which is proportional to a mass accretion rate. We developed ananalytical model that shows the spectral (photon) index of the BH emergentspectrum undergoes an evolution from lower to higher values depending on  a mass accretion rate in the accretion disk. We considered Cygnus X-1 andGRO~J1550-564 as reference sources for which distances, inclination angles andthe BH masses are evaluated by dynamical measurements. Application of thescaling technique for the photon index-mass accretion rate correlation providesan estimate of the black hole mass in NGC 4051 to be more than 6x10^5 solarmasses.

NGC 4051: Black hole mass and photon index-mass accretion rate correlation