NGC 4051: Black hole mass and photon index-mass accretion rate
  correlation
  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 spectral
transition episodes observed in NGC 4051 using XMM/Newton, Suzaku and RXTE. We
applied a scaling technique for a black hole (BH) mass evaluation which uses a
correlation between the photon index and normalization of the seed (disk)
component, which is proportional to a mass accretion rate. We developed an
analytical model that shows the spectral (photon) index of the BH emergent
spectrum undergoes an evolution from lower to higher values depending on
  a mass accretion rate in the accretion disk. We considered Cygnus X-1 and
GRO~J1550-564 as reference sources for which distances, inclination angles and
the BH masses are evaluated by dynamical measurements. Application of the
scaling technique for the photon index-mass accretion rate correlation provides
an estimate of the black hole mass in NGC 4051 to be more than 6x10^5 solar
masses.
