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Analysis of X-ray spectral variability and black hole mass determination of the NLS1 galaxy Mrk 766

We present an XMM-Newton time-resolved spectral analysis of the NLS1 galaxy Mrk 766. We analyse eight available observations of the EPIC-pn camera taken between May 2000 and June 2005 to investigate the X-ray spectral variability as produced by changes in the mass accretion rate. The 0.2-10 keV spectra are extracted in time bins longer than 3 ks to accurately trace the variations of the best fit parameters of our adopted Comptonisation spectral model. We test a bulk-motion Comptonisation (BMC) model which is in general applicable to any physical system powered by accretion onto a compact object, and assumes that soft seed photons are efficiently up-scattered via inverse Compton scattering in a hot and dense electron corona. The Comptonised spectrum has a characteristic power-law shape, whose slope was found to increase for large values of the normalisation of the seed component, that is proportional to the mass accretion rate (in Eddington units). Our baseline spectral model also includes a warm absorber lying on the line of sight and radiation reprocessing from the accretion disk or from outflowing matter in proximity of the central compact object. Our study reveals that the normalisation-slope correlation, observed in Galactic Black Hole sources (GBHs), also holds for Mrk 766: variations of the photon index in the range Gamma~1.9-2.4 are indeed likely to be related to the variations of m-dot, as observed in X-ray binary systems. We finally applied a scaling technique based on the observed correlation to estimate the BH mass in Mrk 766. This technique is commonly and successfully applied to measure masses of GBHs, and this is the first time it is applied in detail to estimate the BH mass in an AGN. We obtain a value of M_{BH}=1.26^{+1.00}_{-0.77}x10^6 M_{sun} that is in very good agreement with that estimated by the reverberation mapping