Black hole mass estimation from X-ray variability measurements in active galactic nuclei

We propose a new method of estimation of the black hole masses in active galactic nuclei (AGN) based on the normalized excess variance, sigma(nxs)(2). We derive a relation between sigma(nxs)(2), the length of the observation, T, the light-curve bin size, Deltat, and the black hole mass, assuming that (i) the power spectrum above the high-frequency break, nu(bf), has a slope of -2, (ii) the high-frequency break scales with black hole mass, (iii) the power-spectrum amplitude (in frequency-power space) is universal and (iv)sigma(nxs)(2) is calculated from observations of length T<1/nu(bf). Values of black hole masses in AGN obtained with this method are consistent with estimates based on other techniques such as reverberation mapping or the M-BH-stellar velocity dispersion relation. The method is formally equivalent to methods based on power spectrum scaling with mass, but the use of sigma(nxs)(2) has the big advantage of being applicable to relatively low-quality data.