The relationship between X-ray variability and the central black hole mass

We assembled a sample of Seyfert 1 galaxies, quasi-stellar objects (QSOs) a nd low-luminosity active galactic nuclei (LLAGNs) observed by ASCA, the cen tral black hole masses of which have been measured. We found that the X-ray variability (which is quantified by the 'excess variance' sigma (2)(rms)) is significantly anti-correlated with the central black hole mass, and it i s likely that a linear relationship of sigma (2)(rms) proportional to M-bh( -1) exists. It can be interpreted that the short time-scale X-ray variabili ty is caused by some global coherent variations in the X-ray emission regio n, which is scaled by the size of the central black hole. Hence the central black hole mass is the driving parameter of the previously established rel ation between X-ray variability and luminosity. Our findings favour the hyp othesis that the narrow-line Seyfert 1 galaxies and QSOs harbour smaller bl ack holes than the broad-line objects, and can also easily explain the obse rvational fact that high-redshift QSOs have greater variability than local AGNs at a given luminosity. Further investigations are needed to confirm ou r findings, and a large sample X-ray variability investigation can give con straints on the physical mechanisms and evolution of AGNs.