Is the anti-correlation between the X-ray variability amplitude and black hole mass of AGNs intrinsic?

Abstract

Aims. Both the black hole mass and the X-ray luminosity of AGNs have been found to be anti-correlated with the normalized excess variance ($\sigma_{\rm rms}^2 $) of the X-ray light curves. We investigate which correlation with $\sigma_{\rm rms}^2 $ is the intrinsic one. Methods. We divide a full sample of 33 AGNs (O' Neill et al. 2005) into two sub-samples. The black hole masses of 17 objects in sub-sample 1 were determined by the reverberation mapping or the stellar velocity dispersion. The black hole masses of the remaining 16 objects were estimated from the relationship between broad line region radius and optical luminosity (sub-sample 2). Then partial correlation analysis, ordinary least squares regression and K-S tests are performed on the full sample and the sub-samples, respectively. Results. We find that $\sigma_{\rm rms}^2 $ seems to be intrinsically correlated with the black hole mass in the full sample. However, this seems to be caused by including the sub-sample 2 into the analysis, which introduces an extra correlation between the black hole mass and the luminosity and strengthens any correlation with the black hole mass artificially. Therefore, the results from the full sample may be misleading. The results from the sub-sample 1 show that the correlation between $\sigma_{\rm rms}^2 $ and the X-ray luminosity may be the intrinsic one and therefore the anti-correlation between $\sigma_{\rm rms}^2 $ and the black hole mass is doubtful.