Astronomy & Astrophysics

A&A 480, 699-701 (2008)
DOI: 10.1051/0004-6361:20078282

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

Y. Liu and S. N. Zhang

Physics Department and Center for Astrophysics, Tsinghua University, Beijing 100084, PR China
e-mail: zhangsn@tsinghua.edu.cn; yuan-liu@mails.tsinghua.edu.cn

(Received 15 July 2007 / Accepted 8 January 2008)

Abstract
Aims.Both the black hole mass and the X-ray luminosity of active galactic nuclei (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 divided a full sample of 33 AGNs (O' Neill et al. 2005, MNRAS, 358, 1405) into two subsamples. The black hole masses of 17 objects in subsample 1 were determined by reverberation mapping or 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 (subsample 2). We then performed partial correlation analysis, ordinary least-squares regression, and K-S tests on the full sample and the subsamples, respectively.
Results.We found 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 subsample 2 in 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 subsample 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.

Key words: X-rays: galaxies -- galaxies: active -- methods: statistical