CAIXA: a catalogue of AGN in the XMM-Newton archive

III. Excess variance analysis^⋆,⋆⋆

¹ School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
e-mail: ponti@iasfbo.inaf.it
² Department of Physics and Institute of Theoretical & Computational Physics, University of Crete, 71003 Heraklion, Greece
³ IESL, Foundation for Research and Technology, 71110 Heraklion, Greece
⁴ Dipartimento di Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁵ XMM-Newton Science Operations Center, European Space Astronomy Center, ESA, Apartado 50727, 28080 Madrid, Spain

Received: 25 October 2011
Accepted: 30 November 2011

Abstract

Context. We report on the results of the first XMM-Newton systematic “excess variance” study of all the radio quiet, X-ray un-obscured AGN. The entire sample consist of 161 sources observed by XMM-Newton for more than 10 ks in pointed observations, which is the largest sample used so far to study AGN X-ray variability on time scales less than a day.

Aims. Recently it has been suggested that the same engine might be at work in the core of every black hole (BH) accreting object. In this hypothesis, the same variability should be observed in all AGN, once rescaled by the M_BH (M_BH) and accretion rate (ṁ).

Methods. We systematically compute the excess variance for all AGN, on different time-scales (10, 20, 40 and 80 ks) and in different energy bands (0.3–0.7, 0.7–2 and 2–10 keV).

Results. We observe a highly significant and tight (~0.7 dex) correlation between σ²_rms and M_BH. The subsample of reverberation mapped AGN shows an even smaller scatter (only a factor of 2–3) comparable to the one induced by the M_BH uncertainties. This implies that X-ray variability can be used as an accurate tool to measure M_BH and this method is more accurate than the ones based on single epoch optical spectra. This allows us to measure M_BH for 65 AGN and estimate lower limits for the remaining 96 AGN. On the other hand, the σ²_rms vs. accretion rate dependence is weaker than expected based on the PSD break frequency scaling. This strongly suggests that both the PSD high frequency break and the normalisation depend on accretion rate in such a way that they almost completely counterbalance each other (PSD_amp ∝ ṁ^-0.8). A highly significant correlation between σ²_rms and 2–10 keV spectral index is observed. The highly significant correlations between σ²_rms and both the L_Bol and the FWHM_Hβ are consistent with being just by-products of the σ²_rms vs. M_BH relation. The soft and medium σ²_rms is very well correlated with the hard σ²_rms, with no deviations from a linear one to one correlation. This suggests that the additional soft components (i.e. soft excess, warm absorber) add a minor contribution to the total variability. Once the variability is rescaled for M_BH and ṁ, no significant difference between narrow-line and broad-line Seyfert 1 is observed.

Conclusions. The results are in agreement with a picture where, to first approximation, all local AGN have the same variability properties once rescaled for M_BH and ṁ.