Active galactic nucleus X-ray variability in the xmm-cosmos survey

G. Lanzuisi, G. Ponti, M. Salvato, G. Hasinger, N. Cappelluti, A. Bongiorno, M. Brusa, E. Lusso, P. K. Nandra, A. Merloni, J. Silverman, J. Trump, C. Vignali, A. Comastri, R. Gilli, M. Schramm, C. Steinhardt, D. Sanders, J. Kartaltepe, D. RosarioB. Trakhtenbrot

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We used the observations carried out by XMM in the COSMOS field over 3.5 yr to study the long term variability of a large sample of active galactic nuclei (AGNs) (638 sources) in a wide range of redshifts (0.1 < z < 3.5) and X-ray luminosities (1041 < L 0.5-10 <10 45.5). Both a simple statistical method to assess the significance of variability and the Normalized Excess Variance () parameter were used to obtain a quantitative measurement of the variability. Variability is found to be prevalent in most AGNs, whenever we have good statistics to measure it, and no significant differences between type 1 and type 2 AGNs were found. A flat (slope -0.23 ± 0.03) anti-correlation between and X-ray luminosity is found when all significantly variable sources are considered together. When divided into three redshift bins, the anti-correlation becomes stronger and evolving with z, with higher redshift AGNs being more variable. We prove, however, that this effect is due to the pre-selection of variable sources: when considering all of the sources with an available measurement, the evolution in redshift disappears. For the first time, we were also able to study long term X-ray variability as a function of M BH and Eddington ratio for a large sample of AGNs spanning a wide range of redshifts. An anti-correlation between and MBH is found, with the same slope of anti-correlation between and X-ray luminosity, suggesting that the latter may be a by-product of the former. No clear correlation is found between and the Eddington ratio in our sample. Finally, no correlation is found between the X-ray and optical variability.

Original languageEnglish
Article number105
JournalAstrophysical Journal
Issue number2
StatePublished - 1 Feb 2014


  • X-rays: galaxies
  • X-rays: general
  • galaxies: active


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