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English{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 {\lt} z {\lt} 3.5) and X-ray luminosities (10$^{41}$ {\lt} L $_{0.5-10}$ {\lt}10$^{45.5}$). Both a simple statistical method to assess the significance of variability and the Normalized Excess Variance ($\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$) 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 {\ndash}0.23 {\plusmn} 0.03) anti-correlation between $\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$ 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 $\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$ 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 $\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$ and M $_{BH}$ is found, with the same slope of anti-correlation between $\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$ and X-ray luminosity, suggesting that the latter may be a by-product of the former. No clear correlation is found between $\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$ and the Eddington ratio in our sample. Finally, no correlation is found between the X-ray $\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$ and optical variability. }
| Titolo: | Active Galactic Nucleus X-Ray Variability in the XMM-COSMOS Survey | |
| Autore/i: | LANZUISI, GIORGIO; G. Ponti; M. Salvato; G. Hasinger; N. Cappelluti; A. Bongiorno; BRUSA, MARCELLA; E. Lusso; P. K. Nandra; A. Merloni; J. Silverman; J. Trump; VIGNALI, CRISTIAN; A. Comastri; R. Gilli; M. Schramm; C. Steinhardt; D. Sanders; J. Kartaltepe; D. Rosario; B. Trakhtenbrot | |
| Autore/i Unibo: | ||
| Anno: | 2014 | |
| Rivista: | ||
| Digital Object Identifier (DOI): | http://dx.doi.org/10.1088/0004-637X/781/2/105 | |
| Abstract: | {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 {\lt} z {\lt} 3.5) and X-ray luminosities (10$^{41}$ {\lt} L $_{0.5-10}$ {\lt}10$^{45.5}$). Both a simple statistical method to assess the significance of variability and the Normalized Excess Variance ($\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$) 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 {\ndash}0.23 {\plusmn} 0.03) anti-correlation between $\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$ 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 $\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$ 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 $\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$ and M $_{BH}$ is found, with the same slope of anti-correlation between $\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$ and X-ray luminosity, suggesting that the latter may be a by-product of the former. No clear correlation is found between $\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$ and the Eddington ratio in our sample. Finally, no correlation is found between the X-ray $\backslash$sigma \^{}$\{$2$\}$\_$\{$rms$\}$ and optical variability. } | |
| Data prodotto definitivo in UGOV: | 2014-03-14 18:02:22 | |
| Appare nelle tipologie: | 1.01 Articolo in rivista |
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http://hdl.handle.net/11585/229690
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