| Autor: |
Lanzuisi, G., Ponti, G., Salvato, M., Hasinger, G., Cappelluti, N., Bongiorno, A., Brusa, M., Lusso, E., Nandra, P. K., Merloni, A., Silverman, J., Trump, J., Vignali, C., Comastri, A., Gilli, R., Schramm, M., Steinhardt, C., Sanders, D., Kartaltepe, J., Rosario, D. |
| Předmět: |
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| Zdroj: |
Astrophysical Journal; 2/1/2014, Vol. 781 Issue 2, p105-118, 14p |
| Abstrakt: |
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 < L0.5-10 <1045.5). 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 MBH 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. [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
| Externí odkaz: |
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