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The XMM deep survey in the CDF-S : X. X-ray variability of bright sources

Falocco, S.; Paolillo, M.; Comastri, A.; Carrera, F. J.; Ranalli, P. LU ; Iwasawa, K.; Georgantopoulos, I.; Vignali, C. and Gilli, R. (2017) In Astronomy and Astrophysics 608.
Abstract

Aims. We aim to study the variability properties of bright hard X-ray selected active galactic nuclei (AGN) in the redshift range between 0.3 and 1.6 detected in the Chandra Deep Field South (XMM-CDFS) by a long (∼3 Ms) XMM observation. Methods. Taking advantage of the good count statistics in the XMM CDFS, we search for flux and spectral variability using the hardness ratio (HR) techniques. We also investigate the spectral variability of different spectral components (photon index of the power law, column density of the local absorber, and reflection intensity). The spectra were merged in six epochs (defined as adjacent observations) and in high and low flux states to understand whether the flux transitions are accompanied by spectral... (More)

Aims. We aim to study the variability properties of bright hard X-ray selected active galactic nuclei (AGN) in the redshift range between 0.3 and 1.6 detected in the Chandra Deep Field South (XMM-CDFS) by a long (∼3 Ms) XMM observation. Methods. Taking advantage of the good count statistics in the XMM CDFS, we search for flux and spectral variability using the hardness ratio (HR) techniques. We also investigate the spectral variability of different spectral components (photon index of the power law, column density of the local absorber, and reflection intensity). The spectra were merged in six epochs (defined as adjacent observations) and in high and low flux states to understand whether the flux transitions are accompanied by spectral changes. Results. The flux variability is significant in all the sources investigated. The HRs in general are not as variable as the fluxes, in line with previous results on deep fields. Only one source displays a variable HR, anti-correlated with the flux (source 337). The spectral analysis in the available epochs confirms the steeper when brighter trend consistent with Comptonisation models only in this source at 99% confidence level. Finding this trend in one out of seven unabsorbed sources is consistent, within the statistical limits, with the 15% of unabsorbed AGN in previous deep surveys. No significant variability in the column densities, nor in the Compton reflection component, has been detected across the epochs considered. The high and low states display in general different normalisations but consistent spectral properties. Conclusions. X-ray flux fluctuations are ubiquitous in AGN, though in some cases the data quality does not allow for their detection. In general, the significant flux variations are not associated with spectral variability: Photon index and column densities are not significantly variable in nine out of the ten AGN over long timescales (from three to six and a half years). Photon index variability is found only in one source (which is steeper when brighter) out of seven unabsorbed AGN. The percentage of spectrally variable objects is consistent, within the limited statistics of sources studied here, with previous deep samples.

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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Galaxies: nuclei, X-rays: General
in
Astronomy and Astrophysics
volume
608
publisher
EDP Sciences
external identifiers
  • scopus:85037043123
  • wos:000416945700012
ISSN
0004-6361
DOI
10.1051/0004-6361/201731722
language
English
LU publication?
yes
id
14a3190a-9207-4ebf-8cbe-e996673ac2ed
date added to LUP
2017-12-18 10:49:58
date last changed
2018-04-17 09:31:28
@article{14a3190a-9207-4ebf-8cbe-e996673ac2ed,
  abstract     = {<p>Aims. We aim to study the variability properties of bright hard X-ray selected active galactic nuclei (AGN) in the redshift range between 0.3 and 1.6 detected in the Chandra Deep Field South (XMM-CDFS) by a long (∼3 Ms) XMM observation. Methods. Taking advantage of the good count statistics in the XMM CDFS, we search for flux and spectral variability using the hardness ratio (HR) techniques. We also investigate the spectral variability of different spectral components (photon index of the power law, column density of the local absorber, and reflection intensity). The spectra were merged in six epochs (defined as adjacent observations) and in high and low flux states to understand whether the flux transitions are accompanied by spectral changes. Results. The flux variability is significant in all the sources investigated. The HRs in general are not as variable as the fluxes, in line with previous results on deep fields. Only one source displays a variable HR, anti-correlated with the flux (source 337). The spectral analysis in the available epochs confirms the steeper when brighter trend consistent with Comptonisation models only in this source at 99% confidence level. Finding this trend in one out of seven unabsorbed sources is consistent, within the statistical limits, with the 15% of unabsorbed AGN in previous deep surveys. No significant variability in the column densities, nor in the Compton reflection component, has been detected across the epochs considered. The high and low states display in general different normalisations but consistent spectral properties. Conclusions. X-ray flux fluctuations are ubiquitous in AGN, though in some cases the data quality does not allow for their detection. In general, the significant flux variations are not associated with spectral variability: Photon index and column densities are not significantly variable in nine out of the ten AGN over long timescales (from three to six and a half years). Photon index variability is found only in one source (which is steeper when brighter) out of seven unabsorbed AGN. The percentage of spectrally variable objects is consistent, within the limited statistics of sources studied here, with previous deep samples.</p>},
  articleno    = {A32},
  author       = {Falocco, S. and Paolillo, M. and Comastri, A. and Carrera, F. J. and Ranalli, P. and Iwasawa, K. and Georgantopoulos, I. and Vignali, C. and Gilli, R.},
  issn         = {0004-6361},
  keyword      = {Galaxies: nuclei,X-rays: General},
  language     = {eng},
  month        = {12},
  publisher    = {EDP Sciences},
  series       = {Astronomy and Astrophysics},
  title        = {The XMM deep survey in the CDF-S : X. X-ray variability of bright sources},
  url          = {http://dx.doi.org/10.1051/0004-6361/201731722},
  volume       = {608},
  year         = {2017},
}