The XMM deep survey in the CDF-S

X. X-ray variability of bright sources

¹ KTH, Department of Physics, and the Oscar Klein Centre, AlbaNova, 106-91 Stockholm, Sweden
e-mail: falocco@kth.se
² University Federico II, Physics Department, via Cintia, 80126 Naples, Italy
³ INFN Napoli, via Cintia, 80126 Naples, Italy
⁴ Agenzia Spaziale Italiana Science Data Center, via del Politecnico, 00133 Roma, Italy
⁵ INAF Osservatorio Astronomico di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
⁶ Instituto de Física de Cantabria (CSIC-UC), Avenida de los Castros, 39005 Santander, Spain
⁷ Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43, 22100 Lund, Sweden
⁸ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Martí i Franquès, 1, 08028 Barcelona, Spain
⁹ ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain
¹⁰ Institute of Astronomy and Astrophysics, National Observatory of Athens, Palaia Penteli, 15236 Athens, Greece
¹¹ University of Bologna, Department of Physics and Astronomy, via Gobetti 93/2, 40129 Bologna, Italy

Received: 5 August 2017
Accepted: 29 September 2017

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 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.