SIMBAD references

We studied the X-ray variability of sources detected in the Chandra Deep Field-South, nearly all of which are low- to moderate-z active galactic nuclei (AGNs). We find that 45% of the sources with more than 100 counts exhibit significant variability on timescales ranging from a day up to a year. The fraction of sources found to be variable increases with observed flux, suggesting that more than 90% of all AGNs possess intrinsic variability. We also find that the fraction of variable sources appears to decrease with increasing intrinsic absorption; a lack of variability in hard, absorbed AGNs could be due to an increased contribution of reflected X-rays to the total flux. We do not detect significant spectral variability (ΔΓ>0.2) in the majority (∼70%) of our sources. In half of the remaining 30%, the hardness ratio is anticorrelated with flux, mimicking the high/soft-low/hard states of galactic sources. The X-ray variability appears anticorrelated with the luminosity of the sources, in agreement with previous studies. High-redshift sources, however, have larger variability amplitudes than expected from extrapolations of their low-z counterparts, suggesting a possible evolution in the accretion rate and/or size of the X-ray-emitting region. Finally, we discuss some effects that may produce the observed decrease in the fraction of variable sources from z=0.5 out to z=2.