X-Ray Properties of AGN in Brightest Cluster Galaxies. I.A Systematic Study of the Chandra Archive in the $0.2<z<0.3$ and $0.55<z<0.75$ Redshift Range

Abstract

We present a search for nuclear X-ray emission in the brightest cluster galaxies (BCGs) of a sample of groups and clusters of galaxies extracted from the Chandra archive. The exquisite angular resolution of Chandra allows us to obtain robust photometry at the position of the BCG, and to firmly identify unresolved X-ray emission when present, thanks to an accurate characterization of the extended emission at the BCG position. We consider two redshift bins (0.2<z<0.3 and 0.55<z<0.75) and analyze all the clusters observed by Chandra with exposure time larger than 20 ks. Our samples have 81 BCGs in 73 clusters and 51 BCGs in 49 clusters in the low- and high-redshift bin, respectively. X-ray emission in the soft (0.5-2 keV) or hard (2-7 keV) band is detected only in 14 and 9 BCGs ($\sim 18$% of the total samples), respectively. The X-ray photometry shows that at least half of the BCGs have a high hardness ratio, compatible with significant intrinsic absorption. This is confirmed by the spectral analysis with a power law model plus intrinsic absorption. We compute the fraction of X-ray bright BCGs above a given hard X-ray luminosity, considering only sources with positive photometry in the hard band (12/5 sources in the low/high-z sample). In the 0.2<z<0.3 interval the hard X-ray luminosity ranges from $10^{42}$ to $7 \times 10^{43}$ erg s$^{-1}$, with most sources found below $10^{43}$ erg s$^{-1}$. In the $0.55<z<0.75$ range, we find a similar distribution of luminosities below $\sim 10^{44}$ erg s$^{-1}$, plus two very bright sources of a few $10^{45}$ erg s$^{-1}$ associated with two radio galaxies. We also find that X-ray luminous BCGs tend to be hosted by cool-core clusters, despite the majority of cool cores do not host nuclear X-ray emission (Abridged).