2018ApJ...859...65Y

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 bins, 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 (∼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 10⁴² to 7 × 10⁴³ erg s^–1, with most sources found below 10⁴³ erg s^–1. In the 0.55 < z < 0.75 range, we find a similar distribution of luminosities below ∼10⁴⁴ erg s^–1, plus two very bright sources of a few 10⁴⁵ erg s^–1 associated with two radio galaxies. We also find that X-ray luminous BCGs tend to be hosted by cool-core clusters, even though the majority of cool cores do not host nuclear X-ray emission. This work shows that our analysis, when extended to the entire Chandra archive, can provide a sizable number of sources, allowing us to probe the evolution of X-ray active galatic nuclei in BCGs as a function of the cosmic epochs.