A Chandra study of X-ray sources in the field of the z=2.16 radio galaxy   MRC 1138-262
  We present results from a Chandra X-ray Observatory study of the field X-ray source population in the vicinity of the radio galaxy MRC 1138-262. Many serendipitous X-ray sources are detected in an area of 8'x8' around the radio source and 90% are identified in our deep VLT images. The space density of such sources is higher than expected on the basis of the statistics of ROSAT and Chandra deep surveys. The most likely explanation is in terms of a concentration of AGN associated with the protocluster at z=2.16 which was found around the radio galaxy in previous studies. Two sources have a confirmed spectroscopic redshift close to that of the radio galaxy, and for three more sources other observations suggest that they are associated with the protocluster. Four of these five X-ray sources form, together with the radio galaxy, a filament in the plane of the sky. The direction of the filament is similar to that of the radio source axis, the large scale distribution of the other protocluster members, the 150 kpc-sized emission-line halo and the extended X-ray emission associated with the radio galaxy.   The majority of optically identified X-ray sources in this field have properties consistent with type I AGN, a few could be soft, low luminosity galaxies, one is probably an obscured (type II) AGN and one is a star. These statistics are consistent with the results of deep X-ray surveys. 
L. Pentericci, J.D. Kurk, C.L.Carilli, D.E. Harris, G.K. Miley and   H.J.A. Rottgering