X-ray detection of the proto supermassive binary black hole at the centre of Abell 400 D. S. Hudson, T. H. Reiprich, T. E. Clarke and C. L. Sarazin A&A, 453 2 (2006) 433-446 Published online: 16 June 2006 DOI: 10.1051/0004-6361:20064955