X-rays from the First Massive Black Holes
  X-ray studies of high-redshift (z > 4) active galaxies have advanced substantially over the past few years, largely due to results from the new generation of X-ray observatories. As of this writing X-ray emission has been detected from nearly 60 high-redshift active galaxies. This paper reviews the observational results and their implications for models of the first massive black holes, and it discusses future prospects for the field. 
W.N. Brandt, C. Vignali, D.P. Schneider, D.M. Alexander, S.F.   Anderson, F.E. Bauer, X. Fan, G.P. Garmire, S. Kaspi, G.T. R...