The Deepest X-Ray Look at the Universe

The origin of the X-ray background, in particular at hard (2-10 keV) energies, is an issue that has been debated for more than 40 years. The Chandra Deep Fields provide the deepest look at the X-ray sky and are the best data set to study the X-ray background. We searched the Chandra Deep Field-South for X-ray sources with the aid of a dedicated wavelet-based algorithm. We are able to reconstruct the log N -log S source distribution in the soft (0.5-2 keV) and hard (2-10 keV) bands down to limiting fluxes of 2 × 10 -17 and 2 × 10 -16 ergs s -1 cm -2 , respectively. These are a factor of ~5 deeper than previous investigations. We find that the soft relation continues along the extrapolation from higher fluxes, almost completely accounting for the soft X-ray background. On the contrary, the hard distribution shows a flattening below ~2 × 10 -14 ergs s -1 cm -2 . Nevertheless, we can account for ≳68% of the hard X-ray background, with the main uncertainty being the sky flux itself.