Faint Source Counts from the Off-source fluctuation Analysis on the   Deepest Chandra Fields
  We show the results of the fluctuation analysis applied to the off-source areas from the two 1 Million second Chandra Deep Fields, including our new results on the Chandra Deep Field-South (CDF-S) in the 0.5-2 keV band in addition to those on the Chandra Deep Field-North (CDF-N), which have already been reported. The distribution of the X-ray counts in cells has been compared with the expectation from the Log N - Log S model to constrain the behavior of the source number density down to a factor of several lower than the source-detection limit. We show that our results are insensitive to the non-uniformity of the non X-ray background (NXB). Our results show that the number counts in the soft band (0.5-2 [keV]) continue to grow down to Sx =7e-18 erg/s/cm2, possibly suggesting the emergence of a new population and they agree well with a prediction of star forming galaxies. 
Takamitsu Miyaji, Richard E. Griffiths