2002ApJ...564L...5M

We report the results of fluctuation analysis of the off-source field from the 1 million second Chandra observation of the Hubble Deep Field-North (HDF-N). The distribution of the counts in cells has been compared with the expectations from the logN-logS model to constrain the behavior of the source number density down to a factor of several lower than the source detection limit. Our results show that the number counts in the soft band (0.5-2 keV) continue to grow down to S_X∼7x10^–18 ergs.s^–1.cm^–2, possibly suggesting the emergence of a new population, and agree well with a prediction of star-forming galaxies by Ptak et al. The fluctuation analysis can loosely constrain the source counts fainter than the detection limit, and we found an upper limit of ≲10,000 sources deg^–2 at S_X∼2x10^–16 ergs.s^–1.cm^–2 in the hard (2-10 keV) band.