Faint-Source Counts from Off-Source Fluctuation Analysis on [ITAL]Chandra[/ITAL] Observations of the Hubble Deep Field–North

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 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. 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⁻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. (2001). For the hard band (2-10 keV), the fluctuation analysis can loosely constrain the source counts fainter than the detection limit and we found an upper limit of 10000 [srcs deg⁻²] at Sx=2e-16 [erg s⁻¹cm⁻²].