1.3. Deep-Survey Number Counts and the Fraction of the Cosmic X-ray Background Resolved

Based on deep surveys with Chandra and XMM-Newton, number-count relations have now been determined down to 0.5-2, 2-8, and 5-10 keV fluxes of about 2.3 × 10^-17, 2.0 × 10^-16, and 1.2 × 10^-15 erg cm^-2 s^-1, respectively (e.g., Brandt et al. 2001b; Hasinger et al. 2001; Cowie et al. 2002; Rosati et al. 2002b; Moretti et al. 2003; Bauer et al. 2004). Figure 3 shows the integral number counts in the 0.5-2 and 2-8 keV bands. At bright fluxes the integral counts have power-law slopes in the range _b 1.6 ± 0.2, depending on the sample selection (compare, e.g., the discussions about bright-end slopes in Hasinger et al. 1998 and Moretti et al. 2003). Toward fainter 0.5-2 and 2-8 keV fluxes, the integral counts show significant cosmological flattening with faint-end slopes of _f 0.4-0.6 and break fluxes of (1-2) × 10^-14 and (3-8) × 10^-15 erg cm^-2 s^-1, respectively. In the 5-10 keV band a flattening has not yet been detected; the faint-end number counts continue rising steeply (_f 1.2-1.4) indicating that a significant fraction of the 5-10 keV CXRB remains unresolved (e.g., Rosati et al. 2002b).

Figure 3. (a) Number of sources, N( > S), brighter than a given flux, S, for the 0.5-2 keV band. The black circles are from the ROSAT Lockman Hole study of Hasinger et al. (1998), the solid black curve is from the Chandra Deep Fields study of Bauer et al. (2004), and the dotted black "fish" region shows the Chandra Deep Field-North fluctuation analysis results of Miyaji & Griffiths (2002). The dashed curves show number counts for AGN (red), only type 1 AGN (blue), and starburst and normal galaxies (green) from Bauer et al. (2004) and Hasinger, Miyaji & Schmidt (2005). (b) N( > S) versus S for the 2-8 keV band. The black circles are from the ASCA Large Sky Survey study of Ueda et al. (1999), the black triangles are from the ChaMP study of Kim et al. (2004), the solid black curve is from the Chandra Deep Fields study of Bauer et al. (2004), and the dotted black "fish" region shows the Chandra Deep Field-North fluctuation analysis results of Miyaji & Griffiths (2002). The dashed curves show number counts for AGN (red) and starburst and normal galaxies (green) from Bauer et al. (2004).

There is some evidence for field-to-field variations of the number counts. Such variations are expected at some level due to "cosmic variance" associated with large-scale structures that have been detected in the X-ray sky (e.g., Barger et al. 2003a; Gilli et al. 2003, 2004; Yang et al. 2003). For example, while the CDF-N and Chandra Deep Field-South (CDF-S) number counts agree in the 0.5-2 keV band and at bright 2-8 keV fluxes, there is up to 3.9 disagreement for 2-8 keV fluxes below 1 × 10^-15 erg cm^-2 s^-1 (Cowie et al. 2002; Rosati et al. 2002b; Bauer et al. 2004). The number counts for the shallower Lockman Hole (Hasinger et al. 2001) and Lynx (Stern et al. 2002a) fields agree with those for the Chandra Deep Fields to within statistical errors, while those for the SSA13 (Mushotzky et al. 2000) field appear to be 40% higher in the 2-8 keV band (see Tozzi et al. 2001). An extensive comparison of field-to-field number counts by Kim et al. (2004) finds little evidence for cosmic variance at 0.5-2 keV (2-8 keV) flux levels of 10^-15-10^-13 erg cm^-2 s^-1 ( 10^-14-10^-12 erg cm^-2 s^-1) in 5-125 ks Chandra observations.

The deepest ROSAT surveys resolved 75% of the 0.5-2 keV CXRB into discrete sources, the major uncertainty in the resolved fraction being the absolute flux level of the CXRB (at low energies it is challenging to separate the CXRB from Galactic emission; see McCammon & Sanders 1990). Deep Chandra and XMM-Newton surveys have now increased this resolved fraction to 90% (e.g., Moretti et al. 2003; Bauer et al. 2004; Worsley et al. 2004). Above 2 keV the situation is complicated by the fact that the 1 background spectrum (Marshall et al. 1980), used as a reference for many years, has a 30% lower normalization than several earlier and later background measurements (see, e.g., Moretti et al. 2003). Recent determinations of the background spectrum with RXTE (Revnivtsev et al. 2003) and XMM-Newton (De Luca & Molendi 2004) strengthen the consensus for a 30% higher normalization, indicating that many past resolved fractions above 2 keV must be scaled down correspondingly. Additionally, X-ray telescopes typically have a large sensitivity gradient across the broad 2-10 keV band. A recent investigation by Worsley et al. (2004), dividing the CDF-N, CDF-S, and XMM-Newton Lockman Hole field into finer energy bins, concludes that the resolved fraction drops from 80-90% at 2-6 keV to 50-70% at 6-10 keV. This is consistent with expectations from the 5-10 keV number counts (see above). In the critical 10-100 keV band, where most of the CXRB energy density resides, only a few percent of the background has been resolved (e.g., Krivonos et al. 2004).

Multiwavelength identification studies indicate that most ( 70%) of the X-ray sources found in deep Chandra and XMM-Newton surveys are AGN (see Section 2.1 for further discussion). The observed AGN sky density in the deepest X-ray surveys, the Chandra Deep Fields, is a remarkable 7200 deg^-2 (e.g., Bauer et al. 2004). This exceptional effectiveness at finding AGN arises largely because X-ray selection (1) has reduced absorption bias, (2) has minimal dilution by host-galaxy starlight, and (3) allows concentration of intensive optical spectroscopic follow-up upon high-probability AGN with faint optical counterparts (i.e., it is possible to probe further down the luminosity function); see Section 2.4 and Mushotzky (2004) for further details on the effectiveness of AGN X-ray selection. The AGN sky density from the Chandra Deep Fields exceeds that found at any other wavelength and is 10-20 times higher than that found in the deepest optical spectroscopic surveys (e.g., Wolf et al. 2003; Hunt et al. 2004); only ultradeep optical variability studies (e.g., Sarajedini, Gilliland & Kasm 2003) may be generating comparable AGN sky densities.