At present, in the 10-100 keV range, where the bulk of the XRB energy resides, the only available measurements are those performed in the late 1970s by HEAO-1 (Marshall et al. 1980; Gruber 1992). The data showed that the XRB spectrum has a characteristic "bell" shape peaking at ~ 30 - 40 keV, which, at lower energies, can be approximated by a power-law with photon index = 1.4. A recent reanalysis of HEAO-1 data by Gruber et al. (1999) confirmed those earlier results, but also showed that the calibration uncertainties among HEAO-1 detectors are of the order of 10% in the overlapping energy bands (Fig. 1). The situation is more complicated at lower energies where a number of measurements have been obtained. While the spectral slope of the 2-10 keV XRB has always been found to have small variations around = 1.4, there are significant discrepancies in the spectral normalization. From the highest (11.5 keV s-1 sr-1 cm-2 keV-1 ; SAX, Vecchi et al. 1999) to the lowest measured value (8 keV s-1 sr-1 cm-2 keV-1 ; HEAO-1, Marshall et al. 1980) the variation is of the order of ~ 40%. The most recent measuments have been obtained by Kushino et al. (2002) from a combination of 91 ASCA GIS fields, with a total sky coverage of 50 deg2, and by Lumb et al. (2002) from a combination of 8 XMM pointings covering 6.5 deg2 in total. The XRB spectral slope is found to be = 1.412 ± 0.032 and = 1.42 ± 0.03 in the ASCA and XMM data, respectively (errors are at 90% confidence level), again very close to = 1.4. The power-law normalizations (9.66 ± 0.07 and 11.1 ± 0.32 keV s-1 sr-1 cm-2 keV-1 for the ASCA and XMM data, respectively, including the resolved bright sources) are within the range already spanned by previous missions, but do not suggest that the measurements of the XRB intensity are converging to a well constrained value. Interestingly, all the measurements of the 2-10 keV XRB intensity since HEAO-1 are higher than the HEAO-1 value.
Figure 1. The extragalactic X-ray background spectrum from 0.2 to 400 keV. Different colors correspond to measurements by different missions/instruments as labeled. The reference list for the shown data is the following: ROSAT 0.25 keV (Warwick & Roberts 1998); ROSAT 0.5-2.4 keV (Georgantopoulos et al. 1996); HEAO-1 A2 HED + A4 LED (Gruber 1992; Gruber et al. 1999); HEAO-1 A4 MED (Kinzer et al. 1997); SAX (Vecchi et al. 1999); ASCA SIS (Gendreau et al. 1995); ASCA GIS (Kushino et al. 2002); XMM (Lumb et al. 2002); CDFS (Tozzi et al. 2001a).