The cosmic X-ray background (XRB), discovered over forty years ago by X-ray instruments aboard a rocket experiment (Giacconi et al. 1962), has been the topic of intense study for the past decades, reinvigorated with the launch of each X-ray astrophysics mission. The observations of this isotropic background with large energy density led researchers to posit that the origin of the background was extragalactic. Figure 2.1shows the spectrum of the XRB from ∼ 3 to 300 keV measured primarily by instruments aboard the High Energy Astronomical Observatory 1 (HEAO 1; Gruber et al. 1999).
2.1.1 The Shape of the XRB
The total diffuse XRB photon spectrum in Figure 2.1 shows that the peak of the curve is at ∼ 30 keV with most of the energy density emitted from 20 to 40 keV. The intensity spectrum (Figure 3 of Gruber et al. 1999) is well fit by a simple exponential at energies of 3− ∼ 60 keV and three summed power-law functions above 60 keV (e.g.,Gruber et al.
1999):
3−60 keV : 7.877 E
keV −0.29
exp
− E 41.13 keV
keV
keV cm2 s sr , (2.1)
Figure 2.1 This plot, from Gruber et al. (1999) illustrates the photon spectrum of the diffuse isotropic component of the extragalactic X-ray background. The data was taken with several detectors on HEAO 1
>60 keV : 0.0259
E 60 keV
−5.5
+ 0.504
E 60 keV
−1.58
+ 0.0288
E 60 keV
−1.05
keV keV cm2 s sr.
(2.2)
The XRB spectrum in the 2 – 10 keV range is well described by a simple power law spectrum with photon index Γ of 1.4: N(E) ∝ E−1.4. About 20 percent of the total energy of the X-ray background is emitted from ∼3−10 keV, whereas, at energies below 3 keV only a few percent of the extragalactic XRB energy is emitted.
The XRB spectrum from∼3 to 45 keV is well matched to the spectrum ofT ∼40 keV thermal bremsstrahlung radiation, or radiation due to free-free interactions. This led some researchers in the late 1970ss and 1980s to suggest that the cosmic X-ray background was produced by thermal bremsstrahlung radiation from an exploding galaxy-heated hot smooth intergalactic medium (IGM) (e.g.,Field & Perrenod 1977;Guilbert & Fabian 1986;
Taylor & Wright 1989). This suggestion was ruled out by the subsequent measurements of the cosmic microwave background (CMB) radiation by theCosmic Background Explorer (COBE)satellite (Mather et al. 1990). TheCOBEanalysis showed that the CMB-spectrum intensity deviates less that 1 percent from a perfect blackbody spectrum withT = 2.735± 0.006 K, while an IGM-produced XRB would cause a distortion of the CMB away from a pure blackbody spectrum due to Compton scattering of the CMB photons off of the hot, dense IGM.Mather et al.(1990) found that the XRB would be constrained to a magnitude of less than 1/36 of its observed value if produced by the IGM. Researchers have dubbed the XRB’s similarity to a∼40 keV bremsstrahlung spectrum a ‘cosmic conspiracy.’
2.1.2 Resolving the XRB
It is now clear that this “diffuse” background is, in fact, largely produced by the integrated light from many discrete sources. The constituent sources are primarily active galactic nuclei: galaxies with central supermassive black holes that are undergoing active accretion.
Several missions from 1978 to 1999 imaged the low-energy (<3.5 keV) X-ray sky with increasing sensitivity and angular resolution. These observatories included the Einstein Observatory, EXOSAT, and ROSAT Results from deep surveys performed by ROSAT, a mission with 300 angular-imaging performance at E ∼< 2.5 keV, found that 70%−80% of
Figure 2.2 Transmission as a function of incident X-ray energy through column densities of 1020, 1021, 1022, 1023, and 1024 cm−2 (increasing column density from left to right). At column densities above∼5×1023cm−2 there is little transmission at energies below 7 keV the 0.5 – 2 keV background can be resolved into discrete sources (Hasinger et al. 1998).
Corresponding optical identifications were also performed, revealing that the majority of the extragalactic sources were unobscured quasars and Seyfert galaxies (Schmidt et al. 1998).
Although these detected sources resolved a large fraction of the low-energy background, the spectrum of these sources, if extrapolated to higher energies, does not match the shape of the X-ray background. Typical unobscured AGN have power-law spectral indices of∼1.9 (Nandra & Pounds 1994), while the spectrum of the X-ray background from 2 to 10 keV is fit with Γ = 1.4, a much flatter slope. This spectral mismatch was explained by invoking a large population of obscured active galaxies.
Photoelectric absorption by dusty material either in the galaxy or surrounding the nu- cleus of an active galaxy will absorb incident X-rays, preferentially so at low energies. Figure 2.2provides an illustration of the transmission of X-rays through column densities ranging fromNH= 1020cm−2toNH= 1024cm−2. For the lowest column density the transmission is nearly unity throughout the entire range (0.5 – 10 keV); as the column density increases the low energy photons are not able to penetrate the material. An active galaxy with intrinsic power-law index Γ of 1.9 but surrounded by obscuring material along the line of sight will appear to have a lower power-law index (if still fit with a single power law).
In the unified model (e.g., Antonucci 1993), the central engine of each active galaxy is surrounded by a torus of obscuring material. Thus, the same object viewed from different angles will present different observational properties depending on what fraction of the torus column is within the observer’s line of sight. In unified-model terms, type-1 AGN (including type-1 quasars and type-1 Seyfert galaxies) are AGN viewed face on, with an unobscured view to the central engine, while type-2 AGN are viewed edge on, such that the soft X-ray emission and doppler-broadened optical emission lines are extinguished. The development of this model was strongly motivated by polarization observations of nearby type-2 Seyfert galaxies (Antonucci 1982; Antonucci & Miller 1985). While the optical spectra of the galaxies were devoid of broad, permitted emission lines, the polarized-light spectra showed these broad lines at equivalent widths typical of type-1 AGN, implying that a hidden Seyfert 1 nucleus was reflecting light into the line of sight (Antonucci & Miller 1985). An example of a nearby active galaxy with the theorized torus-like obscuring material was studied by Jaffe et al. (1993). The authors imaged the nucleus of the galaxy with the Hubble Space Telescope (HST). The image showed an unresolved point source, attributed to a hot, inner accretion disk which is feeding the central SMBH, surrounded by a cooler outer accretion disk (torus) extending ∼ 100 pc from the point source — this observation was another striking piece of evidence for an extended dusty torus as hypothesized in the unified model.
It is this absorptive effect that was invoked when reconciling the results of the ROSAT survey with the shape of the 2 – 10 keV XRB. For example,Comastri et al. (1995) models the XRB over a broad range, from a few keV to∼100 keV, by employing a large population of obscured AGN at various redshifts with intrinsic absorbing column densities ranging from 1021 cm−2 to 1025 cm−2. The Comastri et al. (1995) model is consistent with the fraction of the soft-band XRB resolved byROSAT, reproduces theE ∼>30 keV decline in the XRB (assuming that each individual AGN has a spectral break above 70 keV), and is consistent with the luminosity function of optically selected AGN (Boyle et al. 1993).
While the model ofComastri et al.(1995) and other similar models are able to reproduce the XRB by combining the emission of populations of obscured and unobscured AGN, the existence of this large population of obscured sources that rivals or outnumbers the unobscured sources was speculative. The missing observational tool was a high-spatial- resolution X-ray telescope with survey capabilities and sensitivity to the obscured sources.
In the X-ray, this requires extending surveys above E '2 keV. It was not until 1999 that
capabilities at 2 – 10 keV rivaled the good angular-resolution of ROSAT.
2.1.3 The Advanced Satellite for Cosmology and Astrophysics (ASCA)
The Advanced Satellite for Cosmology and Astrophysics (ASCA) was the first X-ray astron- omy mission to combine imaging capability and a large effective area with a band pass that extended to higher energies (∼10 keV). However, the ∼3 arcminute half-power diameter (HPD) made secure optical-counterpart identifications challenging or impossible for all but the brightest sources. Sources with faint optical counterparts are difficult to identify at all since there can be of order tens or hundreds of optical sources within the error circle of the X-ray source position. For example, Figure2.3displays anR-band image near the position where a 2 – 10 keV source was detected by bothChandra and ASCA. Optical-counterpart search areas typical of ASCA (0.50) and Chandra (1.500) are overlaid in black and red, re- spectively. To securely identify an optical counterpart to a faint ASCA AGN candidate, one must take optical spectra of many of the sources that lie within the X-ray error circle and then determine the correct counterpart based upon the optical spectral features. Not only is this inefficient, it also requires a reliance on assumptions of what AGN optical-counterpart spectral features should be (see further discussion of this in§5.10).