Angular fluctuations in the CXB: is Fe 6.4 keV line tomography of the large-scale structure feasible?

Abstract

AGN are known to account for a major portion, if not all, of the cosmic X-ray background radiation. The dominant sharp spectral feature in their spectra is the 6.4 keV fluorescent line of iron, which may contribute to as much as ~ 5-10 % of the CXB spectral intensity at ~ 2-6 keV. Owing to cosmological redshift, the line photons detected at the energy E carry information about objects located at the redshift z=6.4/E-1. In particular, imprinted in their angular fluctuations is the information about the large-scale structure at redshift z. This opens the possibility of performing the Fe K_alpha line tomography of the cosmic large-scale structure. We show that detection of the tomographic signal at ~100 sigma confidence requires an all-sky survey by an instrument with an effective area of ~10 m^2 and field of view of ~1 deg^2. The signal is strongest for objects located at the redshift z~1 and at the angular scales corresponding to l ~ 100-300, therefore an optimal detection can be achieved with an instrument having a rather modest angular resolution of ~ 0.1-0.5 deg. For such an instrument, the CCD-type energy resolution of ~ 100-200 eV FWHM is entirely sufficient for the optimal separation of the signals coming from different redshifts. The gain in the signal strength that could potentially be achieved with energy resolution comparable to the line width is nullified by the photon counting and AGN discreteness noise. Among the currently planned and proposed missions, these requirements are best satisfied by LOFT, even though it was proposed for an entirely different purpose. Among others, clear detection should be achieved by WFXT (~ 20-35 sigma) and ATHENA (~ 10-20 sigma). eROSITA, in the course of its 4 years all-sky survey, will detect the tomographic signal only marginally.