The unresolved hard X-ray background: the missing source population implied by the Chandra and XMM--Newton deep fields

M A Worsley,H Brunner,A C Fabian,F E Bauer,D M Alexander,W N Brandt,D P Schneider,S Mateos,G Hasinger

doi:10.1111/j.1365-2966.2005.08731.x

Abstract

We extend our earlier work on X-ray source stacking in the deep XMM-Newton observation of the Lockman Hole, to the 2-Ms Chandra Deep Field North (CDF-N) and the 1-Ms Chandra Deep Field South (CDF-S). The XMM-Newton work showed the resolved fraction of the X-ray background (XRB) to be ∼80-100 per cent at ≤ 2 keV but this decreased to only ∼50 per cent above ∼8 keV. The CDF-N and CDF-S probe deeper, and are able to fill in some of the missing fraction in the 4-6 keV range, but the resolved fraction in the 6-8 keV band remains only ∼60 per cent, confirming the trend seen with XMM-Newton. The missing XRB component has a spectral shape that is consistent with a population of highly obscured active galactic nuclei (AGN) at redshifts ∼0.5-1.5 and with absorption column densities of ∼10 23 -10 24 cm -2 .

Highlights

The nature of the X-ray background (XRB) has been a subject of contention since its discovery 40 years ago (Giacconi et al 1962)
This was done for the Chandra Deep Field North (CDF-N), Chandra Deep Field South (CDF-S) and each of the three XMM–Newton instruments in the XMM-LH
Follow the trends observed by Streblyanska et al (2004) in the Chandra Deep Fields (CDFs)-S and Ueda et al (1999) using the ASCA Medium-Sensitivity Survey and, whilst crude, it is important to take into account the fact that the bright-end correction is the result of sources that are significantly softer than those that we have already stacked

Summary

INTRODUCTION

The nature of the X-ray background (XRB) has been a subject of contention since its discovery 40 years ago (Giacconi et al 1962). C 2005 RAS to resolve some ∼60 per cent of the 5–10 keV background in a deep survey of the Lockman Hole, reaching a limiting flux of ∼3 × 10−15 erg cm−2s−1 in this band (Hasinger et al 2001). The substantial resolved fraction in the broad 2–10 keV band is not entirely consistent with this picture; a similar extrapolation of the 2–10 keV log N–log S distribution is, at most, only able to account for some 93 per cent of the total XRB and is only marginally consistent with complete resolution (Moretti et al 2003). Chandra has low effective area at energies exceeding 7 keV, the Chandra Deep Fields (CDFs) probe to fainter flux limits than the XMM–Newton Lockman Hole observation. Using a similar photometric analysis, we sum the flux from resolved CDF sources in a number of narrow energy bands to determine the resolved fraction of the XRB. We attempt to quantify and correct for field-to-field variations and the missing flux, which would arise from the very bright sources that are not sampled in these deep, pencil-beam surveys

Chandra deep field observations

XMM–Newton Lockman Hole observations

Resolving the X-ray background

Correcting for the bright-end population

DISCUSSION

CONCLUSIONS