Large-scale structure studies with the unresolved CXB - Challenges from XBOOTES

The scientific significance of large-scale structure (LSS) studies with X-ray surveys can be greatly enhanced by analysis of the surface brightness fluctuations of the unresolved cosmic X-ray background (CXB). It enables us to study the clustering properties of source populations, which are otherwise inaccessible with clustering studies of resolved sources of current X-ray surveys due to the lack of deep-wide surveys and selection effects.We have conducted the most accurate measurement to date of the brightness fluctuations of the unresolved CXB in the 0.5-2.0keV band for angular scales of <~17', using XBOOTES, the currently largest continuous Chandra survey (~9 deg²).We find that on small angular scales (<~2') the observed power spectrum of the brightness fluctuations is broadly consistent with the conventional AGN clustering model, although with a ~30% deviation. This deviation demonstrates the current poor knowledge of the clustering properties of AGN within their dark matter halo (DMH). We provide possible explanations for this deviation.For angular scales of >~2' we measure a significant excess with up to an order of magnitude difference in comparison to the standard AGN clustering model. We demonstrate that an instrumental origin can be excluded but also that it can neither be explained with any known X-ray source population based on its clustering strength and the shape of its energy spectrum. We speculate that the excess is caused by more than one type of source and that its dominant source appears to have extragalactic origin.Finally, we make predictions on how the eROSITA all-sky survey (eRASS) will be able to advance the studies of the unresolved CXB.