Soft X-ray AGN Luminosity Function from ROSAT Surveys I. Cosmological Evolution and Contribution to the Soft X-ray Background

TL;DR

This paper studies the evolution of the soft X-ray luminosity function of AGN using ROSAT data, models its contribution to the cosmic X-ray background, and examines the evolution of luminous QSOs across redshifts.

Contribution

It introduces two LDDE models that fit the data well and explain different extents of the X-ray background, providing new insights into AGN evolution and background contribution.

Findings

The SXLF evolves rapidly up to z~1.5 and remains constant beyond.

LDDE models fit the data and explain 60-90% of the CXRB.

ROSAT QSOs do not show a clear decline in number density at z>3.

Abstract

We investigate the evolution of the 0.5-2 keV soft X-ray luminosity function (SXLF) of active galactic nuclei (AGN) using results from ROSAT surveys of various depth, ranging from shallow large-area ROSAT All-Sky Survey -based samples to the deepest pointed observation on the Lockman Hole. As previously found, the SXLF evolves rapidly as a function of redshift up to z\sim 1.5 and is consistent with remaining constant beyond this redshift. We found that a form of the Luminosity-Dependent Density Evolution (LDDE) gives an excellent fit to the data. Extrapolating one form of the LDDE model (LDDE1) explains \approx 60% of the estimated soft extragalactic Cosmic X-ray Background (CXRB). We have also found another representation (LDDE2), which produces \approx 90% of the CXRB and still gives an excellent fit to the sample AGNs. These two versions of the LDDE models can be considered two extremes of the possible extrapolations of the SXLF below the flux limit. We have also investigated the evolution of the number density of luminous QSOs. Unlike these QSOs, the ROSAT-selected QSOs do not show evidence for the decrease of the number density in z>3. The statistical significance of the difference is, however, marginal.