A New Method to Determine X-ray Luminosity Functions of AGN and their Evolution with Redshift

TL;DR

This paper introduces a novel fixed rest-frame energy band method for more accurately studying the evolution of X-ray luminosity functions of AGN across cosmic time, addressing limitations of previous observer-frame approaches.

Contribution

The paper presents a new method that constructs XLFs in fixed rest-frame energy bands, improving accuracy in studying AGN evolution over redshift.

Findings

ML best-fit pure luminosity evolution results are consistent across methods

High-luminosity AGN show little absorption, insensitive to energy band choice

Demonstrated viability of the new fixed rest-frame method

Abstract

Almost all massive galaxies today are understood to contain supermassive black holes (SMBH) at their centers. SMBHs grew by accreting material from their surroundings, emitting X-rays as they did so. X-ray Luminosity Functions (XLFs) of Active Galactic Nuclei (AGN) have been extensively studied in order to understand the AGN population's cosmological properties and evolution. We present a new fixed rest-frame method to achieve a more accurate study of the AGN XLF evolution over cosmic time. Normally, XLFs are constructed in a fixed observer-frame energy band, which can be problematic because it probes different rest-frame energies at different redshifts. In the new method, we construct XLFs in the fixed rest-frame band instead, by varying the observed energy band with redshift. We target a rest-frame 2$-$8 keV band using XMM-Newton and HEAO 1 X-ray data, with 7 observer-frame energy bands that vary with redshift for $0 < z < 3$. We produce the XLFs using two techniques; one to construct a binned XLF, and one using a Maximum Likelihood (ML) fit, which makes use of the full unbinned source sample. We find that our ML best-fit pure luminosity evolution (PLE) results for both methods are consistent with each other, suggesting that performing XLF evolution studies with the high-redshift data limited to high-luminosity AGN is not very sensitive to the choice of fixed observer-frame or rest-frame energy band, which is consistent with our expectation that high-luminosity AGN typically show little absorption. We have demonstrated the viability of the new method in measuring the XLF evolution.