Constraints on the X-ray Luminosity Function of AGN at z=5.7-6.4 with the Extragalactic Serendipitous Swift Survey

Abstract

X-ray luminosity functions (XLFs) of Active Galactic Nuclei (AGN) trace the growth and evolution of supermassive black hole populations across cosmic time, however, current XLF models are poorly constrained at redshifts of 𝑧 > 6. In this work we constrain the bright-end of the XLF at 𝑧 = 5.7 − 6.4 using high-redshift AGN identified within the Extragalactic Serendipitous Swift Survey (ExSeSS) catalogue. Within ExSeSS we find one serendipitously detected X-ray selected 𝑧 > 6 AGN, ATLAS J025.6821-33.4627, with an X-ray luminosity of 𝐿 X = 8.47 +3.40 −3.13 × 10 44 erg.s −1 and 𝑧 = 6.31 ± 0.03 making it the highest redshift, spectroscopically confirmed, serendipitously detected X-ray selected quasar known to date. We also calculate an upper limit on the space density at higher luminosities where no additional sources are found, enabling us to place constraints on the shape of the XLF. Our results are consistent with the rapid decline in the space densities of high-luminosity AGN toward high redshift as predicted by extrapolations of existing parametric models of the XLF. We also find that our X-ray based measurements are consistent with estimates of the bolometric quasar luminosity function based on UV measurements at 𝑧 6, although they require a large X-ray to bolometric correction factor (i.e. AGN that are relatively X-ray weak) at these high luminosities.

X-ray emission from a rapidly accreting narrow-line Seyfert 1 galaxy at z = 6.56

“…Models with a milder decline in AGN space density in the highest X-ray luminosity bins are favoured by this detection. These findings are corroborated by an analysis of a sample of X-ray-detected high-redshift AGN in the ExSeSS by Barlow-Hall et al (2022). We note that Barlow-Hall et al (2022) report the detection of ATLAS J025.6821-33.4627, a spectroscopically confirmed quasar at z = 6.31, which had been, until now, the highest-redshift, blindly X-raydetected AGN.…”

“…These findings are corroborated by an analysis of a sample of X-ray-detected high-redshift AGN in the ExSeSS by Barlow-Hall et al (2022). We note that Barlow-Hall et al (2022) report the detection of ATLAS J025.6821-33.4627, a spectroscopically confirmed quasar at z = 6.31, which had been, until now, the highest-redshift, blindly X-raydetected AGN. J0921+0007 was blindly detected in eFEDS, albeit with a low detection-likelihood, and is therefore the highestredshift, X-ray-selected AGN.…”

“…The discrepancy between previous XLF models obtained using smaller pencil-beam or non-contiguous surveys underlines the necessity for wide surveys to obtain a realistic census of the rare, powerful sources at the bright end of the XLF (see e.g. Barlow-Hall et al 2022).…”

“…To date, ∼ 50 quasars at z > 5.7 have been observed in Xrays, mostly via pointed observations with Chandra and XMM-Newton (Brandt et al 2002;Vignali et al 2003;Nanni et al 2017;Vito et al 2019) following their discovery with optical telescopes. Recently, Barlow-Hall et al (2022) reported the blind detection of a quasar spectroscopically confirmed at z = 6.31 in the Extragalactic Serendipitous Swift Survey (ExSeSS). The first unbiased, blind X-ray detections of quasars at z > 5.7 in the performance verification fields and all-sky maps of the extended ROentgen Survey with an Imaging Telescope Array (eROSITA; Predehl et al 2021;Sunyaev et al 2021) were reported by Medvedev et al (2020) and Wolf et al (2021).…”