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

TL;DR

This paper reports the discovery and analysis of a high-redshift quasar at z=6.56, detected in X-rays, revealing properties similar to local narrow-line Seyfert 1 galaxies and providing insights into black hole growth during re-ionisation.

Contribution

First confirmation of a z=6.56 quasar as the most distant X-ray detection, linking high-redshift quasars to narrow-line Seyfert 1 galaxy properties and estimating black hole accretion rates.

Findings

Confirmed the most distant X-ray detected quasar at z=6.56.

Found the quasar has high X-ray luminosity and properties similar to local narrow-line Seyfert 1 galaxies.

Indicated that luminous quasars dominate black hole accretion at z~6.

Abstract

This study aims at identifying luminous quasars at $z>5.7$ among X-ray-selected sources in the eROSITA Final Equatorial-Depth Survey (eFEDS) in order to place a lower limit on black hole accretion well into the epoch of re-ionisation. We confirm the low significance detection with eROSITA of a previously known, optically faint $z=6.56$ quasar from the Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs) survey. We obtained a pointed follow-up observation of the source with the Chandra X-ray telescope in order to confirm the eROSITA detection. Using new near-infrared spectroscopy, we derived the physical properties of the super-massive black hole. Finally, we used this detection to infer a lower limit on the black hole accretion density rate at $z>6$. The Chandra observation confirms the eFEDS source as the most distant blind X-ray detection to date. The derived X-ray luminosity is high with respect to the rest-frame optical emission of the quasar. With a narrow MgII line, low derived black hole mass, and high Eddington ratio, as well as its steep photon index, the source shows properties that are similar to local narrow-line Seyfert 1 galaxies, which are thought to be powered by young super-massive black holes. In combination with a previous high-redshift quasar detection in the field, we show that quasars with $L_{2-10 \, \mathrm{keV}} >10^{45} \, \mathrm{erg \, s^{-1}}$ dominate accretion onto super-massive black holes at $z\sim 6$.