Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) XXII. Chandra observations of narrow-line quasar candidates at z ≥ 6

We report on Chandra X-ray observations of four narrow-line quasar candidates at z ∼ 6, selected from the Subaru high-z exploration of low-luminosity quasars (SHELLQs) project, based on the Subaru Hyper Suprime-Cam survey. These objects are characterised by narrow (FWHM ≤ 310 km s⁻¹), luminous (> 10⁴⁴ erg s⁻¹) Lyα, and faint UV continuum (M₁₄₅₀ = −22 to −21), prompting us to examine whether they are obscured luminous active galactic nuclei (AGNs) at the epoch of reionization. However, none of these objects were detected by Chandra, giving an upper limit to their rest-frame 2−10 keV luminosity (L_X) of 2 × 10⁴⁴ erg s⁻¹ (2σ), assuming a spectral slope of Γ = 2. Subsequent rest-frame optical spectroscopy of these objects by the James Webb Space Telescope, presented in a companion paper, shows weak broad Balmer emission at the base of narrow cores. With the scaling relation for low-redshift AGNs, the observed strong [O III]λ5007 flux of these sources would predict L_X to be around 10⁴⁵ erg s⁻¹, which is well above the Chandra upper limits. These optical spectra and X-ray quietness are reminiscent of JWST-selected broad-line AGNs. We attribute the weak broad Balmer emission to the broad-line regions hidden partially by optically thick obscuring matter that also hides the optical and X-ray continuum emission from the accretion disc. Compton-thick obscuration, which would strongly suppress X-ray emission, could be due to a dense interstellar medium that is often present in galaxies at high redshifts. Alternatively, the same effect could be obtained from an inflated disc at the innermost radii in a supercritical accretion flow, when the disc is viewed at inclined angles.