A. Shlentsova | Searcharxiv

arXiv2026

The X-ray-to-UV relation does not evolve in homogeneous quasar samples

We present a new, highly homogeneous quasar sample with X-ray and UV observations optimized to reliably estimate distances via the non-linear X-ray-to-UV relation. Cross-matching the Sloan Digital Sky Survey DR16 quasar catalog with the XMM-Newton serendipitous catalogue (4XMM--DR14), we employ strict selection criteria to build a robust sample: (1) UV and (2) X-ray colour constraints to avoid, respectively, extinction and absorption; (3) removal of broad absorption line and radio-bright quasars; (4) exclusion of sources at z<0.7 to prevent galactic UV contamination; and (5) rejection of sources with shallow X-ray observations. The latter step, closely related to the Eddington bias, is critical because SDSS data are generally deeper than X-ray data for typical quasar spectral energy distributions: ignoring such a discrepancy introduces a spurious redshift dependence in the X-ray-to-UV relation parameters. Our final sample contains about 2,000 quasars at z=0.7--5. We demonstrate that the X-ray-to-UV relation is constant across this redshift range, with a mean slope of 0.58\,$\pm$\,0.01 and a dispersion of 0.15 dex. Our findings confirm the intrinsic stability of this relation over cosmic time, emphasizing that both homogeneity and robust Eddington bias corrections are vital for flux-limited samples. In fact, the impact of the preferential detection of X-ray brighter-than-average sources near the effective sensitivity limits significantly grows with redshift. Any resulting evolutionary trend in the X-ray-to-UV relation, especially in the form a slope flattening, is therefore either a largely spurious effect, or the result of mixing populations of quasars with intrinsically different spectral properties.

astro-ph.HE↗

arXiv2025

The accretion of quasars at the epoch of reionisation: $JWST$ catches the primeval monsters slowly feasting

Quasars (QSOs) emit an enormous amount of light as a result of the accretion of gas onto supermassive black holes (SMBHs). Thanks to their luminosity, the most distant known QSOs allow us to trace the growth of SMBHs deep into the epoch of reionisation. In this work, we employed $JWST$/NIRSpec observations of eight luminous (log$(L_{3000\,A^{\circ}}/(erg \, s^{-1}))>$45.7) QSOs at $z\geq$5.9 to constrain their accretion properties, namely black hole mass, accretion disc (AD) luminosity, and Eddington ratio ($M_{BH}$, $L_{AD}$, $\lambda_{Edd}$), by fitting the rest-frame UV and optical emission with different AD models. This method provided self-consistent measurements of both $M_{BH}$ and $L_{AD}$. The uncertainties on $M_{BH}$ and $L_{AD}$, obtained within the AD-modelling framework ($\sigma^{AD}_{M_{BH}}\sim$0.2 dex; $\sigma^{AD}_{L_{AD}}\sim$0.1 dex), are significantly smaller than the systematic uncertainties associated with single-epoch $M_{BH}$ ($\sim$0.4 dex) and $L_{AD}$ derived via bolometric corrections ($\sim$0.2 dex). Based on these results, in our sample we found an average Eddington ratio of $\langle \log(\lambda_{Edd}) \rangle=-0.9$, with a dispersion of $\sim$0.2 dex. Assuming that our high-z QSOs are representative of optically-selected bright blue QSOs, we derive a fraction of systems accreting above the Eddington limit of $\sim$0.2%. In conclusion, this work i) demonstrates the suitability of $JWST$ to test AD models on high-redshift ($z\gtrsim$4) QSOs, thanks to the large NIRSpec spectral coverage; ii) shows that AD modelling can yield robust $M_{\rm BH}$ and $L_{\rm AD}$ measurements, with smaller uncertainties than the typical calibrations; and iii) provides compelling evidence for sub-Eddington accretion in bright high-$z$ QSOs, challenging the widespread paradigm of near- or super-Eddington accretion occurring in these sources.

astro-ph.GA↗