A Deep Chandra X-ray Survey of a Luminous Quasar Sample at $z\sim$ 7

Xiangyu Jin, Feige Wang, Jinyi Yang, Xiaohui Fan, Fuyan Bian, Jiang-Tao Li, Weizhe Liu, Yichen Liu, Jianwei Lyu, Maria Pudoka, Wei Leong Tee, Yunjing Wu, Haowen Zhang, Yongda Zhu

First listed 2025-12-03 | Last updated 2025-12-03

Abstract

We present new Chandra observations of seven luminous quasars at $z>6.5$. Combined with archival Chandra observations of all other known quasars, they form nearly complete X-ray observations of all currently known $z\sim7$ quasars with $M_{1450}<-26.5$, except for J0313$-$1806 at $z=7.642$ and J0910$-$0414 at $z=6.636$. Together with existing ground-based NIR spectroscopy and ALMA observations, we investigate the correlations between X-ray emission (the X-ray luminosity $L_{\rm X}$ and the optical/UV-to-X-ray spectral slope $α_{\rm OX}$) and various quasar properties (rest-UV luminosity $L_{\mathrm{2500\ \mathring{A}}}$, bolometric luminosity $L_{\rm bol}$, C IV blueshift, and infrared luminosity $L_{\rm IR}$). We find most $z>6.5$ quasars follow a similar $α_{\rm OX}-L_{\mathrm{2500\ \mathring{A}}}$ relation as $z\sim1-6$ quasars, but also display a large scatter. We find a potential correlation between $α_{\rm OX}$ and the C IV blueshift, suggesting a soft optical/UV-to-X-ray SED shape is frequently associated with fast disk winds. Furthermore, we analyze the X-ray spectrum of 11 quasars at $z>6.5$ with Chandra detection, and find the best-fit photon index $Γ$ is $2.41\pm0.27$, which is likely driven by high accretion rates of $z>6.5$ quasars. In addition, we find there are no significant correlations between either $L_{\rm X}$ and $L_{\rm IR}$, nor $L_{\rm bol}$ and $L_{\rm IR}$, suggesting no strong correlations between quasar luminosity and star formation luminosity for the most luminous quasars at $z>6.5$.

Short digest

Deep Chandra ACIS-S imaging of seven z>6.5 quasars, combined with archival data, delivers a near-complete X-ray census of all known z~7 quasars with M1450<-26.5 (excluding J0313-1806 and J0910-0414). Most sources sit on the familiar α_OX–L_2500 trend seen at z~1–6 but with substantial scatter, and α_OX increases with C IV blueshift, linking softer optical/UV-to-X-ray SEDs to fast disk winds. A joint fit of 11 Chandra-detected objects yields a steep photon index Γ=2.41±0.27, consistent with high accretion rates in the earliest quasars. No significant L_X–L_IR or L_bol–L_IR correlations emerge, implying quasar power and host star-formation luminosity are decoupled in these luminous systems.

Key figures to inspect

• Figure 1: Inspect the 0.5–7 keV cutouts to see which of the seven targets are formally detected (red circles) and which are not (blue dashed), noting band-dependent detections (e.g., J0411−0907 and J1104−2134 in all bands; J0706−2921 in two) and how source apertures and background regions were chosen relative to the P…
• Figure 2: Read off rest-frame 2–10 keV luminosities versus L_bol to gauge X-ray bolometric corrections (10/100/1000 lines) and compare BAL-labeled objects against SDSS references—useful for spotting any X-ray-weak outliers among the z>6.5 sample.
• Figure 3: Place the z>6.5 points on α_OX versus L_2500 alongside the Just+2007, Nanni+2017, and Timlin+2020 relations to visualize the large intrinsic scatter and whether the most UV-luminous objects drive any offsets.
• Figure 4: Examine α_OX versus C IV blueshift and the best-fit relation (with 1σ band), then compare to the fit excluding J1342+0928 and J0923+0402 to see how outliers affect the inferred wind–SED connection.