Widespread steep X-ray spectra of luminous $z=5.85-7.64$ quasars and their implications for future X-ray surveys

A. Bongiorno; A. Comastri; A. Grazian; A. Luminari; A. Tortosa; B. Musiimenta; C. Done; C. Feruglio; C. Piscitelli; C. Vignali

REVIEW

Widespread steep X-ray spectra of luminous $z=5.85-7.64$ quasars and their implications for future X-ray surveys

A. Tortosa, L. Zappacosta, E. Piconcelli, I. Saccheo, G. Lanzuisi, E. Bertola, F. Vito, M. Bischetti

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M. Brazzini, M. Castellano, S. Carniani, A. Comastri, S. Gallerani, R. Gilli, F. La Franca, P. Madau, R. Maiolino, G. Miniutti, B. Musiimenta, C. Piscitelli, F. Salvestrini, F. Tombesi, R. Tripodi, C. Vignali, M. Volonteri, A. Bongiorno, L. Borrelli, M. Brusa, I. V. Chilingaria, F. Civano, S. Cristiani, V. D'Odorico, D. De Cicco, C. Done, M. Elvis, X. Fan, C. Feruglio, F. Fiore, A. Grazian, M. Guainazzi, F. Haardt, A. Luminari, N. Menci, R. Middei, F. Nicastro, M. Paolillo, M. Perna, S. Puccetti, R. Schneider, V. Testa, R. Valiante, L. Vallini, E. Vanzella, G. Vietri

High Energy Astrophysical Phenomena · Submitted 7 Oct 2026

Pith's verdict: The real news is a null result — Gamma versus Ms,Edd at 0.8 sigma, retiring the tentative correlation from T24 — plus genuinely useful count-rate bias numbers for future surveys; the widespread-steepness claim is probably conservative in direction, but the low-Ms,Edd end is thin.

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arXiv ID2610.10522 (version 4)

Pith IDRJ473IUP, Pith's permanent record of this paper and its review

CitationA. Bongiorno, A. Comastri, A. Grazian, A. Luminari, A. Tortosa, B. Musiimenta, C. Done, C. Feruglio, C. Piscitelli, C. Vignali, D. De Cicco, E. Bertola, E. Piconcelli, E. Vanzella, F. Civano, F. Fiore, F. Haardt, F. La Franca, F. Nicastro, F. Salvestrini, F. Tombesi, F. Vito, G. Lanzuisi, G. Miniutti, G. Vietri, I. Saccheo, I. V. Chilingaria, L. Borrelli, L. Vallini, L. Zappacosta, M. Bischetti, M. Brazzini, M. Brusa, M. Castellano, M. Elvis, M. Guainazzi, M. Paolillo, M. Perna, M. Volonteri, N. Menci, P. Madau, R. Gilli, R. Maiolino, R. Middei, R. Schneider, R. Tripodi, R. Valiante, S. Carniani, S. Cristiani, S. Gallerani, S. Puccetti, V. D'Odorico, V. Testa, X. Fan. "Widespread steep X-ray spectra of luminous $z=5.85-7.64$ quasars and their implications for future X-ray surveys." arXiv:2610.10522 (2026). https://pith.science/paper/RJ473IUP

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Editor's note

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The real news here is a null result: in 26 detected quasars at z~5.85–7.5, Gamma shows no correlation with the growth indicator Ms,Edd (rho_P=0.15, 0.8 sigma). That retires the tentative correlation T24 reported, and the five X-LP targets genuinely extend coverage to lower growth efficiencies. The count-rate bias calculations for NewAthena, XMM, and Chandra are a useful practical addition on their own.

The statistical work is careful: W-stat with Poisson background, KB binning, Monte Carlo propagation of asymmetric errors, censored fits, outlier-removal checks, KS tests against the WISSH sample. Homogeneous re-derivation of black hole masses from MgII, H-alpha, and H-beta with consistent scaling relations is the right way to compare growth indicators. The paper is also honest about what it cannot distinguish: steep power laws versus low cutoffs, and flat-but-absorbed versus intrinsically flat sources. It flags the count-threshold issue for the X-ray weakness fraction explicitly.

The soft spot is the population-level step. The sample is not survey-selected: it requires published virial masses and more than 30 net counts, and the X-LP targets were hand-picked for Ms,Edd extremes. The paper quantifies how steep spectra suppress observed counts but never inverts that to correct the measured Gamma distribution. That said, the direction of the selection actually favors the authors: at fixed intrinsic luminosity, steep sources lose counts, so a count-threshold sample should be biased toward flat spectra. Getting Gamma=2.4 out of that selection suggests the widespread-steepness claim is conservative, not inflated. The genuinely thin part is the low-Ms,Edd end: only three detected X-LP sources anchor it, and the two nondetections are exactly where the threshold binds. "Steepness at all growth rates" rests on a small number of sources. Also, the non-HYPERION average Gamma (2.27) is milder than the HYPERION 2.46, in the direction of the old correlation, though not significantly.

Minor: one comparison sample is in preparation; no code or data products ship, though OBSIDs and recipes support re-analysis.

This is for high-z AGN observers and NewAthena survey planners. It deserves a serious referee: the central claim for the studied quasars is well supported, and the generalization is honestly bounded. I'd send it out and ask the authors to fold their own Fig. 9 back into the sample — a back-of-the-envelope correction for what a >30-count threshold does to the observed Gamma distribution would settle how much weight the widespread claim carries.

Referee report

Significance

The paper is valuable and methodologically careful. It is the largest X-ray spectral sample of z>6 QSOs; masses are re-derived on a consistent footing; and the spectral pipeline (W-stat with background Poisson likelihood, KB binning, Monte Carlo propagation with split-normal errors, censored fitting for limits, AIC checks for the intrinsic-absorption degeneracy, outlier-removal robustness tests) is transparent and appropriate. The null Γ–M_s,Edd result, with the quantified bound |A|≲0.27 on the slope, is a genuine and falsifiable update to the earlier T24 hint, and the §4.3 survey-bias calculation is cleanly scoped with explicit caveats about what it does and does not model. External consistency with the independent stacked Chandra measurement of Jin et al. 2026 (Γ=2.41±0.27) supports the mean. The principal limitation is sample representativeness: the defining >30-net-count selection is itself a function of Γ, and several population-level statements outrun what the sample can strictly support, although the direction of that bias is favorable to the qualitative steepness claim.

Major objections (2)

[§2.1, §4.2, §4.3] The sample's defining requirement of >30 net counts in the observed 0.3–7 keV band (§2.1) is a function of the quantity being measured: the authors' own simulations (§4.3, Eq. 1) show that at fixed intrinsic L_2–10 keV the observed-band count rate drops to R≈0.8–0.9 at Γ=2.4 and to R≈0.3–0.5 for Γ=3–3.5 over z=5–10, so a count-threshold sample truncates the steep tail of the Γ distribution. The paper acknowledges this for the X-ray-weak fraction (§4.2: 'this fraction should be regarded as a lower limit') but not for the headline population values: ⟨Γ⟩=2.40, σ=0.38, and the Γ–M_s,Edd null result (§3.2) all inherit the same selection, and the truncation depth varies along the M_s,Edd axis because the HYPERION sources have dedicated 2.4 Ms exposures whereas the X-LP and archival sources are considerably shallower (Table B.1). Because the selection removes steep sources preferentially, the observed mean Γ is plausibly a lower limit on the parent population mean, which works in favor of the qualitative steepness claim; this should be stated explicitly and, where possible, quantified through a forward-model correction using the paper's own simulation machinery. As written, the abstract's formulation ('demonstrating that luminous QSOs at z≳6 exhibit systematically steep X-ray continua') overstates what an un-inverted, non-representative sample establishes.
[§3.2, §2.2, Table B.1] The two sources failing the count threshold, J0313-1806 and PSO J183-12, are excluded a priori from all correlation fits (§3.2), yet they sit at the extremes of the M_s,Edd range probed (log M_s,Edd ≈4.4–4.7 and ≈2.2; Table A.1), and the flux upper limit for J0313-1806 already satisfies the X-ray-weak criterion (Table 1). PSO J183-12's nondetection rests on only ≈29 ks of net PN exposure plus a few tens of ks in MOS (Table B.1), so its exclusion reflects exposure depth as well as intrinsic faintness. The low-M_s,Edd side of the Γ–M_s,Edd null result is therefore supported by only the three detected X-LP sources (SDSS J2054-0005, SDSS J1411+1217, PSO J025-11), and the two excluded extremes are precisely where a steep or X-ray-weak population would be hidden. The inference that steepness persists 'at all mass growth rate regimes' (§5), and the reported null correlation itself, need a quantitative treatment of these exclusions: for example, a censored treatment of the nondetections' Γ using their flux upper limits, or a bracketing exercise showing how the correlation changes if the excluded extremes are assigned steep or flat fiducial values.

Minor objections (5)

[§4.1 vs Appendix D] The KS-test significance for the same best-fit comparison is quoted as 1−P=0.999 in §4.1 and as 1−P=0.9999 in Appendix D; these should be reconciled.
[§3.1, abstract, §5] The restricted mean ⟨E_c⟩_KM=30.0±3.7 keV is carried as a headline value in the abstract and conclusions, but §3.1 itself cautions that E_c values are an alternative parameterization of the spectral curvature rather than an independent coronal measurement, and that the lowest detections (E_c≈3.5–3.9 keV) lie near the edge of the rest-frame fitting range. Since Γ and E_c are degenerate and fitted separately, the headline presentation should carry the same caution to avoid presenting the same curvature twice as independent evidence for cold coronae.
[§3.1, §5] The mean ⟨Γ⟩=2.40 is quoted without an uncertainty; given that the individual measurements carry errors of typically ±0.3–0.8, the authors should quote the Monte-Carlo-propagated uncertainty on the mean from Appendix D alongside σ=0.38 in the conclusions bullet.
[Fig. 6 caption] The z∼3–4 comparison values in Fig. 6 are attributed to 'Borrelli et al., in prep.'; an in-preparation reference cannot be checked, so either replace it with a published source or explicitly mark those points as preliminary.
[§3.1, §3.2, Appendix B.2] The choice to use the bright-state XMM observation of SDSS J1030+0524 while noting its fainter Chandra state is described only in Appendix B.2; this choice should be flagged in the main text when that source contributes to the mean Γ and to the correlation tests, since a transient state could bias its measured coronal parameters.

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The paper

The paper claims

Luminous quasars at the reionization epoch show X-ray spectra systematically steeper, and coronae colder, than equally luminous later quasars — and the steepness does not depend on how fast the black hole grew.

The reading

The paper asks whether the steep X-ray spectra seen in the most extreme early quasars are a quirk of the fastest-growing black holes or a general property of luminous quasars when the Universe was less than a billion years old. Combining the original most rapidly grown quasars (the HYPERION sample) with less extreme systems, including five newly observed targets, it finds no statistically significant correlation between spectral slope and growth efficiency, and a sample-wide mean photon index of $\langle\Gamma\rangle=2.40$ against the value near 1.9 typical of lower-redshift quasars. Steep power-law continua and relatively low high-energy cutoffs both point to coronae colder than those of similar quasars later in cosmic time. If the steepness is widespread, future X-ray surveys of the reionization epoch will see these sources fainter than canonical templates predict at fixed intrinsic luminosity, biasing the census of early black-hole growth.

What carries the argument

The argument is carried by three tools. The first is the growth-efficiency indicator $M_{\rm s,Edd}$, the seed mass that a $z=20$ black hole would need in order to reach its observed mass by continuous Eddington-limited growth; it lets the sample be deliberately spread from very fast to moderate growth at roughly fixed bolometric luminosity. The second is the double parametrization of spectral curvature: a free photon index $\Gamma$ in a power-law fit, and a separate cutoff-power-law fit with $\Gamma$ fixed at 1.9, since low-count spectra cannot tell an intrinsically steep continuum from a canonical slope with a low high-energy cutoff. The third is the statistical machinery that turns noisy measurements into population statements: survival analysis (the Kaplan–Meier estimator) converts the many lower limits on $E_c$ into a restricted mean of $30.0\pm3.7$ keV, Monte Carlo propagation of the asymmetric $\Gamma$ uncertainties feeds a Kolmogorov–Smirnov comparison with the $z\approx2$–4 sample, and count-rate ratios $R(\Gamma)$ and $R(E_c)$ quantify how much steeper slopes or lower cutoffs depress observed-band counts at fixed intrinsic luminosity.

Extended reading notes

Core claim

The paper's central claim is that steep X-ray continua are widespread among luminous quasars at $z\simeq6$–7.5, not a peculiarity of the fastest-growing subsample. Across the 26 detected sources, the mean photon index is $\langle\Gamma\rangle=2.40$ with a scatter of $\sigma=0.38$, the newly observed moderate-growth targets are steep as well, and a Monte Carlo propagation of the individual uncertainties shows the distribution is shifted well above that of equally luminous quasars at $z\approx2$–4 (two-sample Kolmogorov–Smirnov test, median $1-P=0.99$ over $10^4$ realizations). Because low-count spectra cannot separate an intrinsically steep power law from a canonical slope with a low cutoff, the same curvature is parametrized a second way: with $\Gamma$ fixed at 1.9, the survival-analysis restricted mean cutoff energy is $\langle E_c\rangle_{\rm KM}=30.0\pm3.7$ keV, far below the values found in lower-redshift AGN. The photon index shows no significant correlation with the long-term growth-efficiency indicator $M_{\rm s,Edd}$ ($\rho_P=0.15$, 0.8$\sigma$), which the paper reads as evidence that steep, cold coronae are a general property of luminous quasars at the Epoch of Reionization rather than a selection effect tied to rapid growth. The paper further shows that at fixed intrinsic 2–10 keV luminosity, such steep spectra reduce observed count rates by tens of percent at the sample mean and by up to an order of magnitude for the steepest sources, so future X-ray surveys need spectral-shape corrections when forecasting detections.

Load-bearing premise

The load-bearing premise is that 26 quasars selected for having published black-hole masses and more than 30 X-ray counts stand in for the whole luminous $z\gtrsim6$ population, even though steep and X-ray-faint sources are exactly the ones a count threshold tends to drop.

Editorial extensions

If this is right

Luminous $z\gtrsim6$ quasars share steep X-ray continua, with no statistically significant dependence of $\Gamma$ on growth efficiency, black hole mass, or redshift in the expanded sample.
The steepness, read as a low cutoff energy ($\langle E_c\rangle_{\rm KM}=30.0\pm3.7$ keV), implies coronal electron temperatures far below those of lower-redshift luminous AGN, consistent with efficient Compton cooling at high accretion rates.
Future X-ray surveys of the Epoch of Reionization will detect the steepest sources at count rates lower by up to an order of magnitude than canonical $\Gamma=1.9$ templates predict at fixed intrinsic luminosity, with soft 0.3–2 keV bands suffering less bias than the 2–10 keV band.
The correlation between $\Gamma$ and the C IV wind velocity survives in the expanded sample at $>3\sigma$, tying coronal properties to accretion-disc outflows.
The X-ray-weak fraction of about 8–14% is a lower limit because the count-based selection disfavours intrinsically faint sources.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

Selection cuts both ways: because steep spectra reduce observed counts, the same >30-count threshold that the paper flags as making the X-ray-weak fraction a lower limit probably also truncates the steep tail of the $\Gamma$ distribution, so the true population mean may exceed 2.40; stacking the two non-detected targets is a direct way to test this.
If the low-redshift result that $\Gamma$ is luminosity-independent carries to the EoR, the cold-corona state should show up in far less luminous $z>6$ AGN, including JWST-selected objects, and their stacked spectra are a testable extension of the present sample.
The count-rate ratios $R(\Gamma)$ can be folded into a corrected EoR AGN luminosity function once convolved with exposures and detection thresholds — a step the paper explicitly defers — turning the bias estimate into a quantitative survey-yield forecast.

Original abstract

Luminous quasars (QSOs) at the Epoch of Reionization (EoR, $z\gtrsim6$) host billion-solar-mass black holes assembled within the first Gyr of cosmic time. Their X-ray emission probes the innermost accreting regions and constrains the physical state of the corona during early supermassive black hole (SMBH) growth. Previous results from the HYPerluminous quasars at the Epoch of ReionizatION (HYPERION) sample of $z \gtrsim 6$ QSOs, powered by the most rapidly accreted SMBHs, suggested a link between X-ray spectral slope and long-term growth efficiency. We test whether the steep X-ray spectra of luminous z>6 QSOs are a general property of early SMBH growth or a peculiarity of the most efficiently grown SMBHs, using a sample extended mainly to the lowest growth efficiencies. We analysed the largest and most X-ray sensitive sample of 28 luminous QSOs at $z = 5.85-7.64$ (26 used in the correlation analysis), combining HYPERION targets, luminous non-HYPERION sources, and QSOs powered by moderately accreting SMBHs newly observed in our XMM-Newton Large Programme. We re-derived black hole masses and growth indicators for the entire sample homogeneously, incorporating the latest JWST-based spectroscopic measurements to ensure the most consistent and high-quality characterization across all targets. We found no statistically significant correlation between coronal spectral parameters and BH growth indicators: luminous $z \gtrsim 6$ QSOs systematically show steep X-ray continua, with high photon indices and/or low high-energy cutoffs, consistent with cold coronae, possibly linked to rapid accretion. Finally, if steep X-ray slopes are common among early AGN, they may significantly reduce count rates, at fixed intrinsic luminosity, in future surveys of the EoR, affecting the detectability of steep-spectrum X-ray sources and thus the census of the early AGN population.

The compiled record

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Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-10-08: 1 request, 6,899 input tokens.

What the abstract itself states, as probabilities: claims first 0.07 · states limitation 0.33 · human participants 0.01 · controlled comparison 0.29 · randomized assignment 0.01 · states quantitative result 0.84 · compares with prior results 0.94 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; show every role.

Luminous quasars (QSOs) at the Epoch of Reionization (EoR, $z\gtrsim6$) host billion-solar-mass black holes assembled within the first Gyr of cosmic time. Their X-ray emission probes the innermost accreting regions and constrains the physical state of the corona during early supermassive black hole (SMBH) growth. Previous results from the HYPerluminous quasars at the Epoch of ReionizatION (HYPERION) sample of $z \gtrsim 6$ QSOs, powered by the most rapidly accreted SMBHs, suggested a link between X-ray spectral slope and long-term growth efficiency. We test whether the steep X-ray spectra of luminous z>6 QSOs are a general property of early SMBH growth or a peculiarity of the most efficiently grown SMBHs, using a sample extended mainly to the lowest growth efficiencies. We analysed the largest and most X-ray sensitive sample of 28 luminous QSOs at $z \simeq 5.85-7.56$ (26 used in the correlation analysis), combining HYPERION targets, luminous non-HYPERION sources, and QSOs powered by moderately accreting SMBHs newly observed in our XMM-Newton Large Programme. We re-derived black hole masses and growth indicators for the entire sample homogeneously, incorporating the latest JWST-based spectroscopic measurements to ensure the most consistent and high-quality characterization across all targets. We found no statistically significant correlation between coronal spectral parameters and BH growth indicators: luminous $z \gtrsim 6$ QSOs systematically show steep X-ray continua, with high photon indices and/or low high-energy cutoffs, consistent with cold coronae, possibly linked to rapid accretion. Finally, if steep X-ray slopes are common among early AGN, they may significantly reduce count rates, at fixed intrinsic luminosity, in future surveys of the EoR, affecting the detectability of steep-spectrum X-ray sources and thus the census of the early AGN population.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	Luminous quasars (QSOs) at the Epoch of Reionization (EoR, $z\gtrsim6$) host billion-solar-mass black holes assembled within the first Gyr of cosmic time.	background 1.00	0.06	0.02	asserted 1.00
s1	Their X-ray emission probes the innermost accreting regions and constrains the physical state of the corona during early supermassive black hole (SMBH) growth.	background 1.00	0.07	0.02	asserted 1.00
s2	Previous results from the HYPerluminous quasars at the Epoch of ReionizatION (HYPERION) sample of $z \gtrsim 6$ QSOs, powered by the most rapidly accreted SMBHs, suggested a link between X-ray spectral slope and long-term growth efficiency.	background 0.99	0.07	0.02	hedged possibility 0.96
s3	We test whether the steep X-ray spectra of luminous z>6 QSOs are a general property of early SMBH growth or a peculiarity of the most efficiently grown SMBHs, using a sample extended mainly to the lowest growth efficiencies.	contribution 0.71	0.53	0.03	unclear 0.54
s4	We analysed the largest and most X-ray sensitive sample of 28 luminous QSOs at $z \simeq 5.85-7.56$ (26 used in the correlation analysis), combining HYPERION targets, luminous non-HYPERION sources, and QSOs powered by moderately accreting SMBHs newly observed in our XMM-Newton Large Programme.	method 0.83	0.96	0.02	asserted 1.00
s5	We re-derived black hole masses and growth indicators for the entire sample homogeneously, incorporating the latest JWST-based spectroscopic measurements to ensure the most consistent and high-quality characterization across all targets.	method 0.98	0.96	0.02	asserted 0.99
s6	We found no statistically significant correlation between coronal spectral parameters and BH growth indicators: luminous $z \gtrsim 6$ QSOs systematically show steep X-ray continua, with high photon indices and/or low high-energy cutoffs, consistent with cold coronae, possibly linked to rapid accretion.	result 0.99	0.97	0.92	asserted 0.88
s7	Finally, if steep X-ray slopes are common among early AGN, they may significantly reduce count rates, at fixed intrinsic luminosity, in future surveys of the EoR, affecting the detectability of steep-spectrum X-ray sources and thus the census of the early AGN population.	interpretation 0.99	0.14	0.07	hedged possibility 1.00

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 0.89.

What was examined, and what was not

sources read

arXiv listing abstract v1

not examined

full text
figures and tables
references and citing sentences
relations to other papers

levels run

P abstract

not run

S sections
T body sentences
C citations
R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures

**Figure 1.** Distribution of the sample in the MBH–Lbol plane. Symbols are colour-coded by redshift. We compared our sample (MBH values plotted here are, for simplicity, the mean values derived using all the available values reported in Tab. A.1) with the X-ray selected type 1 AGN in XMM-COSMOS 0.04 < z < 4.5 from Lusso et al. 2010 and the values of the X-ray unobscured (NH < 1021 cm−2 ) type I z < 0.1 AGN from the BASS sample from Mejía-Restrepo et al. 2022. lower limits quoted at the 90% confidence level, if not stated otherwise. 2. Sample selection and data presentation 2.1. The sample The…

**Figure 2.** Left panel: plot of Γ vs. Ms,Edd; Right panel: plot of Ec vs Ms,Edd. The black horizontal dashed line marks the mean value of Γ (top) and Ec (bottom) for the sample of detected QSOs. Points with red edges are the one belonging to the HYPERION sample. Violet shaded region marks the typical range of Γ (∼ 1.8 − 2; e.g. Piconcelli et al. 2005; Ricci et al. 2017) and Ec (∼ 100 − 300 keV; e.g. Fabian et al. 2015; Ricci et al. 2018; Lanzuisi et al. 2019) observed in low-z unobscured AGN. We note that the Ms,Edd values plotted here are, for simplicity, the mean values derived using all the…

**Figure 3.** Γ as a function of vC iv. Points are colour coded for Ms,Edd. The horizontal dashed line represents the mean value of Γ. The solid black line indicates the best fitting relation, with the shaded region showing its 1σ uncertainty the reconstructed distribution remains centred at Γ ∼ 2.4 and is significantly shifted toward steeper values than that observed in a comparison sample (

**Figure 5.** High-energy cutoff distribution. Detections are shown in hatched purple, while lower limits are shown in pink. The dashed vertical line marks the restricted mean cutoff energy derived from the Kaplan–Meier estimator ⟨Ec⟩KM = 30.0±3.7 keV. statistically preferred over a simple unabsorbed power-law, the current data quality does not allow us to establish whether these apparently flat-spectrum QSOs are intrinsically steep but moderately obscured systems. Owing to the well-known degeneracy between Γ and Ec in broadband X-ray spectral modelling (e.g. Dadina 2008; Tortosa et al. 2018),…

**Figure 6.** presents the cutoff energy as a function of the 2– 10 keV luminosity. The QSOs analysed in this work are compared with many different samples from the literature, spanning a broad range of redshift, luminosity, and accretion regimes. Compared to lower-z AGN samples, our sources tend to populate the high-L2−10 keV and low-Ec region of the diagram, implying systematically colder coronae as Ec scales with the coronal electron temperature, kTe ∼ Ec/2 − 3 depending on the adopted corona geometry (Petrucci et al. 2000, 2001). They differ significantly also from the limited number of…

**Figure 7.** Left panel:αOX as a function of L2500. Black dashed line refers to the best-fit relation from Lusso & Risaliti 2016, purple dotted-dashed line refers to the best-fit relation from Martocchia et al. 2017; Right Panel: ∆(αOX) = αOX − αOX,RL16 as a function of L2500; blue shaded region includes the X-ray weak QSOs (Luo et al. 2015). In all the panels the sources analysed in this work are the red stars. We plot for comparison the following samples: X-ray selected type 1 AGN in XMM-COSMOS 0.04 < z < 4.5 from Lusso & Risaliti 2016 (gray points), WISSH QSOs at z ∼ 2 − 4 from Degli Agosti…

**Figure 8.** κ x bol as a function of Lbol. The solid black line indicates the relation from Duras et al. 2020, with the shaded region showing its 1σ uncertainty. The sources analysed in this work are the red stars. We plot for comparison the following samples: X-ray selected type 1 AGN in XMM-COSMOS 0.04 < z < 4.5 from Lusso et al. 2010 (green points), WISSH QSOs at z ∼ 2−4 from Degli Agosti et al. 2025 (purple squares), local AGN/NLS1s from Liu et al. 2021 (blue diamonds), nearby 0.40 < z < 0.75 highly accreting AGN from Laurenti et al. 2022 (dark green triangles), SDSS DR7 QSOs at z ≃ 3.0 −

**Figure 9.** Left Panel: The "spectral steepness bias" as a function of photon index (Γ) for NewAthena/WFI; Right Panel: The "spectral cutoff energy bias" as a function of high-energy cutoff (Ec) for NewAthena/WFI. Count rates ratios in the 0.3–2 keV (solid lines) and 2–10 keV (dashed lines) bands are shown for redshift z = 5 (blue) and z = 10 (orange). More detailed curves are shown in Fig. F.1 and Fig. F.2 tion, proper spectral modelling and broad-band coverage will also be essential to accurately characterise the coronal emission and rapid early growth of SMBHs at the EoR. 5. Conclusions and…

Showing 8 of the paper's 9 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:2610.10522 by the authors.

Discussion

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