Guido Risaliti

Quasar accretion models predict a broad range of optical and ultraviolet properties that depend primarily on black hole mass and accretion rate. Yet, most optically selected luminous quasars display strikingly similar continuum spectra. We show that this uniformity can be explained by a nearly constant luminosity-to-mass (‘Eddington’) ratio, λ _EDD and by thermal emission from a standard, optically thick, geometrically thin accretion disc. A standard disc with an Eddington ratio λ _EDD ∼ 0.1 reproduces both the black hole mass–luminosity distribution of Sloan Digital Sky Survey (SDSS) quasars and their principal continuum properties. In this framework, the spectral energy distribution (SED) peaks beyond the observable ultraviolet range for nearly all sources. We show that the few quasars expected to be cold enough to shift the peak into the observable region indeed show this behaviour. This scenario is further supported by an analysis of the relation between the luminosity of the main broad emission lines and the continuum luminosity (i.e. the ‘Baldwin effect’). We find that (1) the observed slopes of the line–continuum relations match the expectations from the standard disc model, if we assume that the line emission is a good proxy of the ionizing luminosity; (2) the dispersions of the line-continuum luminosity relations are very small (as small as 0.13 dex), suggesting that the physics of the disc-broad line region system is dominated by only one parameter (the black hole mass) with a nearly constant Eddington ratio; (3) our hypothesis of constant λ _EDD ∼ 0.1 provides a black hole mass estimate (based on the observed luminosity) with a smaller error than the virial estimate. Finally, we note that these results are strictly valid only for SDSS quasars; sources selected using other criteria may exist in different accretion states.

Accretion onto supermassive black holes (SMBHs) powers the most luminous persistent sources in the Universe, the so-called active galactic nuclei, whose emission is characterized by two distinct spectral components: thermal optical/ultraviolet (UV) radiation from an optically thick accretion disk and a power-law X-ray tail from a corona located in the innermost regions of the accretion flow. Yet, how radiatively efficient accretion disks develop and couple to the hot corona remains poorly understood. Using 6 yr of simultaneous UV and X-ray monitoring of the nearby active galaxy ESO 511-G030, we witness a dramatic evolution of the broadband spectral energy distribution, driven by an increase of the UV flux from the disk by more than 1 order of magnitude over a timescale of less than 3 yr. The overall behavior is unlikely to track an uncovering event and is instead compatible with a progressive recovery of the optically thick component of the accretion flow. At accretion rates higher than approximately 1% of the Eddington limit, UV and X-ray data are tightly coupled and follow the well-defined, nonlinear correlation between disk and corona found in the more luminous quasars. Below this threshold, the relation apparently breaks down, as expected in case of evaporation of the inner accretion disk into a geometrically thick, optically thin hot flow. This is a strong hint of an accretion-state transition analogous to those observed in stellar-mass black holes and confirms the need for a paradigm change in the models of radiatively efficient accretion flows around SMBHs.

Context. Although the population of quasars evolved significantly in the past, the properties of quasars as physical objects are supposed to remain almost unchanged, which makes quasars promising candidates for cosmological tests. The X–UV luminosity relation in particular is widely used for this purpose. However, the potential of other spectral domains for this purpose remains open. Aims. The aim of the analysis we present is to test the parameter space in order to build a well behaving OPT-IR correlation that could serve as a cosmological probe. The main objective is to calibrate the OPT-IR luminosity relation for quasars, focusing on accurate estimations of dusty torus and accretion disk luminosities. We analyzed contaminations related to host galaxies, particularly from polar dust, the interstellar medium, and stellar emission that affect the optical and infrared. Methods. We used a sample of nearly 400 quasars with photometrical observations and spectroscopical redshift divided into four redshift bins (0.7–2.4). Full spectral energy distribution (SED) fitting was performed with the CIGALE code, and results were compared with simplified photometric luminosity estimates. The impact of non-active galactic nucleus components and the role of polar dust in the fitting process were assessed. Results. We show that for sources with a disk luminosity above 10 ⁴⁵ [erg s ⁻¹ ], the photometric estimates are consistent with SED-based values. While polar dust contributes marginally to luminosity, its presence significantly alters SED fitting, particularly the torus opening angle and cold dust properties. In the optical domain, stellar emission is the dominant contamination. In the infrared, disk emission and cold dust play major roles. We propose two empirical calibrations for the OPT–IR relation. Conclusions. We conclude that the optical band is dominated by the accretion disk component above 10 ⁴⁵ or 10 ⁴⁶ [erg s ⁻¹ ] depending on redshift, while IR luminosity is dominated by the dusty torus emission above 1.6 × 10 ⁴⁵ or 2 × 10 ⁴⁶ [erg s ⁻¹ ] depending on the redshift. In this high-luminosity regime, simplified photometric methods yield reliable disk and torus luminosity estimates.

Context. Strong outflows from active galactic nuclei are frequently observed in objects with lower coronal X-ray luminosity. This intrinsic X-ray weakness is considered a requirement for the formation of radiatively driven winds. Aims. To obtain an unbiased view on the connection between X-ray emission and the presence of powerful winds in the most luminous quasar phase, we present an X-ray analysis of a sample of extremely luminous, radio-quiet quasars with signatures of strong outflows in their rest-frame ultraviolet (UV) emission spectra. Methods. We study the Chandra X-ray spectral properties of 10 objects, selected from the Sloan Digital Sky Survey Data Release 16 quasar catalogue based on their UV luminosities and C IV emission line blueshifts, comparing them to typical optically blue quasars. Results. Our analysis reveals that seven out of 10 quasars in our sample have photon indices Γ > 1.7. Only two out of 10 objects exhibiting outflows with velocities exceeding 1400 km/s are X-ray ‘weak’, consistent with the fraction of X-ray ‘weak’ objects generally observed in quasar populations. Notably, one of the objects identified as X-ray ‘weak’ is likely an intrinsically X-ray ‘normal’ quasar that is heavily obscured. We observe a tentative indication at a ∼2 σ confidence level that the correlation between the excessively low X-ray flux level and the presence of C IV emission-line outflows might emerge at wind velocities greater than 3000 km/s. Conclusions. Our study provides additional evidence that the relationship between X-ray emission and the presence of winds is intricate. Our findings emphasise the need for X-ray observations of a larger sample of UV-selected quasars with confirmed strong emission-line outflows to unravel the nuanced interplay between winds and X-ray emission.

The James Webb Space Telescope (JWST) has discovered a large population of active galactic nuclei (AGN) in the early Universe. A large fraction of these AGN revealed a significant lack of X-ray emission; the observed X-ray luminosity upper limits were 2–3 dex lower than expected. We explored the radio emission of these AGN, focusing on the JWST-selected broad-line AGN (BLAGN, or type 1) in the GOODS-N field, one of the fields with the best combination of deep radio observations and statistics of JWST-selected, spectroscopically confirmed BLAGN. We used deep radio data at different frequencies (144 MHz, 1.5 GHz, 3 GHz, 5.5 GHz, 10 GHz), and we find that none of the 37 sources investigated is detected at any of these frequencies. Similarly, the radio stacking analysis does not reveal any detection down to an rms of ∼0.15 μJy beam ⁻¹ , corresponding to a 3 σ upper limit at rest frame 5 GHz of L _5GHz = 2 × 10 ³⁹ erg s ⁻¹ at the mean redshift of the sample z ∼ 5.1. We compared this and the upper limits of the individual sources with expected radio luminosities estimated assuming different AGN scaling relations to check whether they are consistent with the standard BLAGN spectral energy distribution. For most of the sources the radio luminosity upper limits are still compatible with expectations for radio-quiet (RQ) AGN; nevertheless, the more stringent stacking upper limits and the fact that no detection is found might suggest that JWST-selected BLAGN are weaker than standard AGN, even at radio frequencies. The probability of having none of the BLAGN detected in none of the investigated radio images is expected to be on average very low ( P < 10 ⁻⁴ ). We discuss some scenarios that could explain the possible radio weakness, such as free-free absorption from a dense medium or the lack of either a magnetic field or a corona, possibly as a consequence of super-Eddington accretion. These scenarios would also explain the observed X-ray weakness. We also conclude that ∼1 dex more sensitive radio observations are needed to better constrain the level of radio emission (or lack thereof) for the bulk of these sources. The Square Kilometer Array Observatory (SKAO) will likely play a crucial role in assessing the properties of this AGN population.

One of the most puzzling properties of the high-redshift active galactic nucleus (AGN) population recently discovered by JWST, including both broad-line and narrow-line sources, is their X-ray weakness. With very few exceptions, and regardless of the optical classification, they are undetected at the limits of the deepest Chandra fields, even when stacking signals from tens of sources in standard observed-frame energy intervals (soft, hard, and full bands). It has been proposed that their elusive nature in the X-ray band is due to heavy absorption by dust-free gas or an intrinsic weakness, possibly due to high super-Eddington accretion. For this work we performed X-ray stacking in three customized rest-frame energy ranges (1–4, 4–7.25, and 10–30 keV) of a sample of 50 type 1 and 38 type 2 AGN identified by James Webb Space Telescope (JWST) in the Chandra Deep Field South (CDFS) and Chandra Deep Field North (CDFN) fields. For the type 2 subsample, we achieve a total exposure of about 210 Ms, and report a significant detection (∼3 σ ) in the hardest band (10–30 keV rest frame), along with relatively tight upper limits in the rest-frame softer energy bands. The most straightforward interpretation is in terms of heavy obscuration due to gas column densities well within the Compton-thick regime (> 2 × 10 ²⁴ cm ⁻² ) with a large covering factor, approaching 4 π . The same procedure applied to the type 1 subsample returns no evidence of a significant signal in about 140 Ms stacked data in any of the adopted bands. The bolometric correction k _bol to the absorption corrected 2–10 keV X-ray luminosity of type 2 AGN is consistent with the average value obtained for X-ray selected type 2 objects in the literature. The lower limit on k _bol for the type 1 sample is significantly higher and inconsistent with that of type 2 objects. Absorption in the Compton-thick regime or extreme X-ray weakness, would bring the current lower limits closer to the observed values for type 1 X-ray selected AGN. A brief comparison with the current observations and the implications for the evolution of AGN are discussed.

Quasars accretion models predict a broad range of optical and ultraviolet properties that depend primarily on black hole mass and accretion rate. Yet, most optically selected luminous quasars display strikingly similar continuum spectra. We show that this uniformity can be explained by a nearly constant luminosity to mass (Eddington) ratio, L_EDD and by thermal emission from a standard, optically thick, geometrically thin accretion disc. A standard disk with an Eddington ratio L_EDD=0.1 reproduces both the black hole mass/luminosity distribution of Sloan Digital Sky Survey (SDSS) quasars and their principal continuum properties. In this framework, the spectral energy distribution peaks beyond the observable ultraviolet range for nearly all sources. We show that the few quasars, expected to be cold enough to shift the peak into the observable region, indeed show this behaviour. This scenario is further supported by an analysis of the relation between the luminosity of the main broad emission lines and the continuum luminosity (i.e. the Baldwin effect). We find that 1) the observed slopes of the line to continuum relations match the expectations from the standard disk model, if we assume that the line emission is a good proxy of the ionizing luminosity; 2) the dispersions of the line-continuum luminosity relations are very small (as small as 0.13 dex), suggesting that the physics of the disk-broad line region system is dominated by only one parameter (the black hole mass) with a nearly constant Eddington ratio. Finally, we notice that our hypothesis of constant L_EDD=0.1 provides a black hole mass estimate (based on the observed luminosity) with a smaller error than the virial estimate.

The nonlinear relation between the UV and X-ray luminosity in quasars has been studied for decades. However, as no comprehensive model can yet explain it, its investigation still relies on observational efforts. This work focuses on optically selected quasars detected by eROSITA. We present the properties of the sources collected in the eROSITA early data release (eFEDS) and those resulting from the first six months of the eROSITA all-sky survey (eRASS1). We focus on the subset of quasars bright enough in the optical/UV band to avoid an “Eddington bias” toward X-ray brighter-than-average spectral energy distributions. The final samples include 1248 and 519 sources for eFEDS and eRASS1, up to redshift z ≈ 3 and z ≈ 1.5, respectively. We find that the X-ray–UV luminosity relation does not evolve significantly with redshift, and its slope is in perfect agreement with previous compilations of quasar samples. The intrinsic dispersion of the relation is about 0.2 dex, which is small enough for possible cosmological applications. However, due to the limited redshift range and statistics of the current samples, we cannot obtain significant cosmological constraints yet. We show how this is going to change with the future releases of eROSITA data.

In this paper, we address the question of whether the non-linear relation between the X-ray and UV emission of quasars can be used to derive their distances. In previous works of our group, we demonstrated that such a relation does not show any redshift evolution in its slope. The derived distances are in agreement with the concordance (standard) model of cosmology up to z ∼ 1.5, but they show significant deviations at higher redshifts. Yet, several authors have suggested that this discrepancy is due to inconsistencies between the low- and high-redshift sources within the parent sample, or to a redshift evolution of the relation. Here, we discuss these issues through a quantitative comparison with supernova-derived distances in the common redshift range, complemented by simulations showing that all the claimed inconsistencies would naturally arise from any limitation of the cosmological model adopted for the data analysis; that is, from our ignorance of the “true” cosmology. We argue that the reliability of the method can only be based on a cosmology-independent evaluation of the hypothesis of the non-evolution of the X-ray to UV relation at z > 1.5, subsequent to a careful check of the sample selection and of the flux measurements for possible redshift-dependent systematic effects. Since we cannot conceive of any physical reason for a sudden change in the normalisation of the relation at z > 1.5, and we can exclude any severe systematic effect in the data selection and flux measurements, we conclude that the application of the X-ray to UV relation to cosmology is well motivated. To further strengthen this point, we need to achieve a better understanding of the physical process behind the observed relation and/or an independent observational proof possibly confirming the discrepancy with ΛCDM found with quasars, such as future supernova measurements at z ∼ 2 or higher.