arXiv subjects

Angel Ruiz

Publications and source records attributed to Angel Ruiz.

13 recordsLinked to original sources

arXiv2026

X-ray luminosity function of Compton-thick AGN in the early Universe (z > 3). Robustness and biases of the CTK population

The population of Compton-thick (CTK) AGN represents a critical yet elusive phase in the growth of supermassive black holes. Constraining their abundance and evolution at high z is essential for understanding both SMBH growth and the origin of the cosmic X-ray background. We investigate the X-ray luminosity function (XLF) of CTK AGN at z > 3 using one of the largest available samples of X-ray-selected AGN at high z, containing 811 sources from XMM-Newton XXL-N and Chandra CCLS and CDF-S/N surveys. We first selected a subsample of ten high-probability CTK candidates, identified through x-ray spectral fitting. Their multiwavelength properties are examined through SED modelling to assess the robustness of their CTK classification. For most sources, the inferred X-ray luminosities appear overestimated when compared with their IR luminosities. After updating the NH posteriors with IR-informed priors, only three sources remain consistent with the CTK regime. To compute the XLF for the entire CTK AGN population, we used 24 microns photometry to estimate IR luminosities and update the X-ray posteriors for all the remaining sources. Incorporating IR priors systematically reduces the inferred CTK number densities, yielding a more conservative and physically consistent estimate of the XLF. We find that CTK AGN constitute 17 per cent of the total AGN population at 3 < z < 6, consistent with results at lower z. Our analysis reveals no statistically significant evolution in the CTK fraction up to z about 6, suggesting that the most heavily obscured accretion phase remains a persistent component of black hole growth throughout cosmic history. While the overall obscured AGN fraction (NH > 1e23 cm-2) increases toward higher redshifts, the stable CTK fraction supports the interpretation that, at these epochs, the interstellar medium in typical host galaxies cannot produce CTK levels of obscuration.

astro-ph.GA↗

arXiv2025

Revisiting the X-ray-to-UV relation of Quasars in the era of all-sky surveys

The X-ray--to--UV relation of active galactic nuclei (AGNs), commonly parametrized via the monochromatic luminosities at $2500\,\mathring{A}$ and $2\,keV$, reflects the energetic interplay between the accretion disc and the X-ray-emitting corona, and is key for understanding accretion physics. Previous studies suggest that disc-dominated emission becomes more prominent with increasing optical luminosity. However, the redshift evolution of this relation remains debated, and a dependence on Eddington ratio, predicted by accretion flow models, is still observationally unconstrained. We revisit this relation using a large, nearly all-sky sample by combining the SDSS DR16Q QSO catalogue with X-ray data from XMM-Newton and the SRG/eROSITA All-Sky Survey DR1, yielding 136,745 QSOs at redshifts $0.5 \leq z < 3.0$. We introduce a hierarchical Bayesian framework that treats X-ray detections and upper limits uniformly, enabling robust inference from both parametric and non-parametric models. We confirm a tight, sublinear $\log L_X({\rm 2\,keV})$-$\log L_{\nu}({\rm 2500\,\mathring{A}})$ correlation, but with a normalization at the lower end of previous estimates. Contrary to most literature results, we detect a mild but systematic redshift evolution: the relation flattens and its intrinsic scatter decreases at higher redshift. This trend is consistent with disc emission increasingly dominated by scattering and enhanced energy transfer to the X-ray corona, potentially indicating redshift evolution in the X-ray bolometric correction. We find no significant dependence on Eddington ratio, in tension with recent accretion flow models.

astro-ph.GA↗

arXiv2025

Characterising the X-ray variability of QSOs to the highest Eddington ratios and black hole masses with eROSITA light curves

An important diagnostic of the inner structure of accretion flows onto supermassive black holes are the stochastic flux variations at X-ray wavelengths. Despite its significance, a systematic characterisation of the statistical properties of the X-ray variability to the highest Eddington ratios and most massive black holes is still lacking. In this paper we address this issue using SRG/eROSITA 5-epoch light curves to characterise the mean X-ray variability of optically selected SDSS QSOs extending to black holes masses of $10^{10}$ solar and accretion rates close to the Eddington limit. The adopted variability statistic is the ensemble normalised excess variance, which is measured using a novel hierarchical Bayesian model (eBExVar) tailored to the Poisson nature of the X-ray light curves. We find a clear anti-correlation of the ensemble variability with black hole mass, extending previous results to time scales of months. This can be interpreted as evidence for an X-ray corona size and/or physical conditions that scale with black holes mass. We also find an unexpected increase of the ensemble normalised excess variance close to the Eddington limit, which is contrary to the predictions of empirical variability models. This result suggests an additional variability component for fast growing black holes that may be related to systematic variations of the hot corona size with Eddington ratio or shielding of the hot corona by an inner puffed-up disk and/or outflows.

astro-ph.HE↗

arXiv2024

Observational Properties of AGN Obscuration During the Peak of Accretion Growth

We investigated the gas obscuration and host galaxy properties of active galactic nuclei (AGN) during the peak of cosmic accretion growth of supermassive black holes (SMBHs) at redshift 0.8-1.8 using X-ray detected AGN with mid-infrared and far-infrared detection. The sample was classified as type-1 and type-2 AGN using optical spectral and morphological classification while the host galaxy properties were estimated with multiwavelength SED fitting. For type-1 AGN, the black hole mass was determined from MgII emission lines while the black hole mass of type-2 AGN was inferred from the host galaxy's stellar mass. Based on the derived parameters, the distribution of the sample in the absorption hydrogen column density ($N_{\rm H}$) vs. Eddington ratio diagram is examined. Among the type-2 AGN, $28\pm5$\% are in the forbidden zone, where the obscuration by dust torus cannot be maintained due to radiation pressure on dusty material. The fraction is higher than that observed in the local universe from the BAT AGN Spectroscopic Survey (BASS) data release 2 ($11\pm3$\%). The higher fraction implies that the obscuration of the majority of AGN is consistent with the radiation pressure regulated unified model but with an increased incidence of interstellar matter (ISM) obscured AGN. We discuss the possibility of dust-free absorption in type-1 AGN and heavy ISM absorption in type-2 AGN. We also find no statistical difference in the star-formation activity between type-1 and type-2 AGN which may suggest that obscuration triggered by a gas-rich merging is not common among X-ray detected AGN in this epoch.

astro-ph.GA↗

arXiv2023

The intrinsic X-ray luminosity distribution of an optically-selected SDSS quasar population

In active galactic nuclei, the relationship between UV and X-ray luminosity is well studied (often characterised by $\alpha_\text{ox}$) but often with heterogeneous samples. We have parametrized the intrinsic distribution of X-ray luminosity, $L_\text{X}$, for the optically-selected sample of SDSS quasars in the Stripe 82 and XXL fields across redshifts 0.5-3.5. We make use of the available XMM observations and a custom pipeline to produce Bayesian sensitivity curves that are used to derive the intrinsic X-ray distribution in a hierarchical Bayesian framework. We find that the X-ray luminosity distribution is well described by a Gaussian function in ${\log_{10}}L_\text{X}$ space with a mean that is dependent on the monochromatic 2500A UV luminosity, $L_{2500}$. We also observe some redshift dependence of the distribution. The mean of the $L_\text{X}$ distribution increases with redshift while the width decreases. This weak but significant redshift dependence leads to $L_{2500}$-$L_\text{X}$ and $L_{2500}$-$\alpha_\text{ox}$ relations that evolve with redshift, and we produce a redshift- and $L_{2500}$-dependent $\alpha_\text{ox}$ equation. Neither black hole mass nor Eddington ratio appear to be potential drivers of the redshift evolution.

astro-ph.GA↗

arXiv2023

XMM2ATHENA, the H2020 project to improve XMM-Newton analysis software and prepare for Athena

XMM-Newton, a European Space Agency observatory, has been observing the X-ray, ultra-violet and optical sky for 23 years. During this time, astronomy has evolved from mainly studying single sources to populations and from a single wavelength, to multi-wavelength or messenger data. We are also moving into an era of time domain astronomy. New software and methods are required to accompany evolving astronomy and prepare for the next generation X-ray observatory, Athena. Here we present XMM2ATHENA, a programme funded by the European Union's Horizon 2020 research and innovation programme. XMM2ATHENA builds on foundations laid by the XMM-Newton Survey Science Centre (XMM-SSC), including key members of this consortium and the Athena Science ground segment, along with members of the X-ray community. The project is developing and testing new methods and software to allow the community to follow the X-ray transient sky in quasi-real time, identify multi-wavelength or messenger counterparts of XMM-Newton sources and determine their nature using machine learning. We detail here the first milestone delivery of the project, a new online, sensitivity estimator. We also outline other products, including the forthcoming innovative stacking procedure and detection algorithms to detect the faintest sources. These tools will then be adapted for Athena and the newly detected or identified sources will enhance preparation for observing the Athena X-ray sky.

astro-ph.IM↗

arXiv2022

The demographics of obscured AGN from X-ray spectroscopy guided by multiwavelength information

A complete census of Active Galactic Nuclei (AGN) is a prerequisite for understanding the growth of supermassive black holes across cosmic time. A significant challenge toward this goal is the whereabouts of heavily obscured AGN that remain uncertain. This paper sets new constraints on the demographics of this population by developing a methodology that combines X-ray spectral information with priors derived from multiwavelength observations. We select X-ray AGN in the Chandra COSMOS Legacy survey and fit their $2.2-500\mu m$ spectral energy distributions with galaxy and AGN templates to determine the mid-infrared ($6\mu m$) luminosity of the AGN component. Empirical correlations between X-ray and $6\mu m$ luminosities are then adopted to infer the intrinsic accretion luminosity at X-rays for individual AGN. This is used as prior information in our Bayesian X-ray spectral analysis to estimate physical properties, such as line-of-sight obscuration. Our approach breaks the degeneracies between accretion luminosity and obscuration that affect X-ray spectral analysis, particularly for the most heavily obscured (Compton-Thick) AGN with low photon counts X-ray spectra. The X-ray spectral results are then combined with the selection function of the Chandra COSMOS Legacy survey to derive the AGN space density and a Compton-Thick fraction of $21.0^{+16.1}_{-9.9}\%$ at redshifts $z<0.5$. At higher redshift, our analysis suggests upper limits to the Compton-Thick AGN fraction of $\le 40\%$. These estimates are at the low end of the range of values determined in the literature and underline the importance of multiwavelength approaches for tackling the challenge of heavily obscured AGN demographics.

astro-ph.GA↗

arXiv2020

Forward modeling the multiwavelength properties of Active Galactic Nuclei: application to X-ray and WISE mid-infrared samples

An empirical forward-modeling framework is developed to interpret the multiwavelength properties of Active Galactic Nuclei (AGN) and provide insights into the overlap and incompleteness of samples selected at different parts of the electromagnetic spectrum. The core of the model are observationally derived probabilites on the occupation of galaxies by X-ray selected AGN. These are used to seed mock galaxies drawn from stellar-mass functions with accretion events and then associate them with spectral energy distributions that describe both the stellar and AGN emission components. This approach is used to study the complementarity between X-ray and WISE mid-infrared AGN selection methods. We first show that the basic observational properties of the X-ray and WISE AGN (magnitude, redshift distributions) are adequately reproduced by the model. We then infer the level of contamination of the WISE selection and show that this is dominated by non-AGN at redshifts z < 0.5. These are star-forming galaxies that scatter into the WISE AGN selection wedge because of photometric uncertainties affecting their colours. Our baseline model shows a sharp drop in the number density of heavily obscured AGN above the Compton thick limit in the WISE bands. The model also overpredicts by a factor of 1.5 the fraction of X-ray associations in the WISE AGN selection box compared to observations. This suggests a population of X-ray faint sources that is not reproduced by the model. This discrepancy is discussed in the context of either heavily obscured or intrinsically X-ray weak AGN. Evidence is found in favour of the latter.

astro-ph.HE↗

arXiv2015

Status of the Silicon Photomultiplier Telescope FAMOUS for the Fluorescence Detection of UHECRs

An established technique for the measurement of ultra-high-energy-cosmic-rays is the detection of the fluorescence light induced in the atmosphere of the Earth, by means of telescopes equipped with photomultiplier tubes. Silicon photomultipliers (SiPMs) promise an increase in the photon detection efficiency which outperforms conventional photomultiplier tubes. In combination with their compact package, a moderate bias voltage of several ten volt and single photon resolution, the use of SiPMs can improve the energy and spatial resolution of air fluorescence measurements, and lead to a gain in information on the primary particle. Though, drawbacks like a high dark-noise-rate and a strong temperature dependency have to be managed. FAMOUS is a refracting telescope prototype instrumented with 64 SiPMs of which the main optical element is a Fresnel lens of 549.7 mm diameter and 502.1 mm focal length. The sensitive area of the SiPMs is increased by a special light collection system consisting of Winston cones. The total field of view of the telescope is approximately 12 $^\circ$. The frontend electronics automatically compensates for the temperature dependency of the SiPMs and will provide trigger information for the readout. Already for this prototype, the Geant4 detector simulation indicates full detection efficiency of extensive air showers of $E=10^{18}\,\text{eV}$ up to a distance of 6 km. We present the first working version of FAMOUS with a focal plane prototype providing seven active pixels.

astro-ph.IM↗

arXiv2013

A tale of two feedbacks: star-formation in the host galaxies of radio-AGN

Several lines of argument support the existence of a link between activity at the nuclei of galaxies, in the form of an accreting supermassive black hole, and star-formation activity in these galaxies. The exact nature of this link is still under debate. Radio jets have long been argued to be an ideal mechanism that allows AGN to interact with their host galaxy and regulate star-formation. In this context, we are using a sample of radio sources in the North Ecliptic Pole (NEP) field to study the nature of the putative link between AGN activity and star-formation. This is done by means of spectral energy distribution (SED) fitting. We use the excellent spectral coverage of the AKARI infrared space telescope together with the rich ancillary data available in the NEP to build SEDs extending from UV to far-IR wavelengths. Through SED fitting we constrain both the AGN and host galaxy components. We find a significant AGN component in our sample of relatively faint radio-sources ($<$mJy), that increases in power with increasing radio-luminosity. At the highest radio-luminosities, the presence of powerful jets dominates the radio emission of these sources. A positive correlation is found between the luminosity of the AGN component and that of star-formation in the host galaxy, independent of the radio luminosity. By contrast, for a given redshift and AGN luminosity, we find that increasing radio-luminosity leads to a decrease in the specific star-formation rate. The most radio-loud AGN are found to lie on the main sequence of star-formation for their respective redshifts. For the first time, such a two-sided feedback process is seen in the same sample. We conclude that radio jets do suppress star-formation in their host galaxies but appear not to totally quench it. Our results therefore support the maintenance nature of "radio-mode" feedback from radio-AGN jets.

astro-ph.CO↗

arXiv2013

The Chandra Multi-Wavelength Project: Optical Spectroscopy and the Broadband Spectral Energy Distributions of X-ray Selected AGN

From optical spectroscopy of X-ray sources observed as part of ChaMP, we present redshifts and classifications for a total of 1569 Chandra sources from our targeted spectroscopic follow up using the FLWO, SAAO, WIYN, CTIO, KPNO, Magellan, MMT and Gemini telescopes, and from archival SDSS spectroscopy. We classify the optical counterparts as 50% BLAGN, 16% NELG, 14% ALG, and 20% stars. We detect QSOs out to z~5.5 and galaxies out to z~3. We have compiled extensive photometry from X-ray to radio bands. Together with our spectroscopic information, this enables us to derive detailed SEDs for our extragalactic sources. We fit a variety of templates to determine bolometric luminosities, and to constrain AGN and starburst components where both are present. While ~58% of X-ray Seyferts require a starburst event to fit observed photometry only 26% of the X-ray QSO population appear to have some kind of star formation contribution. This is significantly lower than for the Seyferts, especially if we take into account torus contamination at z>1 where the majority of our X-ray QSOs lie. In addition, we observe a rapid drop of the percentage of starburst contribution as X-ray luminosity increases. This is consistent with the quenching of star formation by powerful QSOs, as predicted by the merger model, or with a time lag between the peak of star formation and QSO activity. We have tested the hypothesis that there should be a strong connection between X-ray obscuration and star-formation but we do not find any association between X-ray column density and star formation rate both in the general population or the star-forming X-ray Seyferts. Our large compilation also allows us to report here the identification of 81 XBONG, 78 z>3 X-ray sources and 8 Type-2 QSO candidates. Also we have identified the highest redshift (z=5.4135) X-ray selected QSO with optical spectroscopy.

astro-ph.CO↗

arXiv2012

Simultaneous X-ray and optical observations of true Type 2 Seyfert galaxies

We present the results of a campaign of simultaneous X-ray and optical observations of `true' Type 2 Seyfert galaxies candidates, i.e. AGN without a Broad Line Region (BLR). Out of the initial sample composed by 8 sources, one object, IC1631, was found to be a misclassified starburst galaxy, another, Q2130-431, does show broad optical lines, while other two, IRAS01428-0404 and NGC4698, are very likely absorbed by Compton-thick gas along the line of sight. Therefore, these four sources are not unabsorbed Seyfert 2s as previously suggested in the literature. On the other hand, we confirm that NGC3147, NGC3660, and Q2131-427 belong to the class of true Type 2 Seyfert galaxies, since they do not show any evidence for a broad component of the optical lines nor for obscuration in their X-ray spectra. These three sources have low accretion rates ($\dot{m}= L_{bol}/L_{Edd} \la0.01$), in agreement with theoretical models which predict that the BLR disappears below a critical value of $L_{bol}/L_{Edd}$. The last source, Mrk273x, would represent an exception even of this accretion-dependent versions of the Unification Models, due to its high X-ray luminosity and accretion rate, and no evidence for obscuration. However, its optical classification as a Seyfert 2 is only based on the absence of a broad component of the Hβ, due to the lack of optical spectra encompassing the Hα band.

astro-ph.GA↗

arXiv2011

A Multiwavelength Study of Binary Quasars and Their Environments

We present Chandra X-ray imaging and spectroscopy for 14 quasars in spatially resolved pairs, part of a complete sample of binary quasars with small transverse separations drawn from Sloan Digital Sky Survey (DR6) photometry. We find no significant difference in X-ray properties when compared with large control samples of isolated quasars. We present infrared photometry from our observations with SWIRC at the MMT, and from the WISE Preliminary Data Release, and fit simple spectral energy distributions to all 14 QSOs. We find preliminary evidence that substantial contributions from star formation are required, but possibly no more so than for isolated X-ray-detected QSOs. Sensitive searches of the X-ray images for extended emission, and the optical images for optical galaxy excess show that these binary QSOs are not preferentially found in rich cluster environments. While larger binary QSO samples with richer far-IR and sub-millimeter multiwavelength data might better reveal signatures of merging and triggering, optical color-selection of QSO pairs may be biased against such signatures. X-ray and/or variability selection of QSO pairs, while challenging, should be attempted. We present in our Appendix a primer on X-ray flux and luminosity calculations.

astro-ph.CO↗