arXiv subjects

Tullia Sbarrato

Publications and source records attributed to Tullia Sbarrato.

At least 19 recordsLinked to original sources

arXiv2026

BLAZ4R and the eROSITA view of z>4 blazars

We present BLAZ4R, the first living catalog of confirmed $z>4$ blazars, with a focus on the contribution of eROSITA, on board of the Spectrum Roentgen Gamma (SRG) spacecraft. Blazars at $z>4$ are rare but powerful probes of how active supermassive black holes evolve in connection to relativistic jets, in the first 2 billion years of cosmic history. At these redshifts, X-ray observations are essential for constraining blazars jet power and orientation, enabling effective trace of their parent population. The all-sky surveys provided by eROSITA ensure X-ray detection for BLAZ4R sources and, in some cases, allow spectral and temporal studies of their jetted emission. BLAZ4R includes 54 confirmed blazars, characterized through their X-ray properties, radio spectra and morphology, and multiwavelength profiles. We confirm that jetted sources are significantly more numerous relative to non-jetted counterparts at high-$z$, and that blazars (and therefore the overall jetted population) do not exhibit significantly different features compared to the total active galactic nuclei population in the early Universe. Fast accretion processes that involve relativistic jets are clearly required to justify the existence of fully formed jetted AGN at $z>4$. However, the diverse multiwavelength properties characterizing BLAZ4R do not yet allow us to identify the specific signatures of these processes. We will continue updating BLAZ4R to search for such signatures and ultimately understand the early formation of jetted AGN.

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arXiv2026

BL Lac host galaxies: how to systematically characterise them in optical-NIR spectroscopy

Host galaxies of Active Galactic Nuclei give crucial information on the interaction between accreting Supermassive Black Holes and their surroundings, and on their common evolution. Their study in the case of aligned jetted AGN - BL Lacertae objects in particular - is complicated by the non-thermal jet component, whose bright and multi-frequency emission easily dominates over the whole electromagnetic spectrum. BL Lac host galaxies have thus been sparsely studied, and their elliptical nature is currently a hypothesis supported by few observations. With the broad aim of a systematic analysis of these sources, and in light of the many optical and NIR spectroscopic facilities that are now available, we implement an easily applicable method to determine whether a BL Lac is hosted in an elliptical or spiral galaxy. Building on the only systematic study currently available on BL Lac hosts, we worked on a sample of realistic BL Lac synthetic spectra. We analysed them and characterized their statistics using QSFit, a publicly available spectroscopy software. If BL Lac host galaxies were both elliptical and spiral, our method would be able to discriminate between them, provided that BL Lac jets are fainter than $L_\gamma\sim10^{46}$erg/s. Just two runs of QSFit for each BL Lac spectrum would return a single parameter, that would allow for a first broad distinction between the two classes. We finally discuss the two galaxy types that introduce some uncertainty in their classification, that might lead to possible classification biases.

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arXiv2025

The IXPE and multifrequency polarimetric view of the extreme blazars 1ES 1101-232 and RGB J0710+591

Multiwavelength polarimetry is a powerful tool to probe magnetic field and flow geometries in the relativistic jets of blazars. In this respect, particularly interesting are the sources whose synchrotron emission covers a broad range of frequencies, from radio to X-rays, such as the BL Lac objects of the HSP type. Previous measurements including radio, optical and X-ray data show a clear trend, with the degree of polarization increasing with frequency. Here we report radio, optical and X-ray observations ($Swift$, $Nustar$ and $IXPE$) of 1ES 1101-232 and RGB J0710+591, two blazars belonging to the puzzling subclass of extreme BL Lacs (EHBL). For 1ES 1101-232 we found a strong frequency-dependency of the degree of polarization (with a ratio $\Pi_X/\Pi_O\simeq 5.2$). For RGB J0710+591, IXPE derived a 1$\sigma$ upper limit $\Pi_X<11.6\%$, comparable with the measured optical degree of polarization (average $\Pi_O\sim 12\%$). We discuss the results in the framework of current interpretations and, in particular, we report an improved version of the stratified shock model that is able to reproduce the observed data of both sources.

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arXiv2025

Supermassive black holes and their surroundings: MeV signatures

The gravitational potential of supermassive black holes is so powerful that it triggers some of the most intense phenomena in the Universe. Accretion onto these objects and relativistic jet emission from their vicinity are observable across a wide range of frequencies and throughout cosmic history. However, despite this wealth of data, many aspects of their underlying mechanisms remain elusive. Investigating this phenomena across all frequencies is crucial, yet some energy windows are still poorly explored. One such window is the MeV energy range: many key signatures related to the emission from the SMBH environment - both in quiescent and active phases - are expected to lie between one and several hundreds MeV. In this work, we explore some of the open questions regarding the behavior and emission processes in the surroundings of SMBHs, and how these questions might be approached. From the elusive nature of Fermi bubbles around our Galactic Centre, to the origin of high-energy neutrinos in the nuclei and jets of Active Galactic Nuclei, to the nature and emission mechanisms of the most powerful blazars, the MeV window stands out as a crucial key to understanding SMBH physics.

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arXiv2025

NVSS J151002+570243: accretion and spin of a z > 4 Fermi detected blazar

Active galactic nuclei formation and evolution is currently an open puzzle. Their enormous mass is not explainable via sub-Eddington accretion and the frequent presence of relativistic jets at high-z, commonly linked with spinning black holes, suggest a less effective accretion process. NVSS J151002+570243 is part of this population, being the most distant blazar consistently detected by Fermi/LAT, hence hosting a powerful jet. We tested the hypothesis of a super-Eddington accretion process for this source by modeling its big blue bump with a set of accretion disk emission models. We first tested a standard geometrically thin, optically thick $α$-disk, obtaining a mass of Log$M/M_\odot=8.65\pm0.19$ consistent with virial-based results and a significantly sub-Eddington accretion rate $λ=0.02\pm0.01$. We then focused on the analytic approximations of two numerical models that take into account the General Relativity effects of a spinning black hole (reasonable due to the presence of a jet) and a close-to- or super-Eddington accretion rate (KERBB and SLIMBH). Despite the focus on super-critical accretion, these models confirm a surprisingly low Eddington ratio, of the order of 3\%. The hypothesis of a continuous accretion at this measured rate is unrealistic, since it would imply a seed black hole mass of $\sim10^6-10^8M_\odot$ at redshift z=20. Hence we explore the possibility of a continuous super-critical accretion starting from a $\sim10^2M_\odot$ seed, that would spin up the black hole and eventually contribute in launching the relativistic jet. The measured low accretion rate would thus happen only once the jet is active. This idea would reconcile the black holes with large masses accreting at somewhat slow rates that are observed at z>4, with the need of an extremely fast evolution, by allowing the formation of stellar-size black hole seeds even as late as at $z\sim8$.

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arXiv2024

Revealing faint compact radio jets at redshifts above 5 with very long baseline interferometry

Over the past two decades, our knowledge of the high-redshift (z > 5) radio quasars has expanded, thanks to dedicated high-resolution very long baseline interferometry (VLBI) observations. Distant quasars provide unique information about the formation and evolution of the first galaxies and supermassive black holes in the Universe. Powerful relativistic jets are likely to have played an essential role in these processes. However, the sample of VLBI-observed radio quasars is still too small to allow meaningful statistical conclusions. We extend the list of the VLBI observed radio quasars to investigate how the source structure and physical parameters are related to radio loudness. We assembled a sample of 10 faint radio quasars located at 5 < z < 6 with their radio-loudness indices spanning between 0.9-76. We observed the selected targets with the European VLBI Network (EVN) at 1.7 GHz. The milliarcsecond-scale resolution of VLBI at this frequency allows us to probe the compact innermost parts of radio-emitting relativistic jets. In addition to the single-band VLBI observations, we collected single-dish and low-resolution radio interferometric data to investigate the spectral properties and variability of our sources. The detection rate of this high-redshift, low-flux-density sample is 90%, with only one target (J0306+1853) remaining undetected. The other 9 sources appear core-dominated and show a single, faint and compact radio core on this angular scale. The derived radio powers are typical of FRII radio galaxies and quasars. By extending our sample with other VLBI-detected z > 5 sources from the literature, we found that the core brightness temperatures and monochromatic radio powers tend to increase with radio loudness.

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arXiv2023

The High Energy X-ray Probe (HEX-P): the most powerful jets through the lens of a superb X-ray eye

A fraction of the active supermassive black holes at the centers of galaxies in our Universe are capable of launching extreme kiloparsec-long relativistic jets. These jets are known multiband (radio to $γ$-ray) and multimessenger (neutrino) emitters, and some of them have been monitored over several decades at all accessible wavelengths. However, many open questions remain unanswered about the processes powering these highly energetic phenomena. These jets intrinsically produce soft-to-hard X-ray emission that extends from $E\sim0.1\,\rm keV$ up to $E>100\,\rm keV$. Simultaneous broadband X-ray coverage, combined with excellent timing and imaging capabilities, is required to uncover the physics of jets. Indeed, truly simultaneous soft-to-hard X-ray coverage, in synergy with current and upcoming high-energy facilities (such as IXPE, COSI, CTAO, etc.) and neutrino detectors (e.g., IceCube), would enable us to disentangle the particle population responsible for the high-energy radiation from these jets. A sensitive hard X-ray survey ($F_{8-24\,\rm keV}<10^{-15}\,\rm erg~cm^{-2}~s^{-1}$) could unveil the bulk of their population in the early Universe. Acceleration and radiative processes responsible for the majority of their X-ray emission would be pinned down by microsecond timing capabilities at both soft and hard X-rays. Furthermore, imaging jet structures for the first time in the hard X-ray regime could unravel the origin of their high-energy emission. The proposed Probe-class mission concept High Energy X-ray Probe (HEX-P) combines all these required capabilities, making it the crucial next-generation X-ray telescope in the multi-messenger, time-domain era. HEX-P will be the ideal mission to unravel the science behind the most powerful accelerators in the universe.

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arXiv2022

Blazar nature of high-z radio-loud quasars

We report on the Swift/XRT observation and classification of eleven blazar candidates at $z>4$. These sources were selected as part of a sample of extremely radio-loud quasars, in order to focus on quasars with jets oriented roughly close to our line-of-sight. Deriving their viewing angles and their jets bulk Lorentz factors was crucial for a strict blazar classification, and it was possible only thanks to X-ray observations. Out of eleven sources, five show strong and hard X-ray fluxes, that allow their blazar classification, two are uncertain, three host relativistic jets that we observe just outside their beaming cone (i.e. are not strictly blazars), while one went undetected by Swift/XRT. Following this approach, we were able to trace the $>10^9M_\odot$ active supermassive black hole population hosted in jetted active galactic nuclei. At $z\geq4$ the massive jetted sources are likely predominant in the overall quasar population: this calls for a deep review of our understanding of the first supermassive black holes formation and evolution. Jets are indeed key actors in fast accretion, and must be searched for across the whole high redshift quasar population. A note of caution must be added: radio-loudness and in general radio features at high redshifts seem not to perfectly reflect high-energy properties. A strong effect due to interaction with CMB radiation is surely in place, that quenches the radio emission with respect to the X-rays, but also more frequent occasions for the jet to be bent seem to play a relevant role in this matter. Classifications and population studies thus must be carefully performed, in order not to be confused by these inconsistencies.

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arXiv2021

A small radio galaxy at z=4.026

Less than $200$ radio-loud active galactic nuclei (AGN) are known above redshift $4$. Around $40$ of them have been observed at milliarcsecond (mas) scale resolution with very long baseline interferometry (VLBI) technique. Some of them are unresolved, compact, relativistically beamed objects, blazars with jets pointing at small angles to the observer's line of sight. But there are also objects with no sign of relativistic beaming possibly having larger jet inclination angles. In a couple of cases, X-ray observations indicate the presence of relativistic beaming in contrary to the VLBI measurements made with the European VLBI Network (EVN). J1420$+$1205 is a prominent example, where our $30-100$ mas-scale enhanced Multi Element Remotely Linked Interferometer Network (e-MERLIN) radio observations revealed a rich structure reminiscent of a small radio galaxy. It shows a bright hotspot which might be related to the denser interstellar medium around a young galaxy at an early cosmological epoch.

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arXiv2021

Jetted radio-quiet quasars at z>5

We report on the JVLA observations of three high redshift Active Galactic Nuclei (AGN), having a black hole mass estimated to be among the largest known. Two of them, SDSS J0100+2802 and SDSS J0306+1853 at redshift 6.326 and 5.363 respectively, are radio-quiet AGN according to the classic definition, while the third (B2 1023+25 at z=5.284) is a powerful blazar. The JVLA data clearly show a radio structure in the first source, and a radio emission with a relatively steep radio spectrum in the second one, demonstrating the presence of a radio jet and a diffuse component. Therefore, being radio-quiet does not exclude the presence of a powerful relativistic jet with important consequences on the population studies and on the ratio between jetted and non-jetted AGN. We can estimate the viewing angle of these jets, and this allows us to find, albeit with some uncertainty, the density of black holes with a mass in excess of $10^{10}M_\odot$ at high redshifts. We found that their density in jetted AGN is very large in the redshift bin 5-6, comparable with the overall AGN population of the same optical luminosity. Jets might thus play a crucial role in the fast formation and evolution of the most massive black holes in the early Universe. They are more common than what expected from wide radio surveys with mJy flux sensitivity. Deeper JVLA or VLBI observations are key to discover a possible relativistic jet population hiding in plain sight at very high-redshift. The discovery of powerful relativistic jets associated with the most massive black holes in the early Universe re-opens the question: is the jet instrumental for a rapid growth of the black hole or instead is the black hole mass the main driver for the jet formation?

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arXiv2021

The disk-torus system in active galactic nuclei: possible evidence of highly spinning black holes

We study the ratio $R$ between the luminosity of the torus and that of the accretion disk, inferred from the relativistic model KERRBB for a sample of approximately 2000 luminosity-selected radio-quiet Type I active galactic nuclei from the Sloan Digital Sky Survey catalog. We find a mean ratio $R \approx 0.8$ and a considerable number of sources with $R \gtrsim 1$. Our statistical analysis regarding the distribution of the observed ratios suggests that the largest values might be linked to strong relativistic effects due to a large black hole spin ($a > 0.8$), despite the radio-quiet nature of the sources. The mean value of $R$ sets a constraint on the average torus aperture angle (in the range $30^{\circ} < θ_{\rm T} < 70^{\circ}$) and, for about one-third of the sources, the spin must be $a > 0.7$. Moreover, our results suggest that the strength of the disk radiation (i.e., the Eddington ratio) could shape the torus geometry and the relative luminosity ratio $R$. Given the importance of the involved uncertainties on this statistical investigation, an extensive analysis and discussion have been made to assess the robustness of our results.

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arXiv2021

Big and young supermassive black holes in the early Universe

Blazars are Active Galactic Nuclei characterized by relativistic jets launched in the vicinity of the central engine (i.e. a supermassive black hole), that are oriented close to our line of sight. Their peculiar orientation makes them very efficient tracers of the overall jetted population, and due to their brightness they can be visible up to very high redshifts. A deep knowledge of these objects can provide fundamental clues to the models of formation and growth of the first supermassive black holes, but their search in the early Universe must be careful and follow a systematic approach. The discovery in the last $\sim15$ years of extremely massive blazars at very high redshifts ($M_{\rm BH}>10^9M_\odot, \, z>4$) revolutionized our perception of their earliest evolution: there seem to be different formation epochs for extremely massive black holes hosted in jetted ($z\sim4$) and non-jetted systems ($z\sim2.5$). This is not easy to explain, since one would expect that jetted sources accrete less efficiently. Small differences in the population are also derived from the search of such high-$z$ sources: we will go through the open questions in order to understand where the common knowledge stands and which steps must be undertaken to better understand the formation and common evolution of supermassive black holes and jets in the early Universe.

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arXiv2021

Minute-timescale variability in the X-ray emission of the highest redshift blazar

We report on two Chandra observations of the quasar PSO J0309+27, the most distant blazar observed so far (z=6.1), performed eight months apart, in March and November 2020. Previous Swift-XRT observation showed that this object is one of the brightest X-ray sources beyond redshift 6.0 ever observed so far. This new data-set confirmed the high flux level and unveiled a spectral change occurred on a very short timescale (250s rest-frame), caused by a significant softening of the emission spectrum. This kind of spectral variability, on a such short interval, has never been reported in the X-ray emission of a flat spectrum radio quasar. A possible explanation is given by the emission produced by the inverse Compton scatter of the quasar UV photons by the cold electrons present in a fast shell moving along the jet. Although this bulk comptonization emission should be an unavoidable consequence of the standard leptonic jet model, this would be the first time that it is observed.

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arXiv2021

A new panchromatic classification of unclassified Burst Alert Telescope active galactic nuclei

We collect data at all frequencies for the new sources classified as unknown active galactic nuclei (AGNs) in the latest Burst Alert Telescope (BAT) all-sky hard X-ray catalog. Focusing on the 36 sources with measured redshift, we compute their spectral energy distribution (SED) from radio to $γ$-rays with the aim to classify these objects. We apply emission models that attempt to reproduce the obtained SEDs, including: i) a standard thin accretion disk together with an obscuring torus and a X-ray corona; ii) a two temperature thick advection-dominated flow; iii) an obscured AGN model, accounting for absorption along the line of sight at kiloelectronvolt energies and in the optical band; and iv) a phenomenological model to describe the jet emission in blazar-like objects. We integrate the models with the SWIRE template libraries to account for the emission of the host galaxy. For every source we found a good agreement between data and our model. Considering that the sources were selected in the hard X-ray band, which is rather unaffected by absorption, we expected and found a large fraction of absorbed radio-quiet AGNs (31 out of 36) and some additional rare radio-loud sources (5 out of 36), since the jet emission in hard X-rays is important for aligned jets owing to the boost produced by the beaming effect. With our work we can confirm the hypothesis that a number of galaxies, whose optical spectra lack AGN emission features, host an obscured active nucleus. The approach we used proved to be efficient in rapidly identifying objects, which commonly used methods were not able to classify.

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arXiv2020

VLBI20-30: a scientific roadmap for the next decade -- The future of the European VLBI Network

This white paper describes the science case for Very Long Baseline Interferometry (VLBI) and provides suggestions towards upgrade paths for the European VLBI Network (EVN). The EVN is a distributed long-baseline radio interferometric array, that operates at the very forefront of astronomical research. Recent results, together with the new science possibilities outlined in this vision document, demonstrate the EVN's potential to generate new and exciting results that will transform our view of the cosmos. Together with e-MERLIN, the EVN provides a range of baseline lengths that permit unique studies of faint radio sources to be made over a wide range of spatial scales. The science cases are reviewed in six chapters that cover the following broad areas: cosmology, galaxy formation and evolution, innermost regions of active galactic nuclei, explosive phenomena and transients, stars and stellar masers in the Milky Way, celestial reference frames and space applications. The document concludes with identifying the synergies with other radio, as well as multi-band/multi-messenger instruments, and provide the recommendations for future improvements. The appendices briefly describe other radio VLBI arrays, the technological framework for EVN developments, and a selection of spectral lines of astrophysical interest below 100 GHz. The document includes a glossary for non-specialists, and a list of acronyms at the end.

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arXiv2020

Estimating black hole masses: Accretion disk fitting versus reverberation mapping and single epoch

We selected a sample of 28 Type 1 AGNs for which a black hole mass has been inferred using the reverberation mapping technique and single epoch scaling relations. All 28 sources show clear evidence of the "Big Blue Bump" in the optical-UV band whose emission is produced by an accretion disk (AD) around a supermassive black hole. We fitted the spectrum of these sources with the relativistic thin AD model KERRBB in order to infer the black hole masses and compared them with those from Reverberation mapping and Single epoch methods, discussing the possible uncertainties linked to such a model by quantifying their weight on our results. We find that for the majority of the sources, KERRBB is a good description of the AD emission for a wide wavelength range. The overall uncertainty on the black hole mass estimated through the disk fitting procedure is $\sim 0.45$ dex (which includes the uncertainty on fitting parameters such as e.g. spin and viewing angle), comparable to the systematic uncertainty of reverberation mapping and single epoch methods; however, such an uncertainty can be $\leq 0.3$ dex if one of the parameters of the fit is well constrained. Although all of the estimates are affected by large uncertainties, the masses inferred using the three methods are compatible if the dimensionless scale factor $f$ (linked to the unknown kinematics and geometry of the Broad Line Region) is assumed to be larger than one. For the majority of the sources, the comparison between the results coming from the three methods favors small spin values. To check the goodness of the KERRBB results, we compared them with those inferred with other models, such as AGNSED, a model that also accounts for the emission originating from an X-ray corona: using two sources with a good data coverage in the X band, we find that the masses estimated with the two models differ at most by a factor of $\sim 0.2$ dex.

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arXiv2020

Searching for Super-Eddington Quasars using a Photon Trapping Accretion Disc Model

Accretion onto black holes at rates above the Eddington limit has long been discussed in the context of supermassive black hole (SMBH) formation and evolution, providing a possible explanation for the presence of massive quasars at high redshifts (z$\gtrsim$7), as well as having implications for SMBH growth at later epochs. However, it is currently unclear whether such `super-Eddington' accretion occurs in SMBHs at all, how common it is, or whether every SMBH may experience it. In this work, we investigate the observational consequences of a simplistic model for super-Eddington accretion flows -- an optically thick, geometrically thin accretion disc (AD) where the inner-most parts experience severe photon-trapping, which is enhanced with increased accretion rate. The resulting spectral energy distributions (SEDs) show a dramatic lack of rest-frame UV, or even optical, photons. Using a grid of model SEDs spanning a wide range in parameter space (including SMBH mass and accretion rate), we find that large optical quasar surveys (such as SDSS) may be missing most of these luminous systems. We then propose a set of colour selection criteria across optical and infra-red colour spaces designed to select super-Eddington SEDs in both wide-field surveys (e.g., using SDSS, 2MASS and WISE) and deep & narrow-field surveys (e.g., COSMOS). The proposed selection criteria are a necessary first step in establishing the relevance of advection-affected super-Eddington accretion onto SMBHs at early cosmic epochs.

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arXiv2020

Difficulties in Mid-Infrared selection of AGN in dwarf galaxies

While massive black holes (MBHs) are known to inhabit all massive galaxies, their ubiquitous presence in dwarf galaxies has not been confirmed yet, with only a limited number of sources detected so far. Recently, some studies proposed infrared emission as an alternative way to identify MBHs in dwarfs, based on a similar approach usually applied to quasars. In this study, by accurately combining optical and infrared data taking into account resolution effects and source overlapping, we investigate in detail the possible limitations of this approach with current ground-based facilities, finding a quite low ($\sim$0.4 per cent) fraction of active MBH in dwarfs that are luminous in mid-infrared, consistent with several previous results. Our results suggest that the infrared selection is strongly affected by several limitations that make the identification of MBHs in dwarf galaxies currently prohibitive, especially because of the very poor resolution compared to optical surveys, and the likely contamination by nearby sources, although we find a few good candidates worth further follow-ups. Optical, X-ray and radio observations, therefore, still represent the most secure way to search for MBH in dwarfs.

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