arXiv subjects

Gabriele Ghisellini

Publications and source records attributed to Gabriele Ghisellini.

At least 19 recordsLinked to original sources

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

A bright megaelectronvolt emission line in $γ$-ray burst GRB 221009A

The highly variable and energetic pulsed emission of a long gamma-ray burst (GRB) is thought to originate from local, rapid dissipation of kinetic or magnetic energy within an ultra-relativistic jet launched by a newborn compact object, formed during the collapse of a massive star. The spectra of GRB pulses are best modelled by power-law segments, indicating the dominance of non-thermal radiation processes. Spectral lines in the X-ray and soft $γ$-ray regime for the afterglow have been searched for intensively, but never confirmed. No line features ever been identified in the high energy prompt emission. Here we report the discovery of a highly significant ($> 6 σ$) narrow emission feature at around $10$ MeV in the brightest ever GRB 221009A. By modelling its profile with a Gaussian, we find a roughly constant width $σ\sim 1$ MeV and temporal evolution both in energy ($\sim 12$ MeV to $\sim 6$ MeV) and luminosity ($\sim 10^{50}$ erg/s to $\sim 2 \times 10^{49}$ erg/s) over 80 seconds. We interpret this feature as a blue-shifted annihilation line of relatively cold ($k_\mathrm{B}T\ll m_\mathrm{e}c^2$) electron-positron pairs, which could have formed within the jet region where the brightest pulses of the GRB were produced. A detailed understanding of the conditions that can give rise to such a feature could shed light on the so far poorly understood GRB jet properties and energy dissipation mechanism.

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arXiv2023

The ultraviolet habitable zone of exoplanets

The dozens of rocky exoplanets discovered in the Circumstellar Habitable Zone (CHZ) currently represent the most suitable places to host life as we know it outside the Solar System. However, the presumed presence of liquid water on the CHZ planets does not guarantee suitable environments for the emergence of life. According to experimental studies, the building blocks of life are most likely produced photochemically in presence of a minimum ultraviolet (UV) flux. On the other hand, high UV flux can be life-threatening, leading to atmospheric erosion and damaging biomolecules essential to life. These arguments raise questions about the actual habitability of CHZ planets around stars other than Solar-type ones, with different UV to bolometric luminosity ratios. By combining the "principle of mediocricy" and recent experimental studies, we define UV boundary conditions (UV-habitable Zone, UHZ) within which life can possibly emerge and evolve. We investigate whether exoplanets discovered in CHZs do indeed experience such conditions. By analysing Swift-UV/Optical Telescope data, we measure the near ultraviolet (NUV) luminosities of 17 stars harbouring 23 planets in their CHZ. We derive an empirical relation between NUV luminosity and stellar effective temperature. We find that eighteen of the CHZ exoplanets actually orbit outside the UHZ, i.e., the NUV luminosity of their M-dwarf hosts is decisively too low to trigger abiogenesis - through cyanosulfidic chemistry - on them. Only stars with effective temperature >3900 K illuminate their CHZ planets with enough NUV radiation to trigger abiogenesis. Alternatively, colder stars would require a high-energy flaring activity.

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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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arXiv2022

The Blazar sequence and its Physical Understanding

Introduced in 1998 to attempt a first unified view of the broad-band emission properties of blazars, the blazar sequence has been extensively used in the past 25 years to guide observations as well as physical interpretation of the overall emission from these galaxies. In this review, we describe the evolution of the sequence along with the tremendous advances in the observational field, in particular in the gamma-ray band. A new version of the sequence built on TeV-detected objects is also presented. Two extreme classes of objects (MeV and hard-TeV blazars) are included in the discussion, given their relevance for future observatories. Finally, the current physical understanding at the base of the sequence is presented along with the major criticisms to the blazar sequence.

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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

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

Spectral index-flux relation for investigating the origins of steep decay in $γ$-ray bursts

$γ$-ray bursts (GRBs) are short-lived transients releasing a large amount of energy ($10^{51}-10^{53} $ erg) in the keV-MeV energy range. GRBs are thought to originate from internal dissipation of the energy carried by ultra-relativistic jets launched by the remnant of a massive star's death or a compact binary coalescence. While thousands of GRBs have been observed over the last thirty years, we still have an incomplete understanding of where and how the radiation is generated in the jet. Here we show a relation between the spectral index and the flux found by investigating the X-ray tails of bright GRB pulses via time-resolved spectral analysis. This relation is incompatible with the long standing scenario which invokes the delayed arrival of photons from high-latitude parts of the jet. While the alternative scenarios cannot be firmly excluded, the adiabatic cooling of the emitting particles is the most plausible explanation for the discovered relation, suggesting a proton-synchrotron origin of the GRB emission.

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arXiv2021

Proof of CMB-driven X-ray brightening of high-z radio galaxies

We present a definitive assessment of the role of Inverse Compton scattering of Cosmic Microwave Background photons (IC/CMB) in the context of radio galaxies. Owing to the steep increase of the CMB radiation energy density, IC/CMB is supposed to become progressively more important with respect to radio synchrotron cooling as the redshift increases. For typical energies at play, this process will up-scatter the CMB photons into the X-ray band, and is thus expected to yield a redshift-dependent, concurrent X-ray brightening and radio dimming of the jet-powered structures. Here we show how a conclusive proof of this effect hinges on high-resolution imaging data in which the extended lobes can be distinguished from the compact hot spots where synchrotron-self-Compton dominates the X-ray emission regardless of redshift. We analyze Chandra and Very Large Array data of 11 radio galaxies between 1.3 3 radio galaxies below <1 mJy, IC/CMB alone can not be responsible for a deficit in high-z, radio-loud AGN if--as we argue--such AGN typically have bright, compact hot spots.

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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

Extended X-ray emission from the z=4.26 radio galaxy 4C 63.20

We report on deep Chandra X-ray Telescope imaging observations of 4C 63.20, one of the few known radio galaxies at z>3.5. The X-ray counterpart is resolved into a core plus two off-nuclear sources that (combined) account for close to 30% of the total X-ray flux. Their morphology and orientation are consistent with a diffuse, lobe-like nature, albeit compact hotspots cannot be ruled out. The broadband spectral energy distribution of 4C 63.20 can be reproduced with a jet model where the majority of the radio flux can be ascribed to synchrotron emission from the hotspots, whereas the (non-nuclear) X-ray emission is produced via Inverse Compton (IC) off of Cosmic Microwave Background (CMB) photons within the extended lobes. This scenario is broadly consistent with the expectation from highly magnetized lobes in a hotter CMB, and supports the view that IC/CMB may quench less extreme radio lobes at high redshifts.

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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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arXiv2019

Extra-galactic jets: a hard X-ray view

Extragalactic jets are the most powerful persistent sources of the universe. Those pointing at us are called blazars. Their relativistically boosted emission extends from radio frequencies to TeV energies. They are also suspected to be the sources of energetic neutrinos and high energies cosmic rays. The study of their overall spectrum indicates that most of the emission of powerful blazars is in hard X-rays or in soft gamma-rays. In this band we can find the most powerful jets, visible also at high redshifts. It is found that the jet power is linked to the accretion luminosity, and exceeds it, especially if they produce energetic neutrinos, that require the presence of ultrarelativistic protons.

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arXiv2019

The luminosity--volume test for cosmological Fast Radio Bursts

We perform the luminosity--volume test, also known as $\langle V/V_{MAX}\rangle$, to Fast Radio Bursts (FRBs). We compare the 23 FRBs, recently discovered by ASKAP, with 20 of the FRBs found by Parkes. These samples have different flux limits and correspond to different explored volumes. We assume that their dispersion measure indicates their redshift and apply the appropriate cosmological corrections to the spectrum and rate in order to compute the $\langle V/V_{MAX}\rangle$ for the ASKAP and Parkes samples. For a radio spectrum of FRBs $F_ν\propto ν^{-1.6}$, we find $\langle V/V_{MAX}\rangle=0.66\pm 0.05$ for the ASKAP sample, that includes FRBs up to $z=0.7$, and 0.52$\pm 0.04$ for Parkes, that extends up to $z=2.1$. The ASKAP value suggests that the population of FRB progenitors evolves faster than the star formation rate, while the Parkes value is consistent with it. Even a delayed (as a power law or gaussian) star formation rate cannot reproduce the $\langle V/V_{MAX}\rangle$ of both samples. If FRBs do not evolve in luminosity, the $\langle V/V_{MAX}\rangle$ values of ASKAP and Parkes sample are consistent with a population of progenitors whose density strongly evolves with redshift as $\sim z^{2.8}$ up to $z \sim 0.7$. We discuss possible scenarios accounting for our results.

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arXiv2019

Black hole mass and spin estimates of the most distant quasars

We investigate the properties of the most distant quasars ULASJ134208.10+092838.61 ($z = 7.54$), ULASJ112001.48+064124.3 ($z = 7.08$) and DELSJ003836.10-152723.6 ($z = 7.02$) studying their Optical-UV emission that shows clear evidence of the presence of an accretion disk. We model such emission applying the relativistic disk models KERRBB and SLIMBH for which we have derived some analytical approximations to describe the observed emission as a function of the black hole mass, accretion rate, spin and the viewing angle. We found that: 1] our black hole mass estimates are compatible with the ones found using the virial argument but with a smaller uncertainty; 2] assuming that the virial argument is a reliable method to have a black hole mass measurement (with no systematic uncertainties involved), we found an upper limit for the black hole spin of the three sources: very high spin values are ruled out; 3] our Eddington ratio estimates are smaller than those found in previous studies by a factor $\sim 2$: all sources are found to be sub-Eddington. Using our results, we explore the parameter space (efficiency, accretion rate) to describe the possible evolution of the black hole assuming a $\sim 10^{2-4} M_{\odot}$ seed: if the black hole in these sources formed at redshift $z = 10 - 20$, we found that the accretion has to proceed at the Eddington rate with a radiative efficiency $η\sim 0.1$ in order to reach the observed masses in less than $\sim 0.7$ Gyr.

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arXiv2018

Blazar jets as the most efficient persistent engines

We have not identified for sure what is the mechanism launching, accelerating and collimating relativistic jets. The two most likely possibilities are the gravitational energy of the accreting matter or the rotational energy of a spinning black hole. Even the evaluation of the jet power is not trivial, since the radiation from the jet is enhanced by relativistic beaming, and there are fundamental uncertainties concerning the matter content of the jet (electron--proton or electron--positron plasma). However, in recent years, there have been crucial advances mainly driven by the richness of data in the $γ$-ray band. This is the band where blazars emit most of their electromagnetic power. Furthermore, there are now large sample of $γ$-ray loud blazars covered by optical spectroscopy. For the blazar sub-class of flat spectrum radio quasars (FSRQ) these data provide measurements of the main emission lines and of the underlying continuum. From these data, it is relatively easy to infer the bolometric luminosity of the accretion disk. The relativistic jet emission on one hand, and the disk luminosity on the other hand, allows us to compare the jet power and the accretion luminosity. Although the inferred jet power is subject to a few assumptions and is somewhat model--dependent, it is possible to derive a lower limit to the jet power that is assumption-free and model-independent. Since this lower limit is of the order of the accretion luminosity, we infer that the true jet power is larger.

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