arXiv subjects

Paolo Padovani

Publications and source records attributed to Paolo Padovani.

At least 37 records · Page 2Linked to original sources

arXiv2020

The redshift and the host galaxy of the neutrino candidate 4FGL J0955.1+3551 (3HSP J095507.9+355101)

The BL Lac object 4FGLJ0955.1+3551 has been suggested as a possible source of ultra energetic neutrinos detected by the Icecube observatory. The target was observed in January 2020 at the Large Binocular Telescope. Our spectroscopy (4100-8500 Ang) yields a firm redshift z = 0.557 as deduced by the absorption lines of the host galaxy. The upper limit of the minimum equivalent width on emission lines is 0.3 Ang. From the source image we are able to resolve the host galaxy for which we measure an absolute magnitude M(R) = -22.9 and Re = 8 kpc, that is values which are typical of the host galaxies of BL Lacs.

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arXiv2020

Concluding Remarks at the Multifrequency Behaviour of High Energy Cosmic Sources -- XIII Workshop -- II

I summarise the concluding remarks I gave at the Multifrequency Behaviour of High Energy Cosmic Sources - XIII Workshop. That was not a summary talk and was meant to be provocative. I first give what I think the main message of the workshop was, then provide some (biased) highlights, touch upon the upcoming new facilities and the issues of "quantity vs. quality" and productivity in astronomy, and finally conclude with a look to the future. Astronomers who did not attend the workshop might still find the first two topics appealing and the last two thought-provoking.

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arXiv2019

The 3HSP catalogue of Extreme & High Synchrotron Peaked Blazars

High synchrotron peaked blazars (HSPs or HBLs) play a central role in very high energy (VHE) gamma-ray astronomy, and likely in neutrino astronomy. Currently, the largest compilation of HSP blazars, the 2WHSP sample, includes 1691 sources, but it is not complete neither in the radio nor in the X-ray band. In order to provide a more sizable and more accurate set of HSP blazars that is useful for future statistical studies and to plan for VHE/TeV observations, we present the largest sample of extreme and high synchrotron peaked (EHSP;HSP) blazars and blazar candidates: the 3HSP catalogue. The 3HSP catalogue includes 2013 sources, 88% of which with a redshift estimation, a much larger percentage than in any other list of HSP blazars. All new γ-ray detections are described in the First and Second Brazil ICRANet gamma-ray blazar catalogues (1BIGB & 2BIGB) also taking into account the 4FGL list of γ-ray sources published by the Fermi large area telescope (Fermi-LAT) team. Moreover, the cross-matching between the 2WHSP, 2FHL HSPs, and IceCube neutrinos position has suggested that HSPs are likely counterparts of neutrino events, which implies the 3HSP catalogue is useful also in that respect. The 3HSP catalogue features improved completeness compared to its predecessors, the 1WHSP and 2WHSP catalogues, and follows the track of their increasing relevance for VHE astronomy.

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arXiv2019

High energy neutrino flux from individual blazar flares

Motivated by the recently reported evidence of an association between a high-energy neutrino and a gamma-ray flare from the blazar TXS 0506+056, we calculate the expected high-energy neutrino signal from past, individual flares, from twelve blazars, selected in declinations favourable for detection with IceCube. To keep the number of free parameters to a minimum, we mainly focus on BL Lac objects and assume the synchrotron self-Compton mechanism produces the bulk of the high-energy emission. We consider a broad range of the allowed parameter space for the efficiency of proton acceleration, the proton content of BL Lac jets, and the presence of external photon fields. To model the expected neutrino fluence we use simultaneous multi-wavelength observations. We find that in the absence of external photon fields and with jet proton luminosity normalised to match the observed production rate of ultra-high-energy cosmic rays, individual flaring sources produce a modest neutrino flux in IceCube, $\lesssim10^{-3}$ muon neutrinos with energy exceeding 100 TeV, stacking ten years of flare periods selected in the >800 MeV Fermi energy range from each source. Under optimistic assumptions about the jet proton luminosity and in the presence of external photon fields, we find that the two most powerful sources in our sample, AO 0235+164, and OJ 287, would produce, in total, $\approx 3$ muon neutrinos during ten years of Fermi flaring periods, in future neutrino detectors with total instrumented volume $\sim$ten times larger than IceCube,or otherwise, constrain the proton luminosity of blazar jets.

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arXiv2019

The Origin of Radio Emission from Radio-Quiet AGN

The central nuclei of galaxies, where super-massive black holes (SMBHs) are thought to reside, can experience phases of activity when they become Active Galactic Nuclei (AGN). An AGN can eject winds, jets, and produce radiation across the entire electromagnetic spectrum. The fraction of the bolometric emission in the radio spans a factor of ~10^5 across the different AGN classes. The weakest radio sources, radio-quiet (RQ) AGN, are typically 1,000 times fainter than the radio-loud (RL) AGN, and represent the majority of the AGN population. In RL AGN, radio emission is essentially all produced by synchrotron emission from a relativistic jet. In contrast, in RQ AGN the absence of luminous jets allows us to probe radio emission from a wide range of possible mechanisms, from the host galaxy kpc scale down to the innermost region near the SMBHs: star formation, AGN driven wind, free-free emission from photo-ionized gas, low power jet, and the innermost accretion disc coronal activity. All these mechanisms can now be probed with unprecedented precision and spatial resolution, thanks to the current and forthcoming generation of highly sensitive radio arrays.

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arXiv2017

On the two main classes of Active Galactic Nuclei

Active Galactic Nuclei (AGN) are traditionally divided empirically into two main classes: "radio-loud" and "radio-quiet" sources. These labels, which are more than fifty years old, are obsolete, misleading, and wrong. I argue that AGN should be classified based on a fundamentally physical rather than just an observational difference, namely the presence (or lack) of strong relativistic jets, and that we should use the terms "jetted" and "non-jetted" AGN instead.

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arXiv2017

Connecting blazars with ultra high energy cosmic rays and astrophysical neutrinos

We present a strong hint of a connection between high energy $γ$-ray emitting blazars, very high energy neutrinos, and ultra high energy cosmic rays. We first identify potential hadronic sources by filtering $γ$-ray emitters %from existing catalogs that are in spatial coincidence with the high energy neutrinos detected by IceCube. The neutrino filtered $γ$-ray emitters are then correlated with the ultra high energy cosmic rays from the Pierre Auger Observatory and the Telescope Array by scanning in $γ$-ray flux ($F_γ$) and angular separation ($θ$) between sources and cosmic rays. A maximal excess of 80 cosmic rays (42.5 expected) is found at $θ\leq10^{\circ}$ from the neutrino filtered $γ$-ray emitters selected from the second hard {\it Fermi}-LAT catalogue (2FHL) and for $F_γ\left(>50\:\mathrm{GeV}\right)\geq1.8\times10^{-11}\:\mathrm{ph}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$. The probability for this to happen is $2.4 \times 10^{-5}$, which translates to $\sim 2.4 \times 10^{-3}$ after compensation for all the considered trials. No excess of cosmic rays is instead observed for the complement sample of $γ$-ray emitters (i.e. not in spatial connection with IceCube neutrinos). A likelihood ratio test comparing the connection between the neutrino filtered and the complement source samples with the cosmic rays favours a connection between neutrino filtered emitters and cosmic rays with a probability of $\sim1.8\times10^{-3}$ ($2.9σ)$ after compensation for all the considered trials. The neutrino filtered $γ$-ray sources that make up the cosmic rays excess are blazars of the high synchrotron peak type. More statistics is needed to further investigate these sources as candidate cosmic ray and neutrino emitters.

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arXiv2016

2WHSP: A catalog of HE and VHE gamma-ray blazars and blazar candidates

Aims. High Synchrotron Peaked blazars (HSPs) dominate the -ray sky at energies larger than a few GeV; however, only a few hundred blazars of this type have been catalogued so far. In this paper we present the 2WHSP sample, the largest and most complete list of HSP blazars available to date, which is an expansion of the 1WHSP catalog of gamma-ray source candidates off the Galactic plane. Methods. We cross-matched a number of multi-wavelength surveys (in the radio, infrared and X-ray bands) and applied selection criteria based on the radio to IR and IR to X-ray spectral slopes. To ensure the selection of genuine HSPs we examined the SED of each candidate and estimated the peak frequency of its synchrotron emission ($ν_{\rm peak}$) using the ASDC SED tool, including only sources with Synchrotron peak $ν_{\rm peak} > 10^{15}$ Hz (equivalent to $ν_{\rm peak} > 4$ eV). Results. We have assembled the largest and most complete catalog of HSP blazars to date, which includes 1691 sources. A number of population properties, such as infrared colours, synchrotron peak, redshift distributions, and -ray spectral properties, have been used to characterise the sample and maximize completeness. We also derived the radio logN-logS distribution. This catalog has already been used to provide seeds to discover new very high energy objects within Fermi-LAT data and to look for the counterparts of neutrino and ultra high energy cosmic ray sources, showing its potential for the identification of promising high-energy gamma-ray sources and multi-messenger targets.

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arXiv2016

The faint radio sky: radio astronomy becomes mainstream

Radio astronomy has changed. For years it studied relatively rare sources, which emit mostly non-thermal radiation across the entire electromagnetic spectrum, i.e. radio quasars and radio galaxies. Now it is reaching such faint flux densities that it detects mainly star-forming galaxies and the more common radio-quiet active galactic nuclei. These sources make up the bulk of the extragalactic sky, which has been studied for decades in the infrared, optical, and X-ray bands. I follow the transformation of radio astronomy by reviewing the main components of the radio sky at the bright and faint ends, the issue of their proper classification, their number counts, luminosity functions, and evolution. The overall "big picture" astrophysical implications of these results, and their relevance for a number of hot topics in extragalactic astronomy, are also discussed. The future prospects of the faint radio sky are very bright, as we will soon be flooded with survey data. This review should be useful to all extragalactic astronomers, irrespective of their favourite electromagnetic band(s), and even stellar astronomers might find it somewhat gratifying.

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arXiv2016

The many faces of blazar emission in the context of hadronic models

We present two ways of modeling the spectral energy distribution of blazars in the hadronic context and discuss the predictions of each "hadronic variant" on the spectral shape, the multi-wavelength variability, the cosmic-ray flux, and the high-energy neutrino emission. Focusing on the latter, we then present an application of the hadronic model to individual BL Lacs that were recently suggested to be the counterparts of some of the IceCube neutrinos, and conclude by discussing the contribution of the whole BL Lac class to the observed neutrino background.

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arXiv2015

A simplified view of blazars: contribution to the X-ray and gamma-ray cosmic backgrounds

The "blazar simplified view" is a new paradigm that explains well the diverse statistical properties of blazars observed over the entire electromagnetic spectrum on the basis of minimal assumptions on blazars' physical and geometrical properties. In this paper, the fourth in a series, we extend the predictions of this paradigm below the sensitivity of existing surveys and estimate the contribution of blazars to the X-ray and gamma-ray extragalactic backgrounds. We find that the integrated light from blazars can explain up to 100% of the cosmic background at energies larger than ~10 GeV, and contribute ~40 to 70% of the gamma-ray diffuse radiation between 100 MeV and 10 GeV. The contribution of blazars to the X-ray background, between 1 and 50 keV, is approximately constant and of the order of 4-5%. On the basis of an interpolation between the estimated flux at X-ray and gamma-ray energies we can expect that the contribution of blazars raises to ~10% at 100 keV, and continues to increase with energy until it becomes the dominant component at a few MeV. Finally, we show that a strong dependence of the synchrotron peak frequency on luminosity, as postulated by the "blazar sequence", is ruled out by the observational data as it predicts a gamma-ray background above a few GeV that is far in excess of the observed value.

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arXiv2015

Photohadronic origin of $γ$-ray BL Lac emission: implications for IceCube neutrinos

The recent IceCube discovery of 0.1-1 PeV neutrinos of astrophysical origin opens up a new era for high-energy astrophysics. Although there are various astrophysical candidate sources, a firm association of the detected neutrinos with one (or more) of them is still lacking. A recent analysis of plausible astrophysical counterparts within the error circles of IceCube events showed that likely counterparts for nine of the IceCube neutrinos include mostly BL Lacs, among which Mrk 421. Motivated by this result and a previous independent analysis on the neutrino emission from Mrk 421, we test the BL Lac-neutrino connection in the context of a specific theoretical model for BL Lac emission. We model the spectral energy distribution (SED) of the BL Lacs selected as counterparts of the IceCube neutrinos using a one-zone leptohadronic model and mostly nearly simultaneous data. The neutrino flux for each BL Lac is self-consistently calculated, using photon and proton distributions specifically derived for every individual source. We find that the SEDs of the sample, although different in shape and flux, are all well fitted by the model using reasonable parameter values. Moreover, the model-predicted neutrino flux and energy for these sources are of the same order of magnitude as those of the IceCube neutrinos. In two cases, namely Mrk 421 and H 1914-194, we find a suggestively good agreement between the model prediction and the detected neutrino flux. Our predictions for all the BL Lacs of the sample are in the range to be confirmed or disputed by IceCube in the next few years of data sampling.

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arXiv2015

Exploring AGN Activity over Cosmic Time with the SKA

In this Chapter we present the motivation for undertaking both a wide and deep survey with the SKA in the context of studying AGN activity across cosmic time. With an rms down to 1 $μ$Jy/beam at 1 GHz over 1,000 - 5,000 deg$^2$ in 1 year (wide tier band 1/2) and an rms down to 200 nJy/beam over 10 - 30 deg$^2$ in 2000 hours (deep tier band 1/2), these surveys will directly detect faint radio-loud and radio-quiet AGN (down to a 1 GHz radio luminosity of about $2\times10^{23}$ W/Hz at $z=6$). For the first time, this will enable us to conduct detailed studies of the cosmic evolution of radio AGN activity to the cosmic dawn ($z\gtrsim6$), covering all environmental densities.

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arXiv2015

Studies of Relativistic Jets in Active Galactic Nuclei with SKA

Relativistic jets in active galactic nuclei (AGN) are among the most powerful astrophysical objects discovered to date. Indeed, jetted AGN studies have been considered a prominent science case for SKA, and were included in several different chapters of the previous SKA Science Book (Carilli & Rawlings 2004). Most of the fundamental questions about the physics of relativistic jets still remain unanswered, and await high-sensitivity radio instruments such as SKA to solve them. These questions will be addressed specially through analysis of the massive data sets arising from the deep, all-sky surveys (both total and polarimetric flux) from SKA1. Wide-field very-long-baseline-interferometric survey observations involving SKA1 will serve as a unique tool for distinguishing between extragalactic relativistic jets and star forming galaxies via brightness temperature measurements. Subsequent SKA1 studies of relativistic jets at different resolutions will allow for unprecedented cosmological studies of AGN jets up to the epoch of re-ionization, enabling detailed characterization of the jet composition, magnetic field, particle populations, and plasma properties on all scales. SKA will enable us to study the dependence of jet power and star formation on other properties of the AGN system. SKA1 will enable such studies for large samples of jets, while VLBI observations involving SKA1 will provide the sensitivity for pc-scale imaging, and SKA2 (with its extraordinary sensitivity and dynamic range) will allow us for the first time to resolve and model the weakest radio structures in the most powerful radio-loud AGN.

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arXiv2014

The Interplay between SF and AGN Activity, and its role in Galaxy Evolution

It has become apparent that active galactic nuclei (AGN) may have a significant impact on the growth and evolution of their host galaxies and vice versa but a detailed understanding of the interplay between these processes remains elusive. Deep radio surveys provide a powerful, obscuration-independent tool for measuring both star formation and AGN activity in high-redshift galaxies. Multiwavelength studies of deep radio fields show a composite population of star-forming galaxies and AGN, with the former dominating at the lowest flux densities (S$_{1.4\mathrm{GHz}}<$100~$μ$Jy). The sensitivity and resolution of the SKA will allow us to identify, and separately trace, the total star formation in the bulges of individual high-redshift galaxies, the related nuclear activity and any star formation occurring on larger scales within a disc. We will therefore gain a detailed picture of the apparently simultaneous development of stellar populations and black holes in the redshift range where both star-formation and AGN activity peak (1$\leq$z$\leq$4). In this chapter we discuss the role of the SKA in studying the connection between AGN activity and galaxy evolution, and the most critical technical requirements for such of studies

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arXiv2013

A simplified view of blazars: comparison with multi-frequency observations

We have recently proposed a new scenario where blazars are classified as flat-spectrum radio quasars or BL Lacs according to the prescriptions of unified schemes, and to a varying combination of Doppler boosted radiation from the jet, emission from the accretion disk, the broad line region, and light from the host galaxy. This mix of different components leads to strong selection effects, which are properly taken into account in our scheme. We describe here the main features of our approach, which solves many long-standing issues of blazar research, give the most important results, and discuss its implications and testable predictions.

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arXiv2010

The MicroJy and NanoJy Radio Sky: Source Population and Multi-wavelength Properties

I present simple but robust estimates of the types of sources making up the faint, sub-microJy radio sky. These include, not surprisingly, star-forming galaxies and radio-quiet AGN but also two "new" populations, that is low radio power ellipticals and dwarf galaxies, the latter likely constituting the most numerous component of the radio sky. I then estimate for the first time the X-ray, optical, and mid-infrared fluxes these objects are likely to have, which are very important for source identification and the synergy between the upcoming SKA and its various pathfinders with future missions in other bands. On large areas of the sky the SKA, and any other radio telescope producing surveys down to at least the microJy level, will go deeper than all currently planned (and past) sky surveys, with the possible exception of the optical ones from PAN-STARRS and the LSST. SPICA, JWST, and in particular the Extremely Large Telescopes (ELTs) will be a match to the next generation radio telescopes but only on small areas and above ~ 0.1 - 1 microJy (at 1.4 GHz), while even IXO will only be able to detect a small (tiny) fraction of the microJy (nanoJy) population in the X-rays. On the other hand, most sources from currently planned all-sky surveys, with the likely exception of the optical ones, will have a radio counterpart within the reach of the SKA. JWST and the ELTs might turn out to be the main, or perhaps even the only, facilities capable of securing optical counterparts and especially redshifts of microJy radio sources. Because of their sensitivity, the SKA and its pathfinders will have a huge impact on a number of topics in extragalactic astronomy including star-formation in galaxies and its co-evolution with supermassive black holes, radio-loudness and radio-quietness in AGN, dwarf galaxies, and the main contributors to the radio background.[ABRIDGED]

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arXiv2007

A Search for Synchrotron X-ray Emission in Radio Quasars

This paper presents XMM-Newton and Chandra X-ray spectroscopy of ten flat-spectrum radio quasars (FSRQ) which are candidates to have an X-ray spectrum dominated by jet synchrotron emission. In all these FSRQ, which are less strongly relativistically beamed than blazars, a considerable contribution from a power-law component similar to that present in radio-quiet quasars is required to explain the observed X-ray fluxes and X-ray spectral slopes. And as in radio-quiet quasars, their relatively high optical/UV fluxes can be accounted for by a significant contribution from thermal accretion disk emission. The lack of success in finding radio quasars with synchrotron X-rays is attributed to the adopted selection criteria, which were based on the multiwavelength flux ratios of BL Lacertae (BL Lac) objects. A refined selection technique, which additionally involves radio imaging, is proposed to search for these important candidates for the Gamma Ray Large Area Space Telescope (GLAST). On the other hand, the discovered FSRQ with their strong accretion disk signatures are expected to be important probes for studies of the poorly known accretion disk - jet connection.

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