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Luigi Foschini

Publications and source records attributed to Luigi Foschini.

At least 37 records · Page 2Linked to original sources
arXiv2020

Jetted Narrow-Line Seyfert 1 Galaxies & Co.: where do we stand?

The discovery in 2008 of high-energy gamma-rays from Narrow-Line Seyfert 1 Galaxies (NLS1s) made it clear that there were active galactic nuclei (AGN) other than blazars and radio galaxies that can eject powerful relativistic jets. In addition to NLS1s, the great performance of the Fermi Large Area Telescope made it possible to discover MeV-GeV photons emitted from more classes of AGN, like Seyferts, Compact Steep Spectrum Gigahertz Peaked Sources (CSS/GPS), and disk-hosted radio galaxies. Although observations indicate a variety of objects, their physical characteristics point to a central engine powered by a relatively small-mass black hole (but, obviously, there are interpretations against this view). This essay critically reviews the literature published on these topics during the last eight years and analyses the perspectives for the forthcoming years.

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arXiv2020

High-energy and Very High-energy Constraints from Log-parabolic Spectral Models in Narrow-line Seyfert 1 Galaxies

Narrow-line Seyfert 1 galaxies (NLSy1s) are a well-established class of $γ$-ray sources, showing the presence of a jet like the more common flat-spectrum radio quasars. The evidence of $γ$-ray emission poses the issue of the location of the $γ$-ray emitting zone and of the contribution of the $γ$-$γ$ absorption within the broad-line region (BLR), since such objects have been detected by Fermi-LAT in the MeV-GeV energy range but not by imaging atmospheric Cherenkov telescopes beyond 100 GeV. We discuss how the spectral properties of three NLSy1s (SBS 0846+513, PMN J0948+0022, and PKS 1502+036) derived from the Fermi Large Area Telescope Fourth Source Catalog (4FGL) compare with theoretical models based on the observed properties of the BLR. In particular, we focus on the question on how simple power-law spectral models and log-parabolic ones could be disentangled in $γ$-ray narrow-line Seyfert 1 galaxies by means of current Fermi-LAT or future imaging atmospheric Cherenkov telescopes data. We find that the only possibility for a log-parabolic model to mimic a power-law model in the energy band above $E \sim 100$ GeV is to have a very small value of the curvature parameter $ β\sim 0.05$.

astro-ph.HE↗
arXiv2017

What we talk about when we talk about blazars?

After the discovery of powerful relativistic jets from Narrow-Line Seyfert 1 Galaxies, and the understanding of their similarity with those of blazars, a problem of terminology was born. The word blazar is today associated to BL Lac Objects and Flat-Spectrum Radio Quasars, which are somehow different from Narrow-Line Seyfert 1 Galaxies. Using the same word for all the three classes of AGN could drive either toward some misunderstanding, or to the oversight of some important characteristics. I review the main characteristics of these sources, and finally I propose a new scheme of classification.

astro-ph.HE↗
arXiv2016

Compact steep-spectrum sources as the parent population of flat-spectrum radio-loud NLS1s

Narrow-line Seyfert 1 galaxies (NLS1s) are an interesting subclass of active galactic nuclei (AGN), which tipically does not exhibit any strong radio emission. Seven percent of them, though, are radio-loud and often show a flat radio-spectrum (F-NLS1s). This, along to the detection of $γ$-ray emission coming from them, is usually interpreted as a sign of a relativistic beamed jet oriented along the line of sight. An important aspect of these AGN that must be understood is the nature of their parent population, in other words how do they appear when observed under different angles. In the recent literature it has been proposed that a specific class of radio-galaxies, compact-steep sources (CSS) classified as high excitation radio galaxies (HERG), can represent the parent population of F-NLS1s. To test this hypothesis in a quantitative way,in this paper we analyzed the only two statistically complete samples of CSS/HERGs and F-NLS1s available in the literature. We derived the black hole mass and Eddington ratio distributions, and we built for the first time the radio luminosity function of F-NLS1s. Finally, we applied a relativistic beaming model to the luminosity function of CSS/HERGs, and compared the result with the observed function of F-NLS1s. We found that compact steep-spectrum sources are valid parent candidates and that F-NLS1s, when observed with a different inclination, might actually appear as CSS/HERGs.

astro-ph.GA↗
arXiv2016

The Blandford-Znajek theory revisited

The Blandford-Znajek theory (1977) is generally studied to understand the generation of powerful relativistic jets. However, it could also give important insights about the feedback between a rotating black hole, its ergosphere, and a surrounding accretion disk. The energy flow between these systems could be understood by studying the BZ power as a function of a slip factor, which in turn is defined as a function of the angular velocity of the black hole and the angular velocity of the magnetic field lines (generally anchored to the disk). The distinct stages could explain the different types of observed AGN (with/without jets), but could also help understanding the evolution of the merging of binary AGN.

astro-ph.HE↗
arXiv2015

Parent population of flat-spectrum radio-loud narrow-line Seyfert 1 galaxies

Flat-spectrum radio-loud Narrow-Line Seyfert 1 galaxies (NLS1s) are a recently discovered class of $γ$-ray emitting Active Galactic Nuclei (AGN), that exhibit some blazar-like properties which are explained with the presence of a relativistic jet viewed at small angles. When blazars are observed at larger angles they appear as radio-galaxies, and we expect to observe an analogue parent population for beamed NLS1s. However, the number of known NLS1s with the jet viewed at large angles is not enough. Therefore, we tried to understand the origin of this deficit. Current hypotheses about the nature of parent sources are steep-spectrum radio-loud NLS1s, radio-quiet NLS1s and disk-hosted radio-galaxies. To test these hypotheses we built three samples of candidate sources plus a control sample, and calculated their black hole mass and Eddington ratio using their optical spectra. We then performed a Kolmogorov-Smirnov statistical test to investigate the compatibility of our different samples with a beamed population. Our results indicate that, when the inclination angle increases, a beamed source appears as a steep-spectrum radio-loud NLS1, or possibly even as a disk-hosted radio-galaxy with low black hole mass and high Eddington ratio. Further investigations, involving larger complete samples and observations at radio frequency, are needed to understand the incidence of disk-hosted radio-galaxies in the parent population, and to assess whether radio-quiet NLS1s can play a role, as well.

astro-ph.HE↗
arXiv2014

Understanding the nature of the blazar CGRaBS~J0211$+$1051

The source CGRaBS~J0211$+$1051 (MG1 J021114+1051, z=0.20) flared up on 2011 January 23 in high-energy $γ$-rays as reported by {\it Fermi}/LAT. This event was followed by the increased activity at the UV, optical and radio frequencies as detected by the observing facilities worldwide. The source also showed a high and variable optical polarization based on which it was proposed to be a low-energy peaked BL Lac Object (LBL). Present work reports first comprehensive multi-wavelength study of this source using data in the radio, optical, UV, X- and $γ$-rays and optical polarization. Using these multi-wavelength data on the source, we have estimated various parameters and verified it's classification vis-a-vis blazar sequence. Multi-waveband light-curves are used to discuss flaring events of 2011 January in an attempt to address the nature of the source and pinpoint the possible physical processes responsible for the emission. The light-curves show variations in the high energy $γ$-rays to be correlated with X-ray, UV and optical variations, perhaps indicating to their co-spatial origin. Our optical data, quasi-simultaneous with UV (Swift-UVOT) and X-ray (Swift-XRT) data, enabled us to trace low energy (Synchrotron) component of the spectral energy distribution (SED) for CGRaBS J0211+1051, for the first time. The SED shows the synchrotron peak to lie at $\sim 1.35\times10^{14}$~Hz, confirming CGRaBS~J0211+1051 to be an LBL. Some other parameters, such as local magnetic field ($\sim 5.93 $ Gauss) and black hole mass ($\sim 2.4 \times 10^{8} M_{\odot}$) are also estimated which are in agreement with their typical values for the blazars. Based on the present study, identification of the Fermi/LAT source, 2FGL J0211.2+1050, with its BL Lac counterpart CGRaBS J0211+1051 is confirmed.

astro-ph.HE↗
arXiv2013

The unification of relativistic jets

I report about the unification of relativistic jets from compact objects. The mass range is between 1.4 and 10 billion solar masses (i.e. from neutron stars to supermassive black holes in galaxies).

astro-ph.HE↗
arXiv2013

Evidence of powerful relativistic jets in narrow-line Seyfert 1 galaxies

In 2008, the Fermi Gamma-ray Space Telescope has revealed - for the first time - high-energy (E > 100 MeV) gamma rays from a few Narrow-Line Seyfert 1 Galaxies (NLS1s). Later, in 2009 and 2010, two multifrequency campaigns on one of these sources, PMN J0948+0022 (z=0.585), definitely confirmed the presence, in sources of this type, of a relativistic jet very similar and with comparable power to those in blazars. However, these sources are neither blazars nor radio galaxies, as proven by their optical spectrum and by their very compact radio morphology. Moreover, since NLS1s are generally hosted in spiral galaxies, this casts a significant doubt on the paradigm of the correlation between jets and elliptical host galaxies. These findings pose intriguing challenges to the current knowledge of jet systems and on how these structures are generated. The current status of the researches in this field is reviewed and ongoing work is outlined.

astro-ph.HE↗
arXiv2013

The 2013 multiwavelength campaign on the Narrow-Line Seyfert 1 galaxy 1H 0323+342: a rosetta stone for the jet/disk paradigm

Narrow-Line Seyfert 1 galaxies have been established as a new class of gamma-ray emitting AGN with relatively low black hole masses, but near-Eddington accretion rates. Other extragalactic gamma-ray sources observed so far such as Flat Spectrum Radio Quasars, Radio Galaxies, and BL Lacertae Objects generally exhibit much higher black hole masses and, in the case of BL Lac objects and FRI Radio Galaxies, much lower accretions rates. The multifrequency campaign of 2013 centered on the bright source 1H 0323+342 will provide further insights into the nature of the jets and their gamma ray production mechanisms in a largely unexplored corner of AGN parameter space. Here, we show preliminary results of this campaign and discuss them.

astro-ph.HE↗
arXiv2013

Where the "it from bit" come from?

In his 1989 essay, John Archibald Wheeler has tried to answer the eternal question of existence. He did it by searching for links between information, physics, and quanta. The main concept emerging from his essay is that "every physical quantity, every it, derives its ultimate significance from bits, binary yes-or-no indications". This concept has been summarized in the catchphrase "it from bit". In the Wheeler's essay, it is possible to read several times the echoes of the philosophy of Niels Bohr. The Danish physicist has pointed out how the quantum and relativistic physics - forcing us to abandon the anchor of the visual reference of common sense - have imposed a greater attention to the language. Bohr did not deny the physical reality, but recognizes that there is always need of a language no matter what a person wants to do. To put it as Carlo Sini, language is the first toolbox that man has at hands to analyze the experience. It is not a thought translated into words, because to think is to operate with signs as reminded us by various philosophers from Leonardo da Vinci to Ludwig Wittgenstein. [...]

physics.hist-ph↗
arXiv2013

Powerful relativistic jets in narrow-line Seyfert 1 galaxies

The recent detection of high-energy gamma rays from Narrow-Line Seyfert 1 Galaxies has confirmed that also this type of active galactic nuclei can generate powerful relativistic jets. I outline the evolution of the knowledge in this research field and the implications on the unification of relativistic jets at all scales.

astro-ph.HE↗
arXiv2012

Gamma-ray emission from Narrow-Line Seyfert 1 Galaxies and implications on the jets unification

The recent discovery by Fermi/LAT of high-energy (E>100 MeV) gamma rays from Narrow-Line Seyfert 1 Galaxies (NLS1s) made evident the existence of a third class of gamma-ray emitting Active Galactic Nuclei (AGN), after blazars and radio galaxies. It is now possible to study a rather unexplored range of low masses (10^6-8 Msun) and high accretion rates (up to the Eddington limit) of AGN with relativistic jets. A comparison with the jet emission from Galactic compact objects shows some striking similarities, indicating that NLS1s are the low-mass counterpart of blazars as neutron stars are the low-mass jet systems analogue of stellar mass black holes.

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arXiv2011

On the emission lines in active galactic nuclei with relativistic jets

The effect of the observed continuum emitted from a relativistic jet on the measurement of the full width half maximum (FWHM) of an emission line is analyzed. If the jet contribution is not properly subtracted, the FWHM of the line could seem narrower than what it should be. The cases of emission line detected in BL Lac objects and gamma-ray Narrow-Line Seyfert 1 galaxies (gamma-NLS1s) are addressed. It is shown that the smallness of the observed FWHM of the Ly alpha lines detected in three well-known BL Lacs, is an effect due to the combined action of both the relativistic jet and a weak accretion disc. Once removed the Doppler boosting of the jet continuum, the intrinsic FWHM of the lines are found to be in the usual range. Instead, the narrow permitted lines in gamma-NLS1s are really narrow, since the disc and the lines are much more powerful. This also confirms that gamma-NLS1 is really a new class of gamma-ray emitting AGN, different from blazars and radio galaxies.

astro-ph.GA↗
arXiv2011

Powerful relativistic jets in spiral galaxies

The discovery of high-energy (E>100 MeV) gamma rays from Narrow-Line Seyfert 1 Galaxies (gamma-NLS1s) has confirmed the presence of powerful relativistic jets in this class of active galactic nuclei (AGN). Although the jet emission is similar to that of blazars and radio galaxies, gamma-NLS1s have some striking differences: relatively small masses (10^6-10^8 M_sun), high accretion rates (0.1-1 times the Eddington limit) and are generally hosted by spiral galaxies. It is now possible to study a rather unexplored range of mass and accretion rates of AGN with relativistic jets. Specifically, in this work I present some results obtained by comparing a sample of blazars and gamma-NLS1s with another sample of Galactic binaries with relativistic jets (stellar mass black holes and neutron stars).

astro-ph.HE↗
arXiv2011

Accretion and jet power in active galactic nuclei

The classical diagrams of radio loudness and jet power as a function of mass and accretion rate of the central spacetime singularity in active galactic nuclei are reanalyzed by including the data of the recently discovered powerful relativistic jets in Narrow-Line Seyfert 1 Galaxies. The results are studied in the light of the known theories on the relativistic jets, indicating that while the Blandford-Znajek mechanism is sufficient to explain the power radiated by BL Lac Objects, it fails to completely account the power from quasars and Narrow-Line Seyfert 1. This favors the scenario outlined by Cavaliere & D'Elia of a composite jet, with a magnetospheric core plus a hydromagnetic component emerging as the accretion power increases and the disc becomes radiation-pressure dominated. A comparison with Galactic compact objects is also performed, finding some striking similarities, indicating that as the neutron stars are the low-mass jet systems analogue of black holes, the Narrow-Line Seyfert 1 Galaxies are the low-mass counterpart of the blazars.

astro-ph.HE↗
arXiv2011

What can we say about nature?

In this essay, I review the importance of languages in the study of reality, following the well-known aphorisms by Galilei, Bohr and many others. The emphasis on these aspects helps us to understand that it is not meaningful to ask if the reality if "digital" or "analog", but we have to search what is the best language to study some specific aspects of the reality. This problem is particularly felt in the case of frontier science, like quantum gravity, where, in front of several theories (syntaxes) available, there are presently neither observations nor experiments leading to the building of a convincing semantics. You will not find here recipes for a definitive theory. Just thoughts and questions.

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