1,327 research outputs found
Multiwavelength studies of the blazars detected by AGILE
No full textLa scoperta dell’emissione nei raggi gamma da parti di numerosi Nuclei Galattici Attivi (AGN) con EGRET ed i telescopi Cherenkov è stata una delle più rivoluzionarie scoperte di astrofisica delle alte energie degli ultimi 20 anni, portando all’identificazione di una nuova classe di AGN: i blazar.
I blazar sono la sottoclasse più estrema di AGN, caratterizzata da forte emissione di radiazione non-termica attraverso l'intero spettro elettromagnetico. Questa emissione è interpretata come il risultato della radiazione elettromagnetica da un getto relativistico allineato alla linea di vista dell'osservatore, causando una forte amplificazione relativistica dell’emissione osservata. Considerando che la maggiore frazione della potenza totale dei blazar è emessa nei raggi gamma, le informazioni in questa banda energetica sono fondamentali per studiare i diversi modelli di radiazione. Oltre dieci anni dopo l'epoca di EGRET, il satellite AGILE (e successivamente anche il satellite Fermi) ha colmato la lacuna nella banda MeV-GeV dando ulteriore impulso allo studio dei fenomeni di astrofisica delle alte energie nei blazar.
Tuttavia, nonostante l'importanza delle informazioni fornite dalle osservazioni nei raggi gamma, studi correlati multifrequenza sono la chiave per raggiungere una migliore comprensione della struttura interna del getto, l'origine dei fotoni seme per il processo di Compton inverso ed i meccanismi di emissione che agiscono nei blazar.
Dal suo lancio in Aprile 2007, il satellite AGILE ha rilevato diversi blazar in stato di alta attività: PKS 1510-089, S5 0716+714, 3C 454.3, 3C 273, 3C 279, W Comae, Mrk 421 e PG 1553+113. In questa Tesi saranno presentati i risultati più interessanti dell'analisi multifrequenza di queste sorgenti rilevate da AGILE in raggi gamma, insieme ai dati multifrequenza forniti da altri osservatori come Spitzer, Swift, RXTE, Suzaku, INTEGRAL, MAGIC, VERITAS, nonché dalle osservazioni dal radio all’ottico ottenute da GASP-WEBT e REM.
Questa ampia copertura multifrequenza mi ha offerto l'opportunità di studiare le distribuzioni spettrali di energia di queste sorgenti dal radio ai raggi gamma, le variabilità correlata in diverse bande di energia e di indagare i meccanismi responsabili per la loro emissione, scoprendo in alcuni casi un comportamento più complesso rispetto ai modelli standard.The discovery of emission in the gamma-ray domain from many Active Galactic Nuclei (AGNs) by EGRET onboard Compton Gamma-Ray Observatory and the Cherenkov Telescopes was one of the most breakthrough of high energy astrophysics in the last 20 years, leading to the identification of a new class of AGNs: the blazars.
Blazars are the most extreme subclass of AGNs, characterized by the emission of strong non-thermal radiation across the entire electromagnetic spectrum, from radio to very high gamma-ray energies. This emission is interpreted as the result of the electromagnetic radiation from a relativistic jet that is viewed closely aligned to the line of sight of the observer, thus causing strong relativistic amplification.
Considering that the large fraction of the total power of blazars is emitted in the gamma-rays, information in this energy band is crucial to study the different radiation models. More than ten years after the EGRET era, the AGILE satellite (and subsequently also the Fermi satellite) filled the gap in the MeV-GeV band giving further impulse to the study of the high-energy astrophysics phenomena in blazars.
However, notwithstanding the importance of the information provided by the gamma-ray observations, correlated multiwavelength studies are the key to achieve a better understanding of the structure of the inner jet, the origin of the seed photons for the inverse Compton process and the emission mechanisms at work in blazars.
Since its launch in April 2007, the AGILE satellite detected several blazars in high activity state: PKS 1510–089, S5 0716+714, 3C 454.3, 3C 273, 3C 279, W Comae, Mrk 421 and PG 1553+113. In this Thesis I will present the most interesting results on multifrequency analysis of these sources detected by AGILE in gamma-rays, together with the multiwavelength data from other observatories such as Spitzer, Swift, RXTE, Suzaku, INTEGRAL, MAGIC, VERITAS, as well as radio-to-optical coverage by means of GASP-WEBT and REM.
This large multifrequency coverage gave me the opportunity to study the Spectral Energy Distributions (SEDs) of these sources from radio to gamma-rays, the correlated variability in different energy bands and to investigate the mechanisms responsible for their emission, uncovering in some cases a more complex behaviour with respect to the standard models.
The intense gamma-ray flares of S5 0716+714 observed by AGILE in September and October 2007 are among the highest fluxes detected by a BL Lac object and considering the redshift of the source (z = 0.31) the total power transported in the jet during these episodes approaches or slightly exceeds the maximum power generated by a spinning black hole of 10^9 solar masses, challenging the Blandford-Znajek mechanism and confirming the extreme energetics during these flares.
The modeling of the SEDs of S5 0716+714 indicated as, even if the broad band emission appears in agreement with the synchrotron self Compton (SSC) paradigm, a more complex model with two SSC components is needed to interpret our data. The case of S5 0716+714 is not unique among the BL Lac objects, also for the multifrequency observation of Mrk 421 and W Comae in June 2008 a one-zone SSC model seems to be a good representation of the broad band spectrum, but the observations collected during the multiwavelength campaigns seem to open to more complex interpretations of the data.
Moreover, the dominant emission mechanism in the gamma-ray band for Flat Spectrum Radio Quasars (FSRQs) is the inverse Compton scattering of external photons from the broad line region, but in some particular states also the contribution of seed photons from a hot corona (3C 454.3 in December 2007) or the accretion disk (3C 279) are shown to be important. Therefore, from the modeling of the different SEDs of BL Lacs and FSRQs observed by AGILE seems to emerge that the SSC and the external Compton (EC) frameworks, respectively, are good approximation for describing on average the high activity states of the two flavours of blazars, but going into details of the single observation more complex scenarios sometimes are requested.
The possibility to obtain information over the entire electromagnetic spectrum during the multifrequency campaigns organized by AGILE gave me also the opportunity to investigate in some blazars the presence of Seyfert-like features, such as the little and big blue bumps (PKS 1510-089) and the Compton reflection component (3C 273). Moreover, we revealed in the FSRQ PKS 1510-089 some features typical of High-frequency peaked BL Lac objects, such a X-ray harder-when-brighter behaviour during March 2008 and a shift of the synchrotron peak towards higher frequencies during the huge flare of March 2009.
Emission in optical and gamma-ray bands seems to be correlated during high activity states of blazars, but not strongly, with a possible lag of the gamma-ray flux with respect to optical one less than one day, both for FSRQs (e.g. 3C 454.3) and BL Lacs (S5 0716+714). On the other hand, during March 2009 a possible delay of the optical emission with respect to the gamma-ray one is detected for PKS 1510-089, suggesting a more complex behaviour in the optical/gamma-ray correlation, especially for FSRQs, where also a contribution of the thermal disk emission is clearly visible
A Florentine family in crisis: the Strozzi in the fifteenth century.
No full textPhDIn 1434 the Strozzi lineage had held a leading position in
Florentine society and government for at least one hundred and fifty
years, and was one of the largest and wealthiest of the city's
patrician lineages. The records of the catasto of 1427 and of the
scrutiny of 1433 are used to give a profile of the dominant social,
economic and political position of the Strozzi before the advent of
Medicean dominance. Their record of electoral success, and the
political and cultural leadership of influential and respected men
such as Palla di Nofri and Matteo di Simone, with other factors, put
the Strozzi amongst the greatest enemies of the victorious Medicean
regime of late 1434. The effects of political opposition and exile
on the lineage are examined both directly, through records of office-holding,
and indirectly through such indicators as marriage alliances
and household wealth. The two most prominent lines of the Strozzi
were exiled after 1434. Palla di Nofri's life and preoccupations in
his Paduan exile are examined, together with the lives of his sons;
none of these Strozzi ever returned to Florence, pursued as they were
by the enmity of the Medicean regime. The very different careers of
Filippo di Matteo and his brother Lorenzo are also examined: how they
succeeded in founding a lucrative bank in Naples, and in returning to
Florence to 'rebuild' (rifare) the position of the Strozzi lineage
there. The final decades of the century saw the Strozzi in an
economically more secure position, due substantially to the efforts
of Filippo. Except for a very small number of its members admitted
into the regime, most of the lineage is here shown to have remained
excluded from significant political office until after the fall of
the Medici regime in 1494
Vues présentes sur le celtique cisalpin
No full textPresent views on Cisalpine Celtic.
During the last twenty years, the knowledge of Lepontic and of Italian Gaulish has been notably enriched by very important finds, and also by numerous studies. The author proposes a synthesis of all new linguistical and cultural data provided by epigraphy in this particular resort.Dans les vingt dernières années la connaissance du lépontique et du gaulois d’Italie a été notablement enrichie par certaines trouvailles fort importantes ainsi que par de nombreuses études. Tableau synthétique des nouvelles données linguistiques et culturelles apportées par l’épigraphie de ce domaine.Motta Filippo. Vues présentes sur le celtique cisalpin. In: Etudes Celtiques, vol. 29, 1992. Actes du IXe congrès international d'études celtiques. Paris, 7-12 juillet 1991. Deuxième partie : Linguistique, littératures. pp. 311-318
VLBA observations of radio faint Fermi-LAT sources above 10 GeV
No full textContext. The first Fermi-LAT high-energy source catalog (1FHL), containing γ-ray sources detected above 10 GeV, is an ideal sample for characterizing the physical properties of the most extreme γ-ray sources.
Aims. We investigate the pc scale properties of a subsample of radio faint 1FHL sources with the aim of confirming the proposed blazar associations by revealing a compact high brightness temperature radio core, and we propose new low-frequency counterparts for the unassociated γ-ray sources (UGS). Moreover, we increase the number of 1FHL sources with high-resolution observations to explore the possible connection between radio and γ rays at E> 10 GeV.
Methods. We observed 84 1FHL sources, mostly blazars of high synchrotron peaked (HSP) type, in the northern sky with the Very Long Baseline Array (VLBA) at 5 GHz. These sources lack high-resolution radio observations and have at least one NRAO VLA sky survey counterpart within the 95% confidence radius. For those sources without a well-identified radio counterpart we exploit the VLBA multiple phase-center correlation capability to identify the possible low-frequency candidates.
Results. For ~93% of the sources in our sample we reveal a compact high brightness temperature radio core, which supports their proposed blazar association. The vast majority of the detected sources are radio weak, with a median VLBI flux density value of 16.3 mJy. For the detected sources we obtain an average brightness temperature on the order of 2 × 1010 K. We find a compact component for 16 UGS, for which we propose a new low-frequency association.
Conclusions. We find brightness temperature values that do not require high Doppler factors, and are in agreement with the expected values for the equipartition of energy between particles and magnetic field. We find strong indications for the blazar nature of all of the detected UGS, for which we propose new low-frequency associations. The characterization of the physical properties of this emerging population is relevant in view of the construction of the new generation Cherenkov Telescope Array
Exploring the bulk of the BL Lacertae object population: II. Gamma-ray properties
No full textAims. We are studying an unbiased sample of 42 nearby (z < 0:2) BL Lacertae objects with a multi-wavelength approach. The results of Very Long Baseline Interferometry observations were presented in the first paper of this series. In this paper, we study the -ray properties of the sample. Methods. We analyse data collected by the Fermi-Large Area Telescope (LAT) during its first 8.5 yr of operation in the energy range 0:1300 GeV. Results. We reveal 23 sources with a test statistic greater than 25 (corresponding to 4.6 ) out of 42, with 3 sources not detected in the third LAT active galactic nucleus (AGN) catalogue, and fluxes between 3:5 10 10 and 7:4 10 8 ph cm 2 s 1. The majority of the sources have hard spectra (2), with only four having values in the range 2.1-2.4. The three newly detected sources have fluxes in the range between 0:54 10 9 and 1:35 10 9 ph cm 2 s 1 and photon index 1.7-1.9. Among the 23 LAT-detected sources, 19 are included in the third catalogue of hard Fermi-LAT sources, with a spectrum that connects relatively smoothly from 0.1 GeV to 2 TeV. LAT-detected BL Lacs are more luminous on parsec scales with respect to non-LAT-detected sources and have larger core dominance according to the unified models. Conclusions. The LAT-detected BL Lacs seem to be composed of a bulk of classical sources dominated by Doppler boosting and characterised by compact and bright radio emission as well as hard -ray spectra. Moreover, we have identified a possible population of low-luminosity BL Lacs not detected by LAT, lacking a VLBI core, and with a small Doppler factor. Furthermore, three LATdetected sources show non-classical properties for -ray emitting BL Lacs (no evidence of relativistic jet, low Doppler factor in radio images, relatively low core dominance) and three other sources, while showing radio emission on parsec scales, are not detected in rays so far
The gamma-ray emission from young radio galaxies and quasars
No full textAccording to radiative models, radio galaxies are predicted to produce γ-rays from the earliest stages of their evolution onwards. The study of the high-energy emission from young radio sources is crucial for providing information on the most energetic processes associated with these sources, the actual region responsible for this emission, as well as the structure of the newly born radio jets. Despite systematic searches for young radio sources at γ-ray energies, only a handful of detections have been reported so far. Taking advantage of more than 11 years of Fermi-large area telescope (LAT) data, we investigate the γ-ray emission of 162 young radio sources (103 galaxies and 59 quasars), the largest sample of young radio sources used so far for a γ-ray study. We analyzed the Fermi-LAT data of each source separately to search for a significant detection. In addition, we performed the first stacking analysis of this class of sources in order to investigate the γ-ray emission of the young radio sources that are undetected at high energies. In this note, we present the results of our study and we discuss their implications for the predictions of γ-ray emission from this class of sources
Occhi pieni e mani vaganti. Movimenti, emozioni, astrazioni.
No full textFilippo FIMIANI, Occhi pieni e mani vaganti. Movimenti, emozioni, astrazioni, «Fata Morgana», 12/2010, pp. 147-164. ISSN 1970-5786.
ITALIANO. Una piccola sequenza da un episodio della serie televisiva Mad Men in cui la relazione sensoriale emozionale tra lo spettatore e un corpo in movimento, precisamente un corpo danzante, è l’oggetto di questo articolo, che intende così verificare la cosiddetta naturalizzazione della fenomenologia e del corpo vissuto operata dalle scienze cognitive applicate agli studi visuali. L’autore mostra anche i rapporti tra la cosiddetta simulazione incarnata e una memoria simbolica e iconografica profonda, e discute in particolare la polarità tra proiezione e introiezione, attività e passività nell’esperienza empatica delle immagini in movimento.A short filmic example from the AMC's award-winning series Mad Men in which the sensorial and emotional relationship between the spectator and a moving body, i.e., a dancing body, is explicitly put into play. The essay wants to deal with the so called naturalization of phenomenology and of the living body, trained by the cognitive sciences reading the visuality. The author shows the relationship between the so called ‘embodied Simulation’ and a symbolic and iconographic deep memory of the pictures, and he gives an in-depth attention to the polarity between projections and introjections, activity and passivity of the spectator’s body during the empathic experience of the moving pictures
Young radio sources: the duty-cycle of the radio emission and prospects for gamma-ray emission
No full textThe evolutionary stage of a powerful radio source originated by an AGN is
related to its linear size. In this context, compact symmetric objects (CSOs),
which are powerful and intrinsically small objects, should represent the young
stage in the individual radio source life. However, the fraction of young radio
sources in flux density-limited samples is much higher than what expected from
the number counts of large radio sources.This indicates that a significant
fraction of young radio sources does not develop to the classical
Fanaroff-Riley radio galaxies,suggesting an intermittent jet activity. As the
radio jets are expanding within the dense and inhomogeneous interstellar
medium,the ambient may play a role in the jet growth, for example slowing down
or even disrupting its expansion when a jet-cloud interaction takes place.
Moreover, this environment may provide the thermal seed photons that scattered
by the lobes' electrons may be responsible for high energy emission, detectable
by Fermi-LAT
Multiple political geographies
No full textThis chapter delves into the multifaceted realm of political geography within the context of the journal Political Geography. The author, who has held various editorial positions within the journal, navigates the intricate terrain of diversity and inclusion, exploring the challenges posed by linguistic and cultural biases inherent in academic publishing. The chapter critically examines efforts to mitigate barriers and broaden the scope of political geography, questioning the adequacy of current approaches. Through a reflective analysis, the author elucidates the complex dynamics of political geography as a subdiscipline, shedding light on both the impediments and possibilities it offers to researchers and students
Jet Physics of Accreting Super-Massive Black Holes in the Era of the Fermi Gamma-ray Space Telescope
No full textThe Fermi Gamma-ray Space Telescope with its main instrument on-board, the
Large Area Telescope (LAT), opened a new era in the study of high-energy
emission from Active Galactic Nuclei (AGN). When combined with contemporaneous
ground- and space-based observations, Fermi-LAT achieves its full capability to
characterize the jet structure and the emission mechanisms at work in
radio-loud AGN with different black hole mass and accretion rate, from flat
spectrum radio quasars to narrow-line Seyfert 1 (NLSy1) galaxies.
Here, I discuss important findings regarding the blazar population included
in the third LAT catalog of AGN and the gamma-ray emitting NLSy1. Moreover, the
detection of blazars at redshift beyond three in gamma rays allows us to
constrain the growth and evolution of heavy black holes over cosmic time,
suggesting that the radio-loud phase may be important for a fast black hole
growth in the early Universe. Finally, results on extragalactic objects from
the third catalog of hard LAT sources are presented
- …
