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Spectral evolution of the high energy emission from TeV BL Lac Objects
Full text linkI Blazar sono una delle più enigmatiche classi tra i Nuclei Galattici Attivi (AGN). Secondo lo schema
unificato degli AGN queste sorgenti sono interpretate come radio-sorgenti con un getto relativistico che
è allineato lungo la linea di vista. I Blazar si dividono principalmente in due classi: la prima, costituita
dagli oggetti di tipo BL Lac, con spettri in banda visibile privi di righe, e la seconda composta dai
quasar con spettro radio piatto (FSRQ) che invece presentano righe di emissione. L’emissione dei
Blazar si estende dalla banda radio fino alla banda dei raggi gamma di più alta energia (TeV) e la loro
distribuzione spettrale di energia (SED) presenta due picchi: il primo posizionato tipicamente tra la
bnada infrarossa e quella dei raggi X, ed il secondo dai raggi γ di basse energie fino alla banda del TeV.
Questa distribuzione spettrale è solitamente interpretata in termini di modelli di emissione Synchrotron
Self-Compton, secondo cui i fotoni di sincrotrone, corrispondenti alla prima componente, sono emessi
da una popolazione di elettroni accelerata nei getti relativistici, e producono la seconda componente
attraverso effetto Compton inverso con gli stessi elettroni. Un possibile criterio per classificare gli
oggetti di tipo BL Lac è utilizzando la posizione del prima componente della SED e si indicano come
HBL le sorgenti che presentano il picco della prima componente nella banda dal UV ai raggi X mrentre
LBL quelle in cui questo cade tra l’infrarosso e la banda visibile. Per quanto concerne i FSRQ le
loro SED possono essere anche descritte da modelli External Compton, in cui la seconda componente è
prodotta per effetto Compton inverso degli elettroni relativistici con fotoni di radiazione esterni al getto.
L’obiettivo della Tesi è quello di studiare l’emissione di alta energia (dalla banda dei raggi X fino al
TeV) di tutte gli oggetti di tipo BL Lac osservati al TeV fino ad oggi. Mi sono interessato principalmente
di investigare i meccanismi di accelerazione ed i processi di emissione nei getti relativistici dei BL Lacs.
In particolare, in questo lavoro mi sono occupato dell’evoluzione spettrale delle loro SED nelle bande di
energia considerate. A tal fine ho ridotto ed analizzato gli spettri X di 11 anni di osservazioni di BL Lac
negli archivi di BeppoSAX, XMM-Newton e Swift. Ho sviluppato alcuni codici numerici per riprodurre
le SED dei Blazar. In particolare, ho costruito un codice SSC in grado di calcolare l’emissione per
effetto Compton inverso anche al secondo ordine, ed un codice che riproducesse il meccanismo di
External Compton dove i fotoni seme sono prodotti da un disco di accrescimento. Tutti i codici sono
inoltre in grado di lavorare con diverse distribuzioni in energia degli elettroni. Tra queste distribuzioni
la più utilizzata è quella che proviene da meccanismi di accelerazione di tipo statistico/stocastico ed
ha una forma log-parabolica. Ogni codice ha inoltre una parte di statistica che permette di determinare
i parametri delle SED. Oltre ai codici numerici ho anche sviluppato alcuni calcoli analitici utili per
tenere conto del fenomeno della Catastrofe Compton nell’emissione per effetto Compton inverso. Sono
state individuate per i BL Lac osservati al TeV alcune correlazioni e trend tra i parametri spettrali
dell’emissione X, interpretate in termini di emissione di sincrotrone e meccanismi di accelerazione
statistici e stocastici. Un’analisi dettagliata è stata fatta nel caso della Mrk 421, il cui comportamento
nel paino dei parametri spettrali non sembra essere dominato da variazioni del fattore Doppler come
richiesto dai modelli di “internal shock” per i getti dei BL Lac. E' stata fatta anche una ricerca di questi
trend in tutti negli spettri X degli altri HBL osservati al TeV ed almeno in quattro di loro sono stati
confermati comportamenti nello spazio dei parametri analoghi al caso della Mrk 421 . Alcune SED
degli HBL osservati al TeV sono state confrontate con i calcoli numerici del codice SSC per derivare
i parametri fisici di questa classe di sorgenti. Questi parametri sono stati poi utilizzati per descivere le
SED di un insieme di HBL per verificarne la possibilità di osservarli al TeV. Infine, i calcoli numerici
sono stati anche applicati con successo per descrivere la SED di BL Lacertare, il primo LBL osservato
al TeV. Il lavoro sviluppato in questa tesi è quasi interamente dedicato allo studio dei modelli SSC per
descrivere le SED degli HBL, con l’ eccezione di BL Lacertae. Una simile analisi potrebbe essere
applicata agli oggetti di tipo LBL quando le nuove osservazioni simultanee di entrambe le componenti,
sincrotrone ed Compton inverso, saranno disponibili. Questo avverrà dal prossimo anno quando i nuovi
satelliti AGILE, GLAS T e Planck forniranno un grande archivio di dati riguardanti l’emissione degli
LBL. Il primo periodo di attività di AGILE, durante il quale alcuni LBL ed alcuni FSRQ sono già stati
osservati conferma l’importanza di queste osservazioni nei raggi γ per la comprensione dei Blazars.Blazars are a most enigmatic class of active galactic nuclei (AGNs). In the unification scenario
of active galactic nuclei (AGNs) Blazars fit in as radio-loud sources with a relativistic jet that points
toward us. They are divided in two main classes: the first, less luminous, constitute by the BL Lac objects,
with featurelss optical spectra, and the other one composed by the Flat Spectrum Radio Quasars
(FSRQs) in which, tipically, there are prominent spectral lines. Blazar emissions extends from radio to
TeV energies and their spectral energy distribution (SED) is double bump: the first component tipically
peaks from IR to X-ray band, and the second one in the gamma-rays up to TeV energies. Usually, BL
Lac SEDs are described in terms of Synchrotron Self-Compton models in which synchrotron photons,
emitted by a population of electrons accelerated in the relativistic jets, produce the second component
via inverse Compton (IC) scattering by the same electrons. A possible classification criterium for BL
Lacs is in terms of the SED peak energy position of the first component: high-frequency peaked BL
Lacs (HBLs) comprise sources in which the synchrotron peak is between the UV band and X-rays;
low-frequency peaked BL Lacs (LBLs), when the first bump appears in the IR-optical range. FSRQ
SEDs tipically require other spectral components to be described, as for example soft seed photons
produced in external regions to their jets; so, these emission models are generally named: External
Compton radiation.
The aim of this thesis is to investigate the high energy emission (from X-rays to TeV frequencies)
of all BL Lacs detected at TeV energies, up to now. I am mainly interested in how the synchrotron
emission and inverse Compton radiation work in BL Lac jets. My goal is to interpret the spectral evolution
of their SEDs in the considered energy ranges.
To reach my aim I have reduced and analysed the X-ray spectra of eleven years of archival
and new BL Lac observations performed with BeppoSAX, XMM-Newton and Swift. I have developed
several numerical codes to reproduce the SEDs of these sources. In detail, I have developed a Synchrotron
Self Compton (SSC) code able to reproduce the first and the second order inverse Compton
emission, and the External Compton (EC) code that evaluates the inverse Compton scattering between
the electron jet population and external radiation field from the accretion disk. Several electrons distribution
have been used, in particular, those produced by statistical/stochastical acceleration processes
(Fokker-Planck equation), leading to log-parabolic electron distributions. Each code has also a statistical
section to determine spectral parameters of the BL Lac SEDs. Besides this numerical codes some
analytical calculations have been developed to take into account of the so called Compton Catastrophe
in the inverse Compton emission.
Some correlations and trends between spectral parameters were found in X-ray observations
of the sample of TeV BL Lacs, and they have been interpreted in terms of synchrotron emission and
statistical/stochastical acceleration. A detailed statistical analysis was performed for Mrk 421, whose
behavior, in the plane of spectral parameters, appears not to reflect beaming variations, as required
by “internal shock” models of BL Lac jets. An extensive search of the similar trends was performed
on eleven years of X-ray observations with BeppoSAX, XMM-Newton and Swift. I discovered similar
trends in HBLs detected at TeV energies at least four sources. Numerical simulations provide a good
description of the SEDs of HBLs in my sample. These calculations provide constraints on input parameters
for the numerical simulations on HBLs. The constraints derived have been used to describe a
sample of HBLs candidate as TeV emitters. Finally, I have also simulated the SED of BL Lacertae, the
first LBL observed at TeV energies, discussing among SSC codes and EC simulations.
The work developed in this thesis is quasi entirely dedicated to the study of SSC models
to describe HBL SEDs, with the only exception of BL Lacertae. A similar analysis could be applied
to other LBL objects when the new simultaneous observations of both the synchrotron and the inverse
Compton will be avaiable. These will occur in the next year when the new satellites AGILE, GLAST
and Planck will provide a large number of data on rich LBL samples. The first period of activity of
AGILE in which some LBLs and FSRQs have already been detected confirmed the relevance of these
γ-ray observations for the understanding of the BLazars phenomenon
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Improving angle stability by switching shunt reactors in mixed overhead cable lines. An Italian 400 kV case study
Full text linkStringent environmental constraints make the construction of new transmission overhead lines more and more difficult. Alternatively, today it is possible to use cable lines for high (HV) and extra-high (EHV) voltage systems. The configuration of the so-called mixed lines can create some problems in the operation of the electrical system, both during steady-state and transient conditions. In particular, the system stability is one of the main concerns when analyzing the dynamic response of power systems. In this paper, the study of angular stability of a system containing a mixed line is presented: a specific control logic applied to the shunt reactors of the mixed line is proposed as improvement of the overall system stability. The proposed switching logic is first discussed from a theoretical point of view and validated with two different testing systems. Then, the existing overhead-cable lines connecting Sicily to the rest of continental Europe 400 kV power system are taken as case study for the application of the proposed switching strategy. Several simulations are performed in the power system analysis software NEPLAN360: the results show the fundamental role of the timing of the control actions applied on the shunt reactors in helping the system to keep the stability. The proposed control proves to be an effective support to the system subjected to critical contingencies, contributing decisively to avoid the angular separation between areas and therefore to preserve the stability of the system
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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