ETD

Archivio digitale delle tesi discusse presso l'Università di Pisa

Tesi etd-10092013-064224


Tipo di tesi
Tesi di laurea specialistica
Autore
PREZIUSO, SILVIA
URN
etd-10092013-064224
Titolo
TEV BLAZARS: SCRUTINY OF THE SYNCHROTRON SELF-COMPTON SINGLE-ZONE EMISSION MODEL THROUGH MAGIC AND MULTI-WAVELENGTH OBSERVATIONS
Dipartimento
FISICA
Corso di studi
SCIENZE FISICHE
Relatori
relatore Stamerra, Antonio
relatore Prada Moroni, Pier Giorgio
Parole chiave
  • IBL
  • spectral energy distribution
  • SED
  • HBL
  • AGN
  • BL Lacertae
  • MAGIC
  • synchrotron self-Compton
  • Markarian 421
  • SSC
  • blazars emission models
Data inizio appello
24/10/2013
Consultabilità
Completa
Riassunto
TEV BLAZARS: SCRUTINY OF THE SYNCHROTRON SELF-COMPTON SINGLE-ZONE EMISSION MODEL THROUGH MAGIC AND MULTI-WAVELENGTH OBSERVATIONS


During the development of my thesis, I dealt with the observational study of the emission processes which occur in Active Galactic Nuclei (AGN), very compact extragalactic sources emitting most of their power through non-thermal radiation up to energies of tens of TeV. The subject has been faced through the study of data collected with the MAGIC Cherenkov telescopes system, that
has also been combined with more direct fieldwork. I indeed had the opportunity to gain experience on testing of the electronics developed in Pisa for the Data Acquisition System (DAQ) of MAGIC and with the data taking directly on the site where the MAGIC telescopes are located, the Roche de Los Muchachos observatory, on the Spanish island of La Palma.
MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov Telescopes) is composed by two Imaging Atmospheric Cherenkov telescopes (IACTs) (each one with a diameter of 17 meters), operating in stereo-mode and designed to perform γ-ray astronomy in the Very High Energy (VHE) band (E >100 GeV), in particular in the energy range from ∼50 GeV up to ∼20 TeV.
The AGN category that MAGIC telescopes most profitably study is the one of BL Lacertae type
blazars. They represent a sub-class of radio-loud AGNs (those having Flux(at 5 GHz)/Flux(in B band) > 10) which form a small angle with the observer’s line of sight, are dominated by non-thermal emission and are characterized by an extreme weakness or even a total absence of emission lines in their optical spectra. These sources present continuous emission of non-thermal origin extending over an energy range that overcomes 15 orders of magnitude, from radio band till γ-rays with extremely high energies. Such γ-rays are detected by IACTs in an indirect way because they can’t penetrate terrestrial atmosphere which instead acts for them like a calorimeter. γ-ray photons which reach the earth’s atmosphere react with its constituents and give rise, through recursive interactions, to a cascade of electrons, positrons and photons in form of an Extensive Air Shower (EAS) of electromagnetic type. Charged particles constituting the EAS, having a speed greater than the speed of light in the medium (v = c/n, where c is the speed of light in the vacuum and n is the atmospheric refraction index), emit flashes of light peaked in the near UV. This is the so-called Cherenkov light, the track of the passage of γ-photons that IACTs like MAGIC can perceive.
The Spectral Energy Distribution (SED) of BL Lacs is the main instrument through which their
nature and physical characteristics can be investigated, allowing to verify emission models. SEDs of all these sources tipically show a double humped structure, whose first peak is positioned between radio and X-ray wavelengths whereas the second peak falls inside the energy range between GeV and TeV. From the position of the first peak in this distribution, which is imputable to synchrotron emission (Urry and Padovani 1995), it’s possible to further distinguish blazars in three classes: HBLs (High Frequency Peaked), showing the first peak in the UV/X band, IBLs (Intermediate Frequency Peaked), presenting it instead in IR/UV band and LBLs (Low Frequency Peaked), whose first peak is in IR band or at even lower frequencies.
My thesis is in particular centered on the study of HBLs and IBLs, whose spectral emission is the most appropriate for the application of the so-called single-zone Synchrotron Self-Compton (1Z-SSC) emission model (e.g. Tavecchio, Maraschi and Ghisellini 1998), the one that has been examined throughout my work. This model is based on the fact that in compact sources, like blazars, photons produced by synchrotron processes can be used in subsequent Inverse Compton reactions by the same electrons that generate them: the electrons can therefore lose energy either through synchrotron emission or through Compton scattering of photons. The effects of other particles inhabitating active galaxies, first of all protons, are neglected in the frame of SSC model mostly for mass reasons: for a single particle, the synchrotron emitted power is proportinal to the Thomson cross-section, which in turn grows with the square of the inverse of the particle mass. The result is that, as the particle mass increases, the synchrotron emitted power decreases. This "historically” justifies the recourse to a quite simple model like single-zone SSC (only 7 fitting parameters) that, despite the relative simplicity, is meanwhile effective in modeling a big fraction of the overall known HBLs and IBLs, photographed in different moments of their lives.
The single-zone SSC model, with its assumptions, has certainly also some limits which leave space for alternative models, like the one based on lepto-hadronic, hadronic and/or mesonic reactions (e.g. Aharonian (2000), Mucke and Protheroe (2000)). These models may have a role in some specific cases but don’t have till now reach the large applicability scale of SSC.
The aim of my thesis is to test the actual capability of the single-zone SSC model to explain BL Lacertae emission. I have scrutinized the real applicability field of such modelization and underlined common behaviours as well as lacks of SSC fitting. To achieve this goal, I have cataloged all the presently known TeV-IBLs and HBLs, extracting their model parameters from the published papers. I have then implemented comparative statistical studies on the one-zone SSC model parameters, the main observables connected to them and the VHE spectral indices, source-by-source and single observation-by-single observation, listed in my sample. I have in this way managed to underline which sources, in specific periods of their emission history, deviate from expected or averaged values. I have searched for correlations between some specific parameters, possibly leading to a better comprehension of the physics governing blazars emission or in any case helping to understand which are the main limitations and problems of our current modelization knowledges in this field. In this respect, I have pored on the three main problematic situations for SSC fitting: blazars showing extremely fast flux variability (sometimes even of the order of minutes), blazars with big time lags between lightcurves of the low energy peak and of the high energy peak (referring to SSC model the varibility of the two peaks should happen almost simulataneously) and the so-called extreme blazars (whose first emission peak is in the hard X-ray band). For all the presently known TeV-IBLs and HBLs, I have underlined critical aspects of the one zone SSC modeling.
In this framework, I have then analyzed an extreme outburst of one of the most famous sources
of the TeV sky, Markarian 421, the second source to be ever seen through the Cherenkov imaging
technique and the first source in the extragalactic Universe (discovered during March-June 1992).
Markarian 421’s most recent high energy flare occured in April 2013 and MAGIC has followed its
evolution together with many other instruments investigating other wavelength bands so as to obtain an extensive multiwavelength (MWL) coverage. My analysis has concentrated on the MAGIC data, covering the gamma-ray IC peak of the source’s SED, which were strictly simultaneous to X-ray data taken by the Swift satellite, covering the synchrotron peak. The strict simultaneity allows a critical test of the one zone SSC model.
I have concluded my work depicting possible developments that could come from further and lengthened multiwavelength (MWL) and simultaneous campaigns on specific sources and still more from the VHE sensitivity increase that should be achieved with the forthcoming telescopes array called CTA (Cherenkov Telescope Array), an international project in which the best presently possessed tecnologies in the field of physics based on imaging Cherenkov will merge.
File