Multiwavelength studies of the blazars detected by AGILE

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.

La 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.