Flaring Active Galactic Nuclei. The cases of 3C 279 and PMN J0948+0022 as seen by the Fermi-LAT

Active galactic nuclei (AGNs) exhibit variability across the entire electromagnetic spectrum with distinct flaring episodes at different frequencies. The high sensitivity and nearly uniform sky coverage of the Large Area Telescope on board the Fermi satellite make it a powerful tool for monitoring a large number of AGNs over long timescales. This allowed us to detect several flaring AGNs in gamma rays, triggering dedicated multifrequency campaigns on these sources from radio to TeV energies. We discuss the results for two different types of flaring AGN: the flat spectrum radio quasar 3C 279, in particular the coincidence of a gamma-ray flare from this source with the drastic change of the optical polarization angle, and the first gamma-ray flare from a radio-loud narrow-line Seyfert 1, PMN J0948+0022.Comment: 6 pages, 2 figures. Invited talk presented at the Workshop "SciNeGHE 2010", September 8-10, 2010, Trieste, Italy. To appear in Il Nuovo Cimento C - Colloquia on physics. Title, Summary and Conclusions changed according to the referee's suggestio

Jet physics of accreting super-massive black holes in the era of the Fermi Gamma-ray Space Telescope

The 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.Comment: 9 pages, 2 figures. Proceeding of the conference "Quasars at all cosmic epochs", Padova, 2017 April 2-7. Published on Frontiers in Astronomy and Space Sciences, Volume 4, 53 (2017

Relativistic Jets in Gamma-Ray-Emitting Narrow-Line Seyfert 1 Galaxies

Before the launch of the Fermi Gamma-ray Space Telescope satellite only two classes of active galactic nuclei (AGN) were known to generate relativistic jets and thus to emit up to the $\gamma$-ray energy range: blazars and radio galaxies, both hosted in giant elliptical galaxies. The discovery by the Large Area Telescope (LAT) on-board the Fermi satellite of variable $\gamma$-ray emission from a few radio-loud narrow-line Seyfert 1 galaxies (NLSy1) revealed the presence of an emerging third class of AGN with powerful relativistic jets. Considering that NLSy1 are usually hosted in late-type galaxies with relatively small black hole masses, this finding opened new challenging questions about the nature of these objects, the disc/jet connection, the emission mechanisms at high energies, and the formation of relativistic jets. In this review, I will discuss the broad-band properties of the $\gamma$-ray-emitting NLSy1 included in the Fourth Fermi LAT source catalog, highlighting major findings and open questions regarding jet physics, black hole mass estimation, host galaxy and accretion process of these sources in the Fermi era.Comment: 32 pages, 12 figures, 1 table. Review for the Special Issue 'Particle Acceleration Processes in Astrophysical Jets' of the journal Galaxies; Eds.: B. Kapanadze, P. A. Hughe

Multiwavelength studies of the blazars detected by AGILE

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.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

Flaring Patterns in Blazars

Blazars radiate from relativistic jets launched by a supermassive black hole along our line of sight; the subclass of FSRQs exhibits broad emission lines, a telltale sign of a gas-rich environment and high accretion rate, contrary to the other subclass of the BL Lacertae objects. We show that this dichotomy of the sources in physical properties is enhanced in their flaring activity. The BL Lac flares yielded spectral evidence of being driven by further acceleration of highly relativistic electrons in the jet. Here we discuss spectral fits of multi-lambda data concerning strong flares of the two flat spectrum radio quasars 3C 454.3 and 3C 279 recently detected in gamma rays by the AGILE and Fermi satellites. We find that optimal spectral fits are provided by external Compton radiation enhanced by increasing production of thermal seed photons by growing accretion. We find such flares to trace patterns on the jet power - electron energy plane that diverge from those followed by flaring BL Lacs, and discuss why these occur.Comment: 6 pages, 3 figures, accepted for publication in The Astrophysical Journa

Uncovering the host galaxy of the γ\gamma-ray-emitting narrow-line Seyfert 1 galaxy FBQS J1644+2619

The discovery of $\gamma$-ray emission from radio-loud narrow-line Seyfert 1 (NLSy1) galaxies has questioned the need for large black hole masses (> 10$^8$ M$_{\odot}$) to launch relativistic jets. We present near-infrared data of the $\gamma$-ray-emitting NLSy1 FBQS J1644+2619 that were collected using the camera CIRCE (Canarias InfraRed Camera Experiment) at the 10.4-m Gran Telescopio Canarias to investigate the structural properties of its host galaxy and to infer the black hole mass. The 2D surface brightness profile is modelled by the combination of a nuclear and a bulge component with a S\'ersic profile with index $n$ = 3.7, indicative of an elliptical galaxy. The structural parameters of the host are consistent with the correlations of effective radius and surface brightness against absolute magnitude measured for elliptical galaxies. From the bulge luminosity, we estimated a black hole mass of (2.1$\pm$0.2) $\times$10$^8$ M$_{\odot}$, consistent with the values characterizing radio-loud active galactic nuclei.Comment: 5 pages, 3 figures, 1 table. Monthly Notices of the Royal Astronomical Society Letter, Vol. 469, L11-L1

The Fermi-LAT view of young radio sources

Compact Symmetric Objects (CSO) are considered to be the young version of Fanaroff-Riley type I and type II radio galaxies, with typical sizes smaller than 1 kpc and ages of the order of a few thousand years. Before the launch of the Fermi satellite, young radio sources were predicted to emerge as a possible new γ-ray emitting population detectable by the Large Area Telescope (LAT). After more than 6 years of Fermi operation, the question of young radio sources as γ-ray emitting objects still remains open. In this contribution, we discuss candidate γ-ray emitting CSO and future perspective for detecting young radio sources with Fermi-LAT

Collimation, Acceleration and Recollimation Shock in the Jet of Gamma-Ray-emitting Radio-Loud Narrow-Line Seyfert 1 Galaxy 1H 0323+342

We investigated the detailed radio structure of the jet of 1H 0323+342 using high-resolution multi-frequency Very Long Baseline Array observations. This source is known as the nearest $\gamma$-ray emitting radio-loud narrow-line Seyfert 1 (NLS1) galaxy. We discovered that the morphology of the inner jet is well characterized by a parabolic shape, indicating the jet being continuously collimated near the jet base. On the other hand, we found that the jet expands more rapidly at larger scales, resulting in a conical-like shape. The location of the "collimation break" is coincident with a bright quasi-stationary feature at 7 mas from core (corresponding to a deprojected distance of the order of $\sim$100pc), where the jet width locally contracts together with highly polarized signals, suggesting a recollimation shock. We found that the collimation region is coincident with the region where the jet speed gradually accelerates, suggesting the coexistence of the jet acceleration and collimation zone, ending up with the recollimation shock, which could be a potential site of high-energy $\gamma$-ray flares detected by the Fermi-LAT. Remarkably, these observational features of the 1H 0323+342 jet are overall very similar to those of the nearby radio galaxy M87 and HST-1 as well as some blazars, suggesting that a common jet formation mechanism might be at work. Based on the similarity of the jet profile between the two sources, we also briefly discuss the mass of the central black hole of 1H 0323+342, which is also still highly controversial on this source and NLS1s in general.Comment: Accepted for publication in ApJ. 15pages, 9 figure