88 research outputs found
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 -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 -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 -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
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 -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
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 -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
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