Active Galactic Nuclei (AGN) are among the most energetic sources in the Universe, and they are observed in about 10-20% of the galaxies in the local Universe. Their energy generation mechanism is accretion of matter on a central Supermassive Black Hole (SMBH). According to the most accredited paradigm, the Unified Model (UM), the SMBH is surrounded by an anisotropic uniform absorber of gas and dust, with a toroidal geometry, extending from 1 pc to 100 pc. Even if on a basic level the UM is correct, the need for alternative models was highlighted by the detection of several dishomogeneous absorbers, located at different distances from the SMBH, and at different physical states. Among the arguments in conflict with the UM there is the variability often observed in the X-ray spectra of AGN. When not resultant from an intrinsic variation of the nuclear emission, it implies that the underlying absorber is clumpy and not uniform. Many studies also showed that, besides being clumpy, the absorbing material is not always completely neutral. Significant amounts of outflowing partially-ionized material have been detected in about ∼50% of Type 1 AGN. These ionized absorbers are called “warm absorbers” and have outflow velocities in the range ∼100−1000 km/s. Furthermore, highly ionized absorbers with high outflow velocities (vout>3000 km/s, “ultrafast outflows”) have been observed. This thesis focuses on observations of absorption variability in the X-ray spectra of AGN, as a useful “tool” to test the size, structure and composition of the circumnuclear medium. In detail, I analysed the X-ray spectra of two AGN, NGC 454E and Mrk 348, for which X-ray data from Swift-BAT, Suzaku and XMM-Newton were available. NGC 454E is a red elliptical Seyfert galaxy in an interacting system, at z=0.0122. When comparing the Suzaku and XMM-Newton spectra, observed with 6 months delay, I noticed a dramatic change in the spectral curvature between 3 keV and 6 keV. The origin of the variability was due to a strong change of the column density of a neutral absorber, that allowed to classify NGC 454E as a “changing-look AGN”. Moreover, the XMM-Newton spectrum showed clear signatures of the presence of an ionised absorber, absent during Suzaku observation. An upper limit on its distance was inferred to be ∼0.001 pc. Mrk 348 is a bright Compton-thin Seyfert 2 galaxy at z=0.015. I found that its spectrum is characterized by three different absorbers, one neutral and two ionised, with different densities and ionisation states, but with approximately the same outflow velocity (∼0.05c). The distances of the ionised absorbers from the central emitting region are likely to be different, indeed the upper limits I find are 0.026 pc for the highly ionised absorber, and 2.72 pc for the mildly ionised one. A variation of the column density is present also for Mrk 348, both for the neutral absorber and for the mildly ionized. Both these studies are a step forwards in the definition of an alternative model to the UM, for which more and more evidences are being collected, denoting that the absorbing material is composed by multiple absorbers at different distances, densities and ionization states.

I Nuclei Galattici Attivi (AGN) sono tra le sorgenti più luminose dell’Universo e sono osservati in circa 10-20% delle galassie nell’Universo locale. Il meccanismo di generazione dell’energia di queste sorgenti è l’accrescimento di materia su un buco nero supermassivo (SMBH). Secondo il paradigma del Modello Unificato (UM) il SMBH è circondato da un assorbitore uniforme e anisotropo di gas e polveri, con geometria toroidale, che si estende da 1pc a 100pc. Anche se a livello generale l’UM è corretto, lavori recenti hanno evidenziato la necessità di formulare modelli alternativi per spiegare ad esempio la presenza di molteplici assorbitori disomogenei, a diverse distanze dal SMBH e in diversi stati fisici. Tra le argomentazioni in conflitto con l’UM vi è la variabilità spesso osservata negli spettri X degli AGN. Quando non deriva da una variazione intrinseca dell’emissione nucleare, essa implica che l’assorbitore è ‘’clumpy’’ e non è uniforme. Vari studi hanno anche mostrato che l’assorbitore non è sempre neutro: sono stati osservati outflow di grandi quantità di materiale parzialmente ionizzato (noti come “warm absorbers”) con velocità di outflow tra 100 e 1000 km/s, in circa il 50% degli AGN di tipo 1. Un’ulteriore complessità è rappresentata dalle osservazioni di assorbitori altamente ionizzati con alte velocità di outflow (vout>3000 km/s) noti come “ultrafast outflows”. La mia tesi si occupa della variabilità di assorbimento osservata negli spettri X degli AGN, fondamentale per testare le dimensioni, la struttura e la composizione della materia circumnucleare. In dettaglio, ho analizzato lo spettro X di due AGN, NGC 454E e Mrk 348, per cui sono disponibili dati dei satelliti Swift-BAT, Suzaku e XMM-Newton. NGC 454E è una galassia di Seyfert 2, parte di un sistema di galassie interagenti, a z=0.0122. Confrontando gli spettri di Suzaku e XMM, con distanza temporale di 6 mesi, ho notato un forte cambiamento della curvatura spettrale tra 3 keV e 6 keV. La sua origine è riconducibile a una variazione importante della densità colonnale dell’assorbitore neutro, che ha permesso di classificare NGC 454E come “changing-look AGN”. Inoltre, lo spettro di XMM mostra segni della presenza di un assorbitore ionizzato, assente durante l’osservazione di Suzaku, con un limite superiore sulla distanza di circa 0.001 pc. Mrk 348 è una galassia classificata come Seyfert 2, a z=0.015. Il suo spettro è caratterizzato da tre diversi assorbitori, uno neutro e due ionizzati, con diverse densità e stati di ionizzazione, ma con circa la stessa velocità di outflow (∼0.05c). Le distanze degli assorbitori ionizzati dal SMBH sono probabilmente differenti, in quanto i limiti superiori sono trovati essere di 0.026pc per l’assorbitore di più alta ionizzazione e 2.72pc per quello di media ionizzazione. Anche Mrk 348 presenza una variazione della densità colonnale tra Suzaku e XMM, sia per l’assorbitore neutro che per quello di media ionizzazione. Questi studi costituiscono un passo in avanti per definire un modello alternativo all’UM, per cui ci sono sempre più prove che la regione circumnucleare è composta da diversi assorbitori, a diverse distanze e stati di ionizzazione.

(2015). X-ray absorption in Active Galactic Nuclei. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2015).

X-ray absorption in Active Galactic Nuclei

MARCHESE, ELENA
2015

Abstract

Active Galactic Nuclei (AGN) are among the most energetic sources in the Universe, and they are observed in about 10-20% of the galaxies in the local Universe. Their energy generation mechanism is accretion of matter on a central Supermassive Black Hole (SMBH). According to the most accredited paradigm, the Unified Model (UM), the SMBH is surrounded by an anisotropic uniform absorber of gas and dust, with a toroidal geometry, extending from 1 pc to 100 pc. Even if on a basic level the UM is correct, the need for alternative models was highlighted by the detection of several dishomogeneous absorbers, located at different distances from the SMBH, and at different physical states. Among the arguments in conflict with the UM there is the variability often observed in the X-ray spectra of AGN. When not resultant from an intrinsic variation of the nuclear emission, it implies that the underlying absorber is clumpy and not uniform. Many studies also showed that, besides being clumpy, the absorbing material is not always completely neutral. Significant amounts of outflowing partially-ionized material have been detected in about ∼50% of Type 1 AGN. These ionized absorbers are called “warm absorbers” and have outflow velocities in the range ∼100−1000 km/s. Furthermore, highly ionized absorbers with high outflow velocities (vout>3000 km/s, “ultrafast outflows”) have been observed. This thesis focuses on observations of absorption variability in the X-ray spectra of AGN, as a useful “tool” to test the size, structure and composition of the circumnuclear medium. In detail, I analysed the X-ray spectra of two AGN, NGC 454E and Mrk 348, for which X-ray data from Swift-BAT, Suzaku and XMM-Newton were available. NGC 454E is a red elliptical Seyfert galaxy in an interacting system, at z=0.0122. When comparing the Suzaku and XMM-Newton spectra, observed with 6 months delay, I noticed a dramatic change in the spectral curvature between 3 keV and 6 keV. The origin of the variability was due to a strong change of the column density of a neutral absorber, that allowed to classify NGC 454E as a “changing-look AGN”. Moreover, the XMM-Newton spectrum showed clear signatures of the presence of an ionised absorber, absent during Suzaku observation. An upper limit on its distance was inferred to be ∼0.001 pc. Mrk 348 is a bright Compton-thin Seyfert 2 galaxy at z=0.015. I found that its spectrum is characterized by three different absorbers, one neutral and two ionised, with different densities and ionisation states, but with approximately the same outflow velocity (∼0.05c). The distances of the ionised absorbers from the central emitting region are likely to be different, indeed the upper limits I find are 0.026 pc for the highly ionised absorber, and 2.72 pc for the mildly ionised one. A variation of the column density is present also for Mrk 348, both for the neutral absorber and for the mildly ionized. Both these studies are a step forwards in the definition of an alternative model to the UM, for which more and more evidences are being collected, denoting that the absorbing material is composed by multiple absorbers at different distances, densities and ionization states.
COLPI, MONICA
BRAITO, VALENTINA
galaxies: active, galaxies: individual: Mrk 348, galaxies: individual: Ngc 454, X-rays: galaxies
Nuclei Galattici Attivi
FIS/05 - ASTRONOMIA E ASTROFISICA
English
28-set-2015
Scuola di dottorato di Scienze
FISICA E ASTRONOMIA - 30R
26
2013/2014
open
(2015). X-ray absorption in Active Galactic Nuclei. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2015).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/89114
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