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Gallo, Luigi C. (2004): X-ray Properties of Narrow-line Seyfert 1 Galaxies. Dissertation, LMU München: Faculty of Physics
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Abstract

The X-ray properties of five Narrow-line Seyfert 1 galaxies (NLS1) are analysed and presented in this dissertation. The data were collected with XMM-Newton, and to date, are of the highest quality ever obtained. Themes which have evolved and appear fundamental in understanding NLS1 are: near- or super-Eddington accretion by a "small" supermassive black hole, partial covering, and reflection. Most of the objects presented in this dissertation can adopt these principles. The main results of this work are as follow. Two observations of 1H0707-495 and one observation of IRAS 13224-3809 show sharp, spectral drops above ~7 keV (Chapters 2 and 4). The sharpness of the features, and absence of iron fluorescent emission and K\beta UTA absorption, challenge the possibility that the drops originate from photonionisation. If partial covering from a neutral absorber is adopted then outflows on the order of 0.05-0.15 c are required. On the other hand, if the sharp drops are associated with the blue wings of relativistically broadened Fe K\alpha lines (Chapters 2 and 5) then the interpretation requires light bending close to the black hole to explain the large equivalent widths and variability properties. Both interpretations require an iron overabundance (3-10 times solar), suggesting that supersolar metallicities may simply be characteristic of NLS1, probably due to strong starburst activity close to the nucleus. The general variability properties of NLS1 remain elusive, but advances have been made. All of the Seyferts and quasars discussed here showed rapid and extreme variability at some point during the observations. IRAS 13224-3809 exhibited some of the most remarkable variability (Chapter 3). Lags between the hard and soft energy bands were detected, suggesting that fluctuations at low energies instigated variability at higher energies. However, closer inspection revealed that the lags alternate: sometimes the hard band follows, while at other times it leads. Spectral variability was determined to be correlated with flux variations, but more interestingly was the finding that the spectral variability also lagged flux variations, resulting in flux-induced spectral variability. For over 120 ks (spread over two observations), 1H0707-495 persistently displayed flux variations by at least a factor of four. During the first observation, when 1H0707-495 was in its lowest flux state ever recorded, there was no significant spectral variability. The second observation, when the source was in a higher flux state, revealed strong spectral variability. Flux and spectral fluctuation were never found to be significantly correlated (Chapter 4). The quasar, PHL 1092, exhibited some of the most striking variability considering its high luminosity (Chapter 7). Indeed, an estimate of its radiative efficiency was in excess of that expected from a Schwarzschild black hole. Interestingly, the variability appeared to be entirely concentrated in the soft-excess, with the power-law component appearing quiescent. The closest example of class behaviour seen in the variability of the group was displayed by the two quasars I Zw 1 and NAB 0205+024 (Chapters 6 and 8, respectively). Both objects exhibited a hard X-ray flare which was concentrated at energies higher than ~2 keV and accompanied by spectral variability. A scenario in which the hard X-ray flare originates in the accretion disc corona, and then irradiates the disc itself seem most applicable here. I Zw 1 has been defined as the prototype NLS1 based on its optical properties. In the X-rays it appears anything but prototypical. Significant low-energy intrinsic absorption, a weak soft-excess, and evidence for multiple iron emission lines suggest that other processes are at work (Chapter 6). PHL 1092 exhibited deviations from a power-law fit in the 2-10 keV band (Chapter 7). The difference could be interpreted as an emission line enhanced by light bending close to a Kerr black hole. The complex variability could also be understood in terms of light bending; however, partial covering could not be definitively dismissed due to the modest-quality data. NAB 0205+024 portrayed a broad emission feature at ~5.8 keV which was inconsistent with expected emission from elements in that spectral region. The feature could be described as neutral iron emitted from a narrow annulus on the disc (Chapter 8). The presence of the hard X-ray flare, steep power-law slope, and redshifted iron line provide circumstantial support for the "thundercloud model" proposed by Merloni & Fabian (2001).