X-ray Properties of Narrow-line Seyfert 1 Galaxies
The X-ray properties of five Narrow-line Seyfert 1 galaxies (NLS1)\nare analysed and presented\nin this dissertation. The data were collected with XMM-Newton, and to date,\nare of the highest quality ever obtained.\nThemes which have evolved and appear fundamental in understanding NLS1 are:\nnear- or super-Eddington accretion by a "small" supermassive black hole,\npartial covering, and reflection. Most of the objects presented in\nthis dissertation can adopt these principles.\n\nThe main results of this work are as follow.\n\nTwo observations of 1H0707-495 and one observation of IRAS 13224-3809 show sharp, spectral\ndrops above ~7 keV (Chapters 2 and 4).\nThe sharpness of the features, and absence of iron fluorescent emission and\nK\\beta UTA absorption,\nchallenge the possibility that the drops originate from\nphotonionisation. If partial covering from\na neutral absorber is adopted then outflows on the order of 0.05-0.15 c\nare required. On the other hand, if the sharp drops are associated with the\nblue wings of relativistically broadened Fe K\\alpha lines\n(Chapters 2 and 5) then the interpretation\nrequires light bending close to the black hole to explain the large\nequivalent widths and variability properties.\nBoth interpretations require an iron overabundance\n(3-10 times solar), suggesting that supersolar metallicities may\nsimply be characteristic of NLS1, probably due to strong starburst activity\nclose to the nucleus.\n\nThe general variability properties of NLS1 remain elusive, but\nadvances have been made. All of the Seyferts and quasars discussed here showed\nrapid and extreme variability at some point during the observations.\nIRAS 13224-3809 exhibited some of the most remarkable variability\n(Chapter 3). Lags between the hard and soft energy bands\nwere detected, suggesting that fluctuations at low energies instigated\nvariability at higher energies. However, closer inspection revealed that the\nlags alternate: sometimes the hard band follows, while at other times it leads.\nSpectral variability was determined to be correlated with flux variations,\nbut more interestingly was the finding that the spectral variability also\nlagged flux variations, resulting in flux-induced spectral variability.\n\nFor over 120 ks (spread over two observations), 1H0707-495 persistently displayed\nflux variations by at least a factor of four.\nDuring the first observation, when 1H0707-495 was in its lowest flux state ever\nrecorded,\nthere was no significant spectral variability.\nThe second\nobservation, when the source was in a higher flux state,\nrevealed strong spectral variability. Flux and spectral fluctuation\nwere never found to be significantly correlated (Chapter 4).\n\nThe quasar, PHL 1092, exhibited some of the most striking variability considering\nits high luminosity (Chapter 7).\nIndeed, an estimate of its radiative efficiency was in\nexcess of that expected from a Schwarzschild black hole. Interestingly,\nthe variability appeared to be entirely concentrated in the soft-excess, with\nthe power-law component appearing quiescent.\n\nThe closest example of class behaviour seen in the variability of\nthe group\nwas displayed by the two quasars I Zw 1 and NAB 0205+024\n(Chapters 6 and 8, respectively).\nBoth objects exhibited a hard X-ray flare which was concentrated at energies higher than\n~2 keV and accompanied by spectral variability.\nA scenario in which the hard X-ray flare originates in the accretion disc\ncorona, and then irradiates the disc itself seem most applicable here.\n\nI Zw 1 has been defined as the prototype NLS1 based on its optical properties.\nIn the X-rays it appears anything but prototypical.\nSignificant low-energy intrinsic absorption, a weak soft-excess, and\nevidence for multiple iron emission lines suggest that other\nprocesses are at work (Chapter 6).\n\nPHL 1092 exhibited deviations from a power-law fit in the 2-10 keV band\n(Chapter 7). The difference\ncould be interpreted as an emission line enhanced by light bending close\nto a Kerr black hole.\nThe complex variability could also be understood in terms of\nlight bending; however, partial covering could not be definitively dismissed\ndue to the modest-quality data.\n\nNAB 0205+024 portrayed a broad emission feature at ~5.8 keV which was\ninconsistent with expected emission from elements in that spectral region.\nThe feature could be described as neutral iron\nemitted from a narrow annulus on the disc\n(Chapter 8).\nThe presence of the hard X-ray flare, steep power-law slope, and\nredshifted iron line provide circumstantial support for the "thundercloud\nmodel" proposed by Merloni & Fabian (2001).