X-ray Properties of Narrow-line Seyfert 1 Galaxies

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).