Relativistic disc reflection in the extreme NLS1 IRAS13224-3809
  We present a spectral variability study of the XMM-Newton and Suzaku
observations of one of the most extreme Narrow Line Seyfert 1 galaxies,
IRAS13224-3809. The X-ray spectrum is characterized by two main peculiar
features, i) a strong soft excess with a steep rise below about 1.3 keV and ii)
a deep drop in flux above 8.2 keV. We focus here on a reflection-based
interpretation which interprets both features, as well as the large soft
excess, in terms of partially ionized reflection off the inner accretion disc.
We show that the two peculiar spectral features mentioned above can be
reproduced by two relativistic emission lines due to Fe K and Fe L. The lines
are produced in the inner accretion disc and independently yield consistent
disc parameters. We argue that the high L/K intensity ratio is broadly
consistent with expectations from an ionized accretion disc reflection,
indicating that they belong to a single ionized reflection component. The
spectral shape, X-ray flux, and variability properties are very similar in the
XMM-Newton and Suzaku observations, performed about 5 years apart. The overall
X-ray spectrum and variability can be described by a simple two-component model
comprising a steep power law continuum plus its ionised reflection off the
inner accretion disc. In this model, a rapidly rotating Kerr black hole and a
steep emissivity profile are required to describe the data. The simultaneous
detection of broad relativistic Fe L and K lines in IRAS 13224-3809 follows
that in another extreme NLS1 galaxy, 1H0707-495. Although the data quality for
IRAS13224-3809 does not allow us to rule out competing models as in 1H0707-495,
we show here that our reflection-based interpretation describes in a
self-consistent manner the available data and points towards IRAS13224-3809
being a very close relative of 1H0707-495 in terms of both spectral and
variability properties.
