The origin of the lag spectra observed in AGN: Reverberation and the propagation of X-ray source fluctuations. (arXiv:1212.2213v1 [astro-ph.HE])

The origin of the lag spectra observed in AGN: Reverberation and the propagation of X-ray source fluctuations. (arXiv:1212.2213v1 [astro-ph.HE]):
The X-ray emission from active galactic nuclei (AGN) is highly variable.
Measurements of time lags (characterised by lag spectra) between variability in
the light curves in energy bands corresponding to directly observed continuum
emission from the corona around the black hole and to X-rays reflected from the
accretion disc adds a further dimension to studies of the structure and
energetics of these systems. We seek to understand these measurements in terms
of the physical parameters of the X-ray source (its location, extent, etc.)
through the calculation of theoretical lag spectra for a range of source
parameters in general relativistic ray tracing simulations, combined with
knowledge of the observed variability of the X-ray emission from AGN. Due to
the proximity of the emission to the central black hole, Shapiro delays are
important and the effects of general relativity should be considered when
interpreting the lags as the light travel time between the source and
reflector. We show that it is important to consider dilution of the lag by the
contribution of both the primary and reflected spectral components to the
observed energy bands. We find that the observed lag spectrum of the narrow
line Seyfert 1 galaxy 1H 0707-495 implies an X-ray source extending radially
outwards to around 35rg and at a height of around 2rg above the plane of the
accretion disc, consistent with the constraints obtained independently by
considering the emissivity profile of the accretion disc. By investigating the
influence of the propagation of X-ray luminosity fluctuations through the
source region we find it is possible to reproduce the shape of the low
frequency part of the lag spectrum (where the hard 'primary' band lags behind
the soft 'reflected' band) as the effect of luminosity fluctuations originating
in the centre of the X-ray source, close to the black hole, and propagating
outwards.