Pablo Cassatella, Phil Uttley, Joern Wilms, Juri Poutanen
The X-ray variations of hard state black hole X-ray binaries above 2 keV show
'hard lags', in that the variations at harder energies follow variations at
softer energies, with a time-lag \tau depending on frequency \nu approximately
as \tau \propto \nu^{-0.7}. Several models have so far been proposed to explain
this time delay, including fluctuations propagating through an accretion flow,
spectral variations during coronal flares, Comptonisation in the extended hot
corona or a jet, or time-delays due to large-scale reflection from the
accretion disc. In principle these models can be used to predict the shape of
the energy spectrum as well as the frequency-dependence of the time-lags,
through the construction of energy-dependent response functions which map the
emission as a function of time-delay in the system. Here we use this approach
to test a simple reflection model for the frequency-dependent lags seen in the
hard state of GX 339-4, by simultaneously fitting the model to the
frequency-dependent lags and energy spectrum measured by XMM-Newton in 2004 and
2009. Our model cannot simultaneously fit both the lag and spectral data, since
the relatively large lags require an extremely flared disc which subtends a
large solid angle to the continuum at large radii, in disagreement with the
observed Fe K\alpha emission. Therefore, we consider it more likely that the
lags > 2 keV are caused by propagation effects in the accretion flow, possibly
related to the accretion disc fluctuations which have been observed previously.
View original:
http://arxiv.org/abs/1202.4881
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