6 High-Frequency Variability in Neutron-Star Low-Mass X-ray Binaries
309
Fig. 6.30 Time lags of the lower kHz QPO in 4U 1608−52 (top (a); originally published as
Figure 6 in [30]) and the upper kHz QPO in 4U 1728−34 (bottom (b); originally published as
Figure 6 in [36]) as a function of energy, with the best-fitting reverberation model
spectrum, but a fraction of those corona photons will illuminate back the disc,
increasing the disc temperature [55]. If the photon flux that is originally produced
in the disc is modulated (e.g., at the QPO frequency), the flux that returns to the
disc after being scattered in the corona will also be modulated and, as those photons
reheat the disc, the disc temperature will vary at the frequency of that modulation. In
this feedback loop, the photons that reach the observer from the corona through the
309
Fig. 6.30 Time lags of the lower kHz QPO in 4U 1608−52 (top (a); originally published as
Figure 6 in [30]) and the upper kHz QPO in 4U 1728−34 (bottom (b); originally published as
Figure 6 in [36]) as a function of energy, with the best-fitting reverberation model
spectrum, but a fraction of those corona photons will illuminate back the disc,
increasing the disc temperature [55]. If the photon flux that is originally produced
in the disc is modulated (e.g., at the QPO frequency), the flux that returns to the
disc after being scattered in the corona will also be modulated and, as those photons
reheat the disc, the disc temperature will vary at the frequency of that modulation. In
this feedback loop, the photons that reach the observer from the corona through the