6 High-Frequency Variability in Neutron-Star Low-Mass X-ray Binaries
293
Fig. 6.17 Top (a): rms spectrum of the single kHz QPO in EXO 0748−676 (originally published
as Figure 4 in [57]), and the lower and upper kHz QPOs in 4U 1608−52 [24, 110]. Bottom (b): rms
(red) and covariance (black) spectrum of the upper kHz QPO in six sources (originally published
as Figure 5 in [157])
QPOs depend upon QPO frequency, and therefore one has to consider that to draw
conclusions from the comparisons). For instance, for 4U 1608−52 the amplitude
of the upper kHz QPO increases from ∼2–4% at ∼3 keV to ∼20% at ∼20 keV
[24, 110]. On the other hand, there is no evidence that the frequency or the width of
either of the kHz QPOs change with energy [121].
Figure 6.18 shows the rms spectrum of the lower and the upper kHz QPO
plotted in different colours for different QPO frequencies. For both kHz QPOs the
293
Fig. 6.17 Top (a): rms spectrum of the single kHz QPO in EXO 0748−676 (originally published
as Figure 4 in [57]), and the lower and upper kHz QPOs in 4U 1608−52 [24, 110]. Bottom (b): rms
(red) and covariance (black) spectrum of the upper kHz QPO in six sources (originally published
as Figure 5 in [157])
QPOs depend upon QPO frequency, and therefore one has to consider that to draw
conclusions from the comparisons). For instance, for 4U 1608−52 the amplitude
of the upper kHz QPO increases from ∼2–4% at ∼3 keV to ∼20% at ∼20 keV
[24, 110]. On the other hand, there is no evidence that the frequency or the width of
either of the kHz QPOs change with energy [121].
Figure 6.18 shows the rms spectrum of the lower and the upper kHz QPO
plotted in different colours for different QPO frequencies. For both kHz QPOs the