272
M. Méndez and T. M. Belloni
(a)
(b)
Fig. 6.2 Left panel (a): Frequency of the lower kHz QPO in 4U 1608−52 vs. the count rate of
the source in the 2–16 keV band. Each point corresponds to a measurement over intervals of ∼64–
128 s. The frequency and the source count rate are significantly correlated over intervals of a
few thousand seconds, producing each of the tracks in the plot, but different observations, made
a few days or weeks apart, produce different tracks. This plot shows the so-called parallel tracks
(originally published as Figure 2 in [109]). Right panel (b): the distribution of the observations
with kHz QPOs on the colour-colour diagram of 4U 1636−53 [188]. Black points correspond
to observations in which only the upper kHz QPO was detected, while red points correspond to
observations that showed only the lower, or both the lower and the upper, kHz QPO. The grey
points indicate observations where no kHz QPO were detected. The parameter S a gives the length
along the solid line, which parametrises the position of the source in this diagram. High values of
S a correspond to high values of inferred ˙
M
Figure 6.2b shows the colour-colour diagram of 4U 1636−53. The red and black
points indicate, respectively, the observations in which the lower and the upper kHz
QPOs were detected. The solid line parameterises the position of the source in this
diagram, through the variable S a . High values of S a correspond to high values
of inferred ˙
M. The frequency of the lower kHz QPO increases as S a increases.
The upper kHz QPO, on the contrary, covers a broader range in the colour-colour
diagram, with the frequency of the QPO increasing as the source moves from
the low-luminosity hard state, via the transitional intermediate state to the highluminosity soft state (increasing value of S a ). This is the sense in which mass
accretion rate is inferred to increase in these sources. For completeness, the grey
points mark observations in which no kHz QPO was detected. The observations
with no kHz QPOs are at the extremes of the C-shaped figure traced by the source
in the colour-colour diagram; at the top right the source is in the low-luminosity
hard state, where the inferred ˙
M is the lowest, while at the bottom right it is in the
high-luminosity soft state, where the inferred ˙
M is the highest.
Although not apparent from the plot, there are several observations in which both
QPOs were detected simultaneously. This can be seen in Fig. 6.3; the top panel of
that Figure shows the frequency of both kHz QPOs as a function of the hard colour,
M. Méndez and T. M. Belloni
(a)
(b)
Fig. 6.2 Left panel (a): Frequency of the lower kHz QPO in 4U 1608−52 vs. the count rate of
the source in the 2–16 keV band. Each point corresponds to a measurement over intervals of ∼64–
128 s. The frequency and the source count rate are significantly correlated over intervals of a
few thousand seconds, producing each of the tracks in the plot, but different observations, made
a few days or weeks apart, produce different tracks. This plot shows the so-called parallel tracks
(originally published as Figure 2 in [109]). Right panel (b): the distribution of the observations
with kHz QPOs on the colour-colour diagram of 4U 1636−53 [188]. Black points correspond
to observations in which only the upper kHz QPO was detected, while red points correspond to
observations that showed only the lower, or both the lower and the upper, kHz QPO. The grey
points indicate observations where no kHz QPO were detected. The parameter S a gives the length
along the solid line, which parametrises the position of the source in this diagram. High values of
S a correspond to high values of inferred ˙
M
Figure 6.2b shows the colour-colour diagram of 4U 1636−53. The red and black
points indicate, respectively, the observations in which the lower and the upper kHz
QPOs were detected. The solid line parameterises the position of the source in this
diagram, through the variable S a . High values of S a correspond to high values
of inferred ˙
M. The frequency of the lower kHz QPO increases as S a increases.
The upper kHz QPO, on the contrary, covers a broader range in the colour-colour
diagram, with the frequency of the QPO increasing as the source moves from
the low-luminosity hard state, via the transitional intermediate state to the highluminosity soft state (increasing value of S a ). This is the sense in which mass
accretion rate is inferred to increase in these sources. For completeness, the grey
points mark observations in which no kHz QPO was detected. The observations
with no kHz QPOs are at the extremes of the C-shaped figure traced by the source
in the colour-colour diagram; at the top right the source is in the low-luminosity
hard state, where the inferred ˙
M is the lowest, while at the bottom right it is in the
high-luminosity soft state, where the inferred ˙
M is the highest.
Although not apparent from the plot, there are several observations in which both
QPOs were detected simultaneously. This can be seen in Fig. 6.3; the top panel of
that Figure shows the frequency of both kHz QPOs as a function of the hard colour,