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M. Méndez and T. M. Belloni
it is difficult to assign a characteristic frequency (time scale) to it, whereas relatively
narrow QPOs give good frequencies which are easier to extract and follow over time,
and can be treated in a model-independent way. This approach has been tried in a
few cases, and interesting correlations among the properties of those (sometimes
weak) Lorentzians have emerged. We will mention some of those in the coming
sections.
The Rossi RXTE mission yielded thousands of high-sensitivity observations
of X-ray binaries and revolutionised our knowledge of these objects. RXTE
observations covered a large range of states of dozens of X-ray binaries, unveiling
details of the variability of these objects that helped us understand them more
deeply. One of the discoveries of RXTE was the existence of very-high frequency
quasi-periodic variability components, up to ∼1200 Hz, in several NS LMXBs.
These variability components are what we call the kHz QPOs. Other variability
components were also studied with RXTE, including low-frequency QPOs and
broad-band noise components. We will mention some of those in passing when
necessary, but here we will concentrate mainly on the properties of the kHz QPOs.
At a very basic level, NS LMXBs can be subdivided into three classes: (i)
persistent sources at high luminosity, historically called “Z” sources, that can reach
luminosities close to the Eddington limit for a neutron star, (ii) persistent and
transient sources that can become rather bright but, with top luminosities of 0.1–
0.2 Eddington, do not reach the same high luminosities as the Z sources, historically
called “atoll” sources, and (iii) faint sources which, even when transient, remain at
low luminosities, below 0.01 Eddington.
The power spectrum of the three classes of sources mentioned above show
several differences. For example, the amplitude of the variability components in
Z sources is generally (but not for all variability components) lower than in atoll
and low-luminosity sources. In Fig. 6.1 we show examples of the PDS of a lowluminosity (Fig. 6.1a; the NS LMXB 1E 1724−3045 in the globular cluster Terzan
2) and an atoll source (Fig. 6.1b; the NS LMXB 4U 1636−53). As it is apparent in
that Figure, all the PDS show a broad-band noise component extending up to ∼10–
20 Hz; above that frequency the power drops as the frequency increases, except for
a few relatively narrow features peaking at some specific frequencies. The narrow
peaks appearing above ∼400 Hz are the kHz QPOs. Notice also that the scales in the
y axis of the two Figures are different, and that the PDS on the Figure on the right
approaches a power level of 2 at high frequencies, whereas the one the left drops to
0 (the y axis on the left panel is in a log scale). The difference is that the powers in
Fig. 6.1a are in units of fractional rms 2 per Hz (see above), whereas in Fig. 6.1b the
power has not been converted to rms units. Furthermore, in Fig. 6.1a the contribution
of the constant level due to the Poisson nature of the counting process was subtracted
from the PDS, leaving only the signal from the source.
The first two sources to show kHz QPOs were the Z source Sco X-1 [164] and
the atoll source 4U 1728−34 [152]. Both sources displayed (sometimes) two QPOs
appearing simultaneously in the PDS at frequencies between ∼700 and ∼1100 Hz.
The two QPOs were then labeled “lower” and “upper” kHz QPO according to their
frequencies, such that ν upper > ν lower . If observed frequently enough, most sources
M. Méndez and T. M. Belloni
it is difficult to assign a characteristic frequency (time scale) to it, whereas relatively
narrow QPOs give good frequencies which are easier to extract and follow over time,
and can be treated in a model-independent way. This approach has been tried in a
few cases, and interesting correlations among the properties of those (sometimes
weak) Lorentzians have emerged. We will mention some of those in the coming
sections.
The Rossi RXTE mission yielded thousands of high-sensitivity observations
of X-ray binaries and revolutionised our knowledge of these objects. RXTE
observations covered a large range of states of dozens of X-ray binaries, unveiling
details of the variability of these objects that helped us understand them more
deeply. One of the discoveries of RXTE was the existence of very-high frequency
quasi-periodic variability components, up to ∼1200 Hz, in several NS LMXBs.
These variability components are what we call the kHz QPOs. Other variability
components were also studied with RXTE, including low-frequency QPOs and
broad-band noise components. We will mention some of those in passing when
necessary, but here we will concentrate mainly on the properties of the kHz QPOs.
At a very basic level, NS LMXBs can be subdivided into three classes: (i)
persistent sources at high luminosity, historically called “Z” sources, that can reach
luminosities close to the Eddington limit for a neutron star, (ii) persistent and
transient sources that can become rather bright but, with top luminosities of 0.1–
0.2 Eddington, do not reach the same high luminosities as the Z sources, historically
called “atoll” sources, and (iii) faint sources which, even when transient, remain at
low luminosities, below 0.01 Eddington.
The power spectrum of the three classes of sources mentioned above show
several differences. For example, the amplitude of the variability components in
Z sources is generally (but not for all variability components) lower than in atoll
and low-luminosity sources. In Fig. 6.1 we show examples of the PDS of a lowluminosity (Fig. 6.1a; the NS LMXB 1E 1724−3045 in the globular cluster Terzan
2) and an atoll source (Fig. 6.1b; the NS LMXB 4U 1636−53). As it is apparent in
that Figure, all the PDS show a broad-band noise component extending up to ∼10–
20 Hz; above that frequency the power drops as the frequency increases, except for
a few relatively narrow features peaking at some specific frequencies. The narrow
peaks appearing above ∼400 Hz are the kHz QPOs. Notice also that the scales in the
y axis of the two Figures are different, and that the PDS on the Figure on the right
approaches a power level of 2 at high frequencies, whereas the one the left drops to
0 (the y axis on the left panel is in a log scale). The difference is that the powers in
Fig. 6.1a are in units of fractional rms 2 per Hz (see above), whereas in Fig. 6.1b the
power has not been converted to rms units. Furthermore, in Fig. 6.1a the contribution
of the constant level due to the Poisson nature of the counting process was subtracted
from the PDS, leaving only the signal from the source.
The first two sources to show kHz QPOs were the Z source Sco X-1 [164] and
the atoll source 4U 1728−34 [152]. Both sources displayed (sometimes) two QPOs
appearing simultaneously in the PDS at frequencies between ∼700 and ∼1100 Hz.
The two QPOs were then labeled “lower” and “upper” kHz QPO according to their
frequencies, such that ν upper > ν lower . If observed frequently enough, most sources