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M. Méndez and T. M. Belloni
the possibility to probe some of the most extreme predictions of General Relativity,
such as dragging of inertial frames and periastron precession at rates that are 16
orders of magnitude faster than those observed in the solar system and, ultimately,
the existence of a minimum distance at which a stable orbit around a compact object
is possible. Here we review the last 20 years of research on kHz QPOs, and we
discuss the prospects for future developments in this field.
6.1 Introduction
Fast time variability from accreting X-ray binaries has become in the past decades a
very important tool for our understanding of the process of accretion onto compact
objects. As emission properties change on time scales well below a second, it is
impossible to ignore variability while concentrating solely on spectral analysis. It
was the Rossi X-ray Timing Explorer (RXTE) satellite with its large-area PCA
instrument that allowed us to probe very fast time scales, below 10 ms. In this
regime, we are exploring the accreting flow very close to the compact object,
whether it is a black hole or a neutron star. So deep into the potential well the
effects of General Relativity in the strong-field regime can be observable and timing
analysis is a very direct way to explore them.
In neutron-star binaries, the phenomenology is particularly rich and complex,
with the presence of Quasi-Periodic Oscillations (QPOs) at frequencies of hundreds
of Hz, and even faster than 1 kHz. The frequencies of these oscillations are linked
to fundamental frequencies in a gravitational field, allowing us to probe General
Relativity in extreme gravitational fields. Most of what we learned comes from
the RXTE satellite, which ended in 2012, although new information is provided
by current missions such as Astrosat and NICER. New, much more sensitive,
instruments are being planned, like the Chinese-European satellite eXTP, and when
they become operative we expect a real explosion of new results. This will take
several years; here we review the current state of research for neutron-star binaries,
concentrating on high-frequency oscillations.
6.2 History
The history of aperiodic variability from X-Ray Binaries began in the early
days of X-ray astronomy. After several detections of spurious pulse periods from
Cygnus X-1 were reported, the idea that the observed variability was the result
of an incoherent process (shot noise) was put forward [155]. Further observations
with more advanced instrumentation led to the production of the first statistically
significant Power Density Spectra (PDS) from which the strong noise from this
black hole candidate was defined and found to be more complex than a simple shot
noise (see e.g. [123]).
M. Méndez and T. M. Belloni
the possibility to probe some of the most extreme predictions of General Relativity,
such as dragging of inertial frames and periastron precession at rates that are 16
orders of magnitude faster than those observed in the solar system and, ultimately,
the existence of a minimum distance at which a stable orbit around a compact object
is possible. Here we review the last 20 years of research on kHz QPOs, and we
discuss the prospects for future developments in this field.
6.1 Introduction
Fast time variability from accreting X-ray binaries has become in the past decades a
very important tool for our understanding of the process of accretion onto compact
objects. As emission properties change on time scales well below a second, it is
impossible to ignore variability while concentrating solely on spectral analysis. It
was the Rossi X-ray Timing Explorer (RXTE) satellite with its large-area PCA
instrument that allowed us to probe very fast time scales, below 10 ms. In this
regime, we are exploring the accreting flow very close to the compact object,
whether it is a black hole or a neutron star. So deep into the potential well the
effects of General Relativity in the strong-field regime can be observable and timing
analysis is a very direct way to explore them.
In neutron-star binaries, the phenomenology is particularly rich and complex,
with the presence of Quasi-Periodic Oscillations (QPOs) at frequencies of hundreds
of Hz, and even faster than 1 kHz. The frequencies of these oscillations are linked
to fundamental frequencies in a gravitational field, allowing us to probe General
Relativity in extreme gravitational fields. Most of what we learned comes from
the RXTE satellite, which ended in 2012, although new information is provided
by current missions such as Astrosat and NICER. New, much more sensitive,
instruments are being planned, like the Chinese-European satellite eXTP, and when
they become operative we expect a real explosion of new results. This will take
several years; here we review the current state of research for neutron-star binaries,
concentrating on high-frequency oscillations.
6.2 History
The history of aperiodic variability from X-Ray Binaries began in the early
days of X-ray astronomy. After several detections of spurious pulse periods from
Cygnus X-1 were reported, the idea that the observed variability was the result
of an incoherent process (shot noise) was put forward [155]. Further observations
with more advanced instrumentation led to the production of the first statistically
significant Power Density Spectra (PDS) from which the strong noise from this
black hole candidate was defined and found to be more complex than a simple shot
noise (see e.g. [123]).