266
M. Méndez and T. M. Belloni
contains an instrument similar to the main one on board RXTE [3] and of the NICER
experiment on board the International Space Station [53] have opened a new window
onto these phenomena, while future missions like eXTP are being studied.
6.3 Basic Frequencies Close to a Neutron Star
The accretion flow around a neutron star is a very complex physical system. In
order to study the time variability of the emitted flux, it is important to consider the
expected characteristic time scales that might be observed, leaving aside the issue
of the mechanism that will give rise to flux variability.
• Neutron stars in LMXBs are expected to be rapidly rotating, based on evolutionary scenarios [154]. An obvious characteristic time to consider is the rotational
period of the central object, which would manifest itself in the form of a coherent
signal. Rotational frequencies higher than 100 Hz are known for 26 systems, with
the fastest being currently 620 Hz (see chapter by Patruno and Watts and [179]).
• A particle orbiting a compact object defines an obvious time scale, that of the
period of its orbit (hereafter dynamical time scale t K ). In the vicinity of a neutron
star, the space time is affected by the presence of the compact object and an
expression from General Relativity has to be used.
• The accretion flow around a compact object is made of different components
whose physical nature and emission properties are very varied, more for a neutron
star than for a black hole (see e.g. [94], and references therein). Depending on
the model and on the source state, we have: (a) the surface of the neutron star,
onto which the accreting matter is deposited; (b) a boundary layer between the
star and the accretion flow, where the speed of the material in the disc needs
to drop rather quickly to adjust to the slower rotation speed of the neutron-star
surface; (c) a geometrically thin accretion disc; (d) a Comptonising medium
whose spatial location is not yet firmly established; (e) a relativistic jet where
matter is ejected from the system at a speed close to that of light. In addition,
although the magnetic field of the neutron star in a LMXB is expected to be low,
of the order of 10 8 G, nevertheless the presence of a magnetosphere has influence
onto the accretion flow.
Close the surface of the neutron star surface, matter orbits with a speed close to
half the speed of light, c. A number of fundamental time scales can be identified. The
light-crossing time, t LC , is shorter than a millisecond and is potentially detectable
in time delays between signals. For sub-Keplerian flows, the free-fall time scale t ff
can become important. In an optically thick and geometrically thin disc, in addition
to the shortest characteristic time scale corresponding to the dynamical timescale t K
(see above), other important time scales are the viscous time scale, t disc , on which
matter diffuses through the disc due to viscosity, the vertical time scale, t z , on which
vertical deviations from the hydrostatic equilibrium are damped, and the thermal
time scale, t th . on which deviations from thermal equilibrium are damped (see [52]).
M. Méndez and T. M. Belloni
contains an instrument similar to the main one on board RXTE [3] and of the NICER
experiment on board the International Space Station [53] have opened a new window
onto these phenomena, while future missions like eXTP are being studied.
6.3 Basic Frequencies Close to a Neutron Star
The accretion flow around a neutron star is a very complex physical system. In
order to study the time variability of the emitted flux, it is important to consider the
expected characteristic time scales that might be observed, leaving aside the issue
of the mechanism that will give rise to flux variability.
• Neutron stars in LMXBs are expected to be rapidly rotating, based on evolutionary scenarios [154]. An obvious characteristic time to consider is the rotational
period of the central object, which would manifest itself in the form of a coherent
signal. Rotational frequencies higher than 100 Hz are known for 26 systems, with
the fastest being currently 620 Hz (see chapter by Patruno and Watts and [179]).
• A particle orbiting a compact object defines an obvious time scale, that of the
period of its orbit (hereafter dynamical time scale t K ). In the vicinity of a neutron
star, the space time is affected by the presence of the compact object and an
expression from General Relativity has to be used.
• The accretion flow around a compact object is made of different components
whose physical nature and emission properties are very varied, more for a neutron
star than for a black hole (see e.g. [94], and references therein). Depending on
the model and on the source state, we have: (a) the surface of the neutron star,
onto which the accreting matter is deposited; (b) a boundary layer between the
star and the accretion flow, where the speed of the material in the disc needs
to drop rather quickly to adjust to the slower rotation speed of the neutron-star
surface; (c) a geometrically thin accretion disc; (d) a Comptonising medium
whose spatial location is not yet firmly established; (e) a relativistic jet where
matter is ejected from the system at a speed close to that of light. In addition,
although the magnetic field of the neutron star in a LMXB is expected to be low,
of the order of 10 8 G, nevertheless the presence of a magnetosphere has influence
onto the accretion flow.
Close the surface of the neutron star surface, matter orbits with a speed close to
half the speed of light, c. A number of fundamental time scales can be identified. The
light-crossing time, t LC , is shorter than a millisecond and is potentially detectable
in time delays between signals. For sub-Keplerian flows, the free-fall time scale t ff
can become important. In an optically thick and geometrically thin disc, in addition
to the shortest characteristic time scale corresponding to the dynamical timescale t K
(see above), other important time scales are the viscous time scale, t disc , on which
matter diffuses through the disc due to viscosity, the vertical time scale, t z , on which
vertical deviations from the hydrostatic equilibrium are damped, and the thermal
time scale, t th . on which deviations from thermal equilibrium are damped (see [52]).