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D. K. Galloway and L. Keek
higher accretion rates [142]. There is some evidence [207, 208] linking the presence
of oscillations in the burst tails with the characteristic evolution of the blackbody
normalisation (proportional to the emitting area, up to a factor of the colour
correction; cf. with Fig. 5.4). However, since the spectral evolution is also correlated
with the persistent spectral state (e.g. [91, 152]), the causal relationship between
these phenomena remains unclear.
At the same time, theoretical studies have revisited the microphysics of the burning taking place in bursts. Simulations of flame propagation for helium detonations,
with possible application to intermediate-duration bursts (see Sect. 5.7.1) have been
performed [165, 210]; however, most bursts are likely deflagrations rather than
detonations. Other simulations have modelled the propagation of the burning front
(e.g. [21]), and the influence of the latitude of the ignition point and the neutron
star spin. Spreading is understood to occur at different speeds in the latitudinal and
longitudinal directions, and even at different speeds towards and away from the
equator [22, 166]. The discovery of Terzan 5 X-2, a burster with a spin rate of 11 Hz
(more than an order of magnitude slower than the next fastest burster) has provided
an additional avenue to such studies [20].
Along with accretion-powered pulsations, burst oscillations have held the
promise for constraining neutron star mass and radius, via measurements of the
Doppler shift of the burst spectrum. However, the most recent studies remain
inconclusive [5], suggesting that significantly higher signal-to-noise is required,
as might be provided by a next-generation mission such as the Enhanced X-ray
Timing and Polarization mission (eXTP; [209]), currently in phase A study with a
launch expected from 2025 or beyond. Another proposed mission offering effective
area significantly in excess of the RXTE/PCA is the Spectroscopic Time-Resolving
Observatory for Broadband Energy X-rays (STROBE-X; [197])
5.6 mHz Oscillations and Marginally Stable Burning
Quasi-periodic oscillations (QPOs) at mHz frequencies have been detected from
≈5% of bursting sources. The behaviour of the mHz QPOs is related to the
occurrence of bursts, and thus the QPOs are thought to result from an oscillatory
burning mode, as expected from marginally stable burning (regime VI in Table 5.1).
Numerical simulations of this burning regime broadly reproduce the observed properties of mHz QPOs, although some of the details do not match. The discrepancies
are principally related to the difficulty in matching theoretically-predicted burning
regimes to the observed mass accretion rates, and mHz QPOs may provide important
insight in how to improve theoretical expectations. Here we review the observations
of the mHz QPOs and the theory of marginally stable burning, and discuss to what
extent they match.
D. K. Galloway and L. Keek
higher accretion rates [142]. There is some evidence [207, 208] linking the presence
of oscillations in the burst tails with the characteristic evolution of the blackbody
normalisation (proportional to the emitting area, up to a factor of the colour
correction; cf. with Fig. 5.4). However, since the spectral evolution is also correlated
with the persistent spectral state (e.g. [91, 152]), the causal relationship between
these phenomena remains unclear.
At the same time, theoretical studies have revisited the microphysics of the burning taking place in bursts. Simulations of flame propagation for helium detonations,
with possible application to intermediate-duration bursts (see Sect. 5.7.1) have been
performed [165, 210]; however, most bursts are likely deflagrations rather than
detonations. Other simulations have modelled the propagation of the burning front
(e.g. [21]), and the influence of the latitude of the ignition point and the neutron
star spin. Spreading is understood to occur at different speeds in the latitudinal and
longitudinal directions, and even at different speeds towards and away from the
equator [22, 166]. The discovery of Terzan 5 X-2, a burster with a spin rate of 11 Hz
(more than an order of magnitude slower than the next fastest burster) has provided
an additional avenue to such studies [20].
Along with accretion-powered pulsations, burst oscillations have held the
promise for constraining neutron star mass and radius, via measurements of the
Doppler shift of the burst spectrum. However, the most recent studies remain
inconclusive [5], suggesting that significantly higher signal-to-noise is required,
as might be provided by a next-generation mission such as the Enhanced X-ray
Timing and Polarization mission (eXTP; [209]), currently in phase A study with a
launch expected from 2025 or beyond. Another proposed mission offering effective
area significantly in excess of the RXTE/PCA is the Spectroscopic Time-Resolving
Observatory for Broadband Energy X-rays (STROBE-X; [197])
5.6 mHz Oscillations and Marginally Stable Burning
Quasi-periodic oscillations (QPOs) at mHz frequencies have been detected from
≈5% of bursting sources. The behaviour of the mHz QPOs is related to the
occurrence of bursts, and thus the QPOs are thought to result from an oscillatory
burning mode, as expected from marginally stable burning (regime VI in Table 5.1).
Numerical simulations of this burning regime broadly reproduce the observed properties of mHz QPOs, although some of the details do not match. The discrepancies
are principally related to the difficulty in matching theoretically-predicted burning
regimes to the observed mass accretion rates, and mHz QPOs may provide important
insight in how to improve theoretical expectations. Here we review the observations
of the mHz QPOs and the theory of marginally stable burning, and discuss to what
extent they match.