5 Thermonuclear X-ray Bursts
217
5.1.2 Status of Burst Observations
Alongside the developments in theory have been commensurate improvements in
our understanding of the phenomenology of bursts, enabled in part by the assembly
of large samples from long-duration X-ray missions.
A clear distinction has been established between sources that accrete mixed
H/He, and those which accrete pure (or almost pure) He, as from an “ultracompact”
companion (e.g. [53]). Sources accreting mixed H/He characteristically show
regular, consistent bursts with long (≈5 s) rise times and decays of a few minutes,
understood to be powered by rp-process burning (e.g. [159]; Fig. 5.3). However, for
most sources such bursts are only seen episodically, within the low/hard spectral
state (see below; a notable exception is the so-called “Clocked Burster”, GS 182624 [183]). At high accretion rates, mixed H/He accretors tend to show bursts with
much shorter rise and decay times, often with burst oscillations (see Sect. 5.5)
and photospheric radius expansion (PRE; [114, 182]). This latter phenomenon is
detectable by a temporary rise in the apparent (blackbody) radius (Fig. 5.4; see also
Sect. 5.3.1) around the peak of the burst, accompanied by a decrease in temperature
sufficient to maintain a roughly constant luminosity. This behaviour is understood
to arise when the burst luminosity reaches the (local) Eddington limit at the surface,
resulting in a temporary expansion of the photosphere. When the energy input from
the burning can no longer support the expanded photosphere, it falls back onto the
star, resulting in a secondary increase in temperature (the “touchdown point”) and
followed by cooling at roughly constant radius.
Mixed H/He accretors are also the only sources that exhibit short recurrence
time bursts (e.g. [99]), weak events occurring just a few minutes after a brighter
event. These events are significant because the interval since the previous burst is
insufficient to reach the critical conditions for burst ignition (see Sect. 5.2.2).
Burst sources where the accreted fuel is H-deficient, consistently show bursts
characteristic of largely He-fuel, with short (1 s) rise times and durations of 10–
20 s (Fig. 5.3; see also Sect. 5.7). One of the best-studied examples is 3A 1820−30,
which consists of a neutron star in an 11-min orbit with an evolved companion (e.g.
[32]). PRE bursts are more common, except at high accretion rates.
A secondary distinction has emerged between bursts that occur in the low (hard)
persistent spectral state, and those in the high (soft) spectral state. These states
are defined both by their spectral shape, and their periodic (and aperiodic) timing
features (e.g. [69]). The persistent spectral states are thought to indicate different
characteristic accretion rate regimes; the low (hard) state, a truncated disk with the
innermost accretion occurring through an optically thin, spherical flow; and the high
(soft) state, with a disk extending to the NS surface and terminating in a boundary
layer providing most of the persistent X-ray emission (e.g. [40]). Remarkably, these
different accretion states also seem to give rise to markedly different burst behaviour.
217
5.1.2 Status of Burst Observations
Alongside the developments in theory have been commensurate improvements in
our understanding of the phenomenology of bursts, enabled in part by the assembly
of large samples from long-duration X-ray missions.
A clear distinction has been established between sources that accrete mixed
H/He, and those which accrete pure (or almost pure) He, as from an “ultracompact”
companion (e.g. [53]). Sources accreting mixed H/He characteristically show
regular, consistent bursts with long (≈5 s) rise times and decays of a few minutes,
understood to be powered by rp-process burning (e.g. [159]; Fig. 5.3). However, for
most sources such bursts are only seen episodically, within the low/hard spectral
state (see below; a notable exception is the so-called “Clocked Burster”, GS 182624 [183]). At high accretion rates, mixed H/He accretors tend to show bursts with
much shorter rise and decay times, often with burst oscillations (see Sect. 5.5)
and photospheric radius expansion (PRE; [114, 182]). This latter phenomenon is
detectable by a temporary rise in the apparent (blackbody) radius (Fig. 5.4; see also
Sect. 5.3.1) around the peak of the burst, accompanied by a decrease in temperature
sufficient to maintain a roughly constant luminosity. This behaviour is understood
to arise when the burst luminosity reaches the (local) Eddington limit at the surface,
resulting in a temporary expansion of the photosphere. When the energy input from
the burning can no longer support the expanded photosphere, it falls back onto the
star, resulting in a secondary increase in temperature (the “touchdown point”) and
followed by cooling at roughly constant radius.
Mixed H/He accretors are also the only sources that exhibit short recurrence
time bursts (e.g. [99]), weak events occurring just a few minutes after a brighter
event. These events are significant because the interval since the previous burst is
insufficient to reach the critical conditions for burst ignition (see Sect. 5.2.2).
Burst sources where the accreted fuel is H-deficient, consistently show bursts
characteristic of largely He-fuel, with short (1 s) rise times and durations of 10–
20 s (Fig. 5.3; see also Sect. 5.7). One of the best-studied examples is 3A 1820−30,
which consists of a neutron star in an 11-min orbit with an evolved companion (e.g.
[32]). PRE bursts are more common, except at high accretion rates.
A secondary distinction has emerged between bursts that occur in the low (hard)
persistent spectral state, and those in the high (soft) spectral state. These states
are defined both by their spectral shape, and their periodic (and aperiodic) timing
features (e.g. [69]). The persistent spectral states are thought to indicate different
characteristic accretion rate regimes; the low (hard) state, a truncated disk with the
innermost accretion occurring through an optically thin, spherical flow; and the high
(soft) state, with a disk extending to the NS surface and terminating in a boundary
layer providing most of the persistent X-ray emission (e.g. [40]). Remarkably, these
different accretion states also seem to give rise to markedly different burst behaviour.