5 Thermonuclear X-ray Bursts
243
After the superburst has decayed, it continues to have an effect on the neutron
star envelope by quenching the occurrence of regular bursts for several weeks [107].
Heat deposited by the superburst in the envelope causes freshly accreted hydrogen
and helium to burn in a stable manner (regime VII in Table 5.1). Even after the
superburst has decayed, the stable burning can sustain itself for several weeks. Once
the envelope has cooled down sufficiently, the burning mode transitions back to
producing bursts [100].
Superbursts ignite close to the outer crust, and are sensitive to the thermal properties at that depth. This region is rather poorly understood, with observations of cooling quiescent sources suggest the presence of an unknown heat source [18, 38], and
where Urca neutrino cooling may play an important role in regulating the temperature [161]. In recent years superburst candidates have been detected from transient
sources [97, 164]. These observations pose several challenges. First, the persistent
flux may evolve on similar timescales to a superburst, and it is difficult to make the
definite determination of the thermonuclear nature of the event. Second, the superbursts occur within months or sometimes days of the start of an accretion outburst [3,
163]. Current theory predicts that the outer crust of the neutron star is not heated sufficiently at that time to ignite a carbon flash. Yet, superbursts are observed, suggesting the presence of an unknown heat source near the superburst ignition depth. This
may be related to the shallow heat source inferred for cooling quiescent sources [39].
It is challenging to explain the origin of the carbon fuel. Comparison of the
X-ray light curves to cooling models finds that the carbon mass fraction of the
superburst fuel is 15–30% [32]. The rest of the fuel may be iron and heavier elements
produced during the rp-process. If a substantial fraction is heavy isotopes near the
end point of the rp-process, their photodisintegration could account for as much as
50% of the superburst energetics [160]. However, simulations typically predict most
heavy isotopes to be near the iron-group [90, 201]. Carbon is typically not accreted
in substantial quantities, and therefore has to be produced by nuclear burning of
the accreted hydrogen and helium. 3α burning of helium during “normal” (mixed
H/He) bursts produces carbon, but the high temperatures reached during these events
enable α- and proton-captures also to destroy carbon. Detailed simulations predict
that the net carbon production by normal bursting activity is at most a mass fraction
of ≈5% [201]. Stable burning of hydrogen and helium can produce large quantities
of carbon [168], but this burning has also been thought to only take place at high
accretion rates, where the temperature in the envelope again is high enough to
allow for the reactions that destroy carbon. A new stable regime has recently been
identified in simulations [95], where stable burning produces copious quantities
of carbon at lower mass accretion rates near 0.1 ˙
M Edd , similar to where most
superbursts are observed. All superbursting sources also exhibit short hydrogen or
helium flashes, but they have a high value of the α-parameter [77]. This suggests
that a substantial part of hydrogen and helium are burned in a stable manner in
between the short bursts. Future studies will need to determine if this involves the
newly suggested stable regime as a source of the carbon fuel for the superbursts.
In the absence of spectroscopic information, superbursts provide the only
observational constraints on the composition of the ashes of hydrogen/helium
243
After the superburst has decayed, it continues to have an effect on the neutron
star envelope by quenching the occurrence of regular bursts for several weeks [107].
Heat deposited by the superburst in the envelope causes freshly accreted hydrogen
and helium to burn in a stable manner (regime VII in Table 5.1). Even after the
superburst has decayed, the stable burning can sustain itself for several weeks. Once
the envelope has cooled down sufficiently, the burning mode transitions back to
producing bursts [100].
Superbursts ignite close to the outer crust, and are sensitive to the thermal properties at that depth. This region is rather poorly understood, with observations of cooling quiescent sources suggest the presence of an unknown heat source [18, 38], and
where Urca neutrino cooling may play an important role in regulating the temperature [161]. In recent years superburst candidates have been detected from transient
sources [97, 164]. These observations pose several challenges. First, the persistent
flux may evolve on similar timescales to a superburst, and it is difficult to make the
definite determination of the thermonuclear nature of the event. Second, the superbursts occur within months or sometimes days of the start of an accretion outburst [3,
163]. Current theory predicts that the outer crust of the neutron star is not heated sufficiently at that time to ignite a carbon flash. Yet, superbursts are observed, suggesting the presence of an unknown heat source near the superburst ignition depth. This
may be related to the shallow heat source inferred for cooling quiescent sources [39].
It is challenging to explain the origin of the carbon fuel. Comparison of the
X-ray light curves to cooling models finds that the carbon mass fraction of the
superburst fuel is 15–30% [32]. The rest of the fuel may be iron and heavier elements
produced during the rp-process. If a substantial fraction is heavy isotopes near the
end point of the rp-process, their photodisintegration could account for as much as
50% of the superburst energetics [160]. However, simulations typically predict most
heavy isotopes to be near the iron-group [90, 201]. Carbon is typically not accreted
in substantial quantities, and therefore has to be produced by nuclear burning of
the accreted hydrogen and helium. 3α burning of helium during “normal” (mixed
H/He) bursts produces carbon, but the high temperatures reached during these events
enable α- and proton-captures also to destroy carbon. Detailed simulations predict
that the net carbon production by normal bursting activity is at most a mass fraction
of ≈5% [201]. Stable burning of hydrogen and helium can produce large quantities
of carbon [168], but this burning has also been thought to only take place at high
accretion rates, where the temperature in the envelope again is high enough to
allow for the reactions that destroy carbon. A new stable regime has recently been
identified in simulations [95], where stable burning produces copious quantities
of carbon at lower mass accretion rates near 0.1 ˙
M Edd , similar to where most
superbursts are observed. All superbursting sources also exhibit short hydrogen or
helium flashes, but they have a high value of the α-parameter [77]. This suggests
that a substantial part of hydrogen and helium are burned in a stable manner in
between the short bursts. Future studies will need to determine if this involves the
newly suggested stable regime as a source of the carbon fuel for the superbursts.
In the absence of spectroscopic information, superbursts provide the only
observational constraints on the composition of the ashes of hydrogen/helium