5 Thermonuclear X-ray Bursts
211
features in the burst spectra; the phenomenon of mHz QPOs, and the link with steady
burning; the role of nuclear experimental physics, and the growing dialogue between
the different communities; progress in simulating bursts; and the growing role for
bursts in probing the environment around the neutron star.
This chapter serves as an update to a series of thorough previous reviews,
beginning in 1993 with [115] and the subsequent work developed from it [116];
the 1998 review [14], covering the current state of the theory and the unexpected
results from the European X-ray Observatory Satellite (EXOSAT); the 2006 review
(published initially in 2003), with its focus on new phenomena discovered in the
preceding decade, including burst oscillations [171]; and the 2012 review of burst
oscillations and related phenomena [193].
5.1.1 Theory of Burst Ignition and Nuclear Burning Regimes
Soon after their discovery, measurements of the ratio of burst to accretion energy
(the so-called α-parameter) established that X-ray bursts are produced by thermonuclear burning of accreted material [127, 199]. Here we introduce the mechanism of
runaway burning and present the classical picture of the different burning regimes.
5.1.1.1 Fuel Accretion from a Binary Companion Star
The fuel for X-ray bursts originates from the outer layers of a binary companion star.
Typically, it is expected to have a composition similar to the Sun, predominantly
hydrogen and helium, as well as small amounts of CNO and other metals. An
exception are the ultra-compact X-ray binaries (UCXBs; e.g., [79]) where most of
the accreted material is helium, with at most a small mass fraction of hydrogen
(10%; [31]). The material is transferred by Roche-lobe overflow to the neutron
star via an accretion disk at rates up to the Eddington limit, of ˙
M Edd = 3.0 ×
10 −8 M year −1 (1+X) −1 (R/10 km), with X the hydrogen mass fraction and R the
neutron star radius (ignoring general relativistic corrections). ˙
M can vary by orders
of magnitude on time scales of days to decades. In transient systems, accretion
occurs episodically, and is effectively switched off (“quiescence”) for long periods
of time in between. We will see that the properties of X-ray bursts depend on ˙
M,
and therefore a wide range of bursting behaviour may be observed from a single
accreting neutron star, as its ˙
M changes with time.
Material falling on the neutron star is expected to rapidly spread across its
surface, due to the lateral pressure gradients imposed by the strong (≈10 14 cm s −2 )
gravity. The presence of accretion-powered pulsations in some 16 low-mass binaries
(of which 8 are also burst sources; [148, 173]) implies that the magnetic field
strength is sometimes sufficient to confine the accreted fuel. For most bursting
sources the magnetic field is thought to be dynamically unimportant.
211
features in the burst spectra; the phenomenon of mHz QPOs, and the link with steady
burning; the role of nuclear experimental physics, and the growing dialogue between
the different communities; progress in simulating bursts; and the growing role for
bursts in probing the environment around the neutron star.
This chapter serves as an update to a series of thorough previous reviews,
beginning in 1993 with [115] and the subsequent work developed from it [116];
the 1998 review [14], covering the current state of the theory and the unexpected
results from the European X-ray Observatory Satellite (EXOSAT); the 2006 review
(published initially in 2003), with its focus on new phenomena discovered in the
preceding decade, including burst oscillations [171]; and the 2012 review of burst
oscillations and related phenomena [193].
5.1.1 Theory of Burst Ignition and Nuclear Burning Regimes
Soon after their discovery, measurements of the ratio of burst to accretion energy
(the so-called α-parameter) established that X-ray bursts are produced by thermonuclear burning of accreted material [127, 199]. Here we introduce the mechanism of
runaway burning and present the classical picture of the different burning regimes.
5.1.1.1 Fuel Accretion from a Binary Companion Star
The fuel for X-ray bursts originates from the outer layers of a binary companion star.
Typically, it is expected to have a composition similar to the Sun, predominantly
hydrogen and helium, as well as small amounts of CNO and other metals. An
exception are the ultra-compact X-ray binaries (UCXBs; e.g., [79]) where most of
the accreted material is helium, with at most a small mass fraction of hydrogen
(10%; [31]). The material is transferred by Roche-lobe overflow to the neutron
star via an accretion disk at rates up to the Eddington limit, of ˙
M Edd = 3.0 ×
10 −8 M year −1 (1+X) −1 (R/10 km), with X the hydrogen mass fraction and R the
neutron star radius (ignoring general relativistic corrections). ˙
M can vary by orders
of magnitude on time scales of days to decades. In transient systems, accretion
occurs episodically, and is effectively switched off (“quiescence”) for long periods
of time in between. We will see that the properties of X-ray bursts depend on ˙
M,
and therefore a wide range of bursting behaviour may be observed from a single
accreting neutron star, as its ˙
M changes with time.
Material falling on the neutron star is expected to rapidly spread across its
surface, due to the lateral pressure gradients imposed by the strong (≈10 14 cm s −2 )
gravity. The presence of accretion-powered pulsations in some 16 low-mass binaries
(of which 8 are also burst sources; [148, 173]) implies that the magnetic field
strength is sometimes sufficient to confine the accreted fuel. For most bursting
sources the magnetic field is thought to be dynamically unimportant.