4 Accreting Millisecond X-ray Pulsars
189
accretor. This model also requires that the NSs in most LMXBs are more massive
that those in AMXPs. Although possible, this is difficult to reconcile with the fact
that most AMXPs have probably accreted for a long time (see for example [230]).
The model also requires some special configuration of the accretion column, which
existed for only a very short time and has never recurred, to explain the short pulsed
episode of Aql X-1. Finally, the light bending effect never suppresses completely
oscillation amplitudes since the Doppler shift due to the rapid NS rotation introduces
anisotropies in the emission pattern and thus pulsations [277, 344].
Magnetic and Rotational Axes Alignment The possibility that the magnetic poles
of NS migrate was first suggested by Ruderman in 1991 [298]. Vortex/fluxoid
coupling in the NS interior links the spin and magnetic axes. Once the NS is
spun-up by accretion, crustal shear stress breaks the crust into “plates” that drift
towards the rotational poles. The phenomenology of an AMXP with a nearly
aligned magnetic and rotational axis was developed recently [178] and explains
fairly well the observed correlations between pulse amplitude, pulse phase and Xray flux. The model can explain intermittency [179], but implies that the persistent
AMXPs should also show occasional “oscillation dropouts”, a sudden and brief
disappearance of pulsations due to sporadic alignment of the hot spot and rotational
axis. Such dropouts have not been observed, although the timescale over which this
might happen may be too short compared to timescale over which the pulsations are
measured (i.e., hundreds of seconds). The model also predicts correlations between
excess background noise produced by the hot spot motion which is anti-correlated
with the pulse amplitude. This prediction could in principle be easily verified in the
future with observational tests.
MHD Instabilities Accretion onto a magnetized NS can proceed via channeled
accretion but also via the Schwarzschild-Kruskal instability (the magnetic version
of the Rayleigh-Taylor instability) in the disk equatorial plane [10, 85]. This latter
mode of accretion has now been observed in 3D-MHD simulations [172] and
predicts a decoherence of the X-ray signal at high accretion rates, when several
“tongues” of plasma penetrate the magnetosphere and impact the NS surface at
random positions. This model explains why bright LMXBs do not pulsate, and
might explain why intermittent sources are on average brighter than persistent
AMXPs. However, the intermittent AMXP SAX J1748.9-2021 has shown pulsations
that appear and disappear in a broad range of accretion rates [253] (rather than at a
sharp threshold). It is also hard to explain why tongues never develop in any other
AMXP when the outbursts reach peak luminosity, during which the accretion rates
are comparable or higher than observed in intermittent AMXPs.
4.7 Thermonuclear Bursts
Type I X-ray bursts are thermonuclear explosions triggered by unstable burning
of hydrogen or helium on the layers that build up on the surfaces of accreting
NSs. The basic cause is an imbalance between nuclear heating and radiative
189
accretor. This model also requires that the NSs in most LMXBs are more massive
that those in AMXPs. Although possible, this is difficult to reconcile with the fact
that most AMXPs have probably accreted for a long time (see for example [230]).
The model also requires some special configuration of the accretion column, which
existed for only a very short time and has never recurred, to explain the short pulsed
episode of Aql X-1. Finally, the light bending effect never suppresses completely
oscillation amplitudes since the Doppler shift due to the rapid NS rotation introduces
anisotropies in the emission pattern and thus pulsations [277, 344].
Magnetic and Rotational Axes Alignment The possibility that the magnetic poles
of NS migrate was first suggested by Ruderman in 1991 [298]. Vortex/fluxoid
coupling in the NS interior links the spin and magnetic axes. Once the NS is
spun-up by accretion, crustal shear stress breaks the crust into “plates” that drift
towards the rotational poles. The phenomenology of an AMXP with a nearly
aligned magnetic and rotational axis was developed recently [178] and explains
fairly well the observed correlations between pulse amplitude, pulse phase and Xray flux. The model can explain intermittency [179], but implies that the persistent
AMXPs should also show occasional “oscillation dropouts”, a sudden and brief
disappearance of pulsations due to sporadic alignment of the hot spot and rotational
axis. Such dropouts have not been observed, although the timescale over which this
might happen may be too short compared to timescale over which the pulsations are
measured (i.e., hundreds of seconds). The model also predicts correlations between
excess background noise produced by the hot spot motion which is anti-correlated
with the pulse amplitude. This prediction could in principle be easily verified in the
future with observational tests.
MHD Instabilities Accretion onto a magnetized NS can proceed via channeled
accretion but also via the Schwarzschild-Kruskal instability (the magnetic version
of the Rayleigh-Taylor instability) in the disk equatorial plane [10, 85]. This latter
mode of accretion has now been observed in 3D-MHD simulations [172] and
predicts a decoherence of the X-ray signal at high accretion rates, when several
“tongues” of plasma penetrate the magnetosphere and impact the NS surface at
random positions. This model explains why bright LMXBs do not pulsate, and
might explain why intermittent sources are on average brighter than persistent
AMXPs. However, the intermittent AMXP SAX J1748.9-2021 has shown pulsations
that appear and disappear in a broad range of accretion rates [253] (rather than at a
sharp threshold). It is also hard to explain why tongues never develop in any other
AMXP when the outbursts reach peak luminosity, during which the accretion rates
are comparable or higher than observed in intermittent AMXPs.
4.7 Thermonuclear Bursts
Type I X-ray bursts are thermonuclear explosions triggered by unstable burning
of hydrogen or helium on the layers that build up on the surfaces of accreting
NSs. The basic cause is an imbalance between nuclear heating and radiative