4 Accreting Millisecond X-ray Pulsars
191
been a comprehensive study assessing whether the bursts from the AMXPs have
properties that differ in any statistically significant way from the non-pulsing
sources. However just as for the non-pulsing bursters, bursts from the AMXPs
show variable recurrence times, peak fluxes and fluences, and durations, even when
accretion rate seems to be relatively stable. There are nonetheless some features of
AMXP bursts that are worthy of note, in particular when they are compared with
bursting sources with higher magnetic fields like the mildly recycled pulsar IGR
J17480-2466.
Magnetic Fields and Bursting Bursts from AMXPs confirm that magnetic fields
∼10 8 –10 10 G (Table 4.1) are no impediment to thermonuclear bursting.
Burst Recurrence Times Many neutron star low mass X-ray binaries show burst
recurrence times of less than an hour, something that is problematic for theoretical
models [162]. The only AMXP in this short recurrence time group is the highly
intermittent source Aql X-1. By contrast the mildly recycled pulsar IGR J174802466 has the shortest recurrence time between thermonuclear bursts yet recorded, at
3.3 min [221], a possible sign of confined burning.
Rotation and Bursting The AMXPs, which have independent estimates of spin
rate, confirm that bursting can occur for spin rates of up to a few hundred Hz
(although the fastest rotating burster, 4U 1608-522, is a burst oscillation source
not an accretion-powered pulsar, so the spin measure is more indirect [122]). The
mildly-recycled accreting pulsar IGR J17480-2466 is instead the bursting source
with burst oscillations with the lowest measured rotation rate, at 11 Hz. For a
star rotating at ∼10 Hz, the Coriolis force is not dynamically relevant [46, 349],
indicating that Coriolis force induced confinement of the igniting patch is not
essential to the development of X-ray bursts [310] or to very short burst recurrence
times (see previous point).
Cooling as a Signature of Thermonuclear Bursts Cooling during the tails of bursts
(inferred from blackbody fits to the burst spectra) has long been regarded as a
signature trait of thermonuclear Type I (rather than accretion-powered Type II)
bursts. Detailed studies of the bursts from the mildly recycled pulsar IGR J174802466 have now shown that cooling is a sufficient but not necessary condition for
the identification of thermonuclear bursts: under certain circumstances, particularly
at high accretion rates, the cooling signature may not be readily detectable [48, 54,
193, 196]. Bursts from the AMXPs do however show cooling.
Marginally Stable Burning The mildly recycled pulsar IGR J17480-2466 shows
mHz QPOs that are interpreted as marginally stable burning of H and He on the NS
surface [128, 196]. The mHz QPOs are observed when the accretion rate rises and
the source transitions from unstable burning (X-ray bursts) to stable nuclear burning
(no bursts). The only AMXP to have shown mHz QPOs is the highest accretion rate
source, the intermittent pulsar Aql X-1 [286].
Pure Helium Bursts SAX J1808.4-3658 has very bright, short bursts that are
thought to be the only secure example of bursting occurring in a layer of pure
191
been a comprehensive study assessing whether the bursts from the AMXPs have
properties that differ in any statistically significant way from the non-pulsing
sources. However just as for the non-pulsing bursters, bursts from the AMXPs
show variable recurrence times, peak fluxes and fluences, and durations, even when
accretion rate seems to be relatively stable. There are nonetheless some features of
AMXP bursts that are worthy of note, in particular when they are compared with
bursting sources with higher magnetic fields like the mildly recycled pulsar IGR
J17480-2466.
Magnetic Fields and Bursting Bursts from AMXPs confirm that magnetic fields
∼10 8 –10 10 G (Table 4.1) are no impediment to thermonuclear bursting.
Burst Recurrence Times Many neutron star low mass X-ray binaries show burst
recurrence times of less than an hour, something that is problematic for theoretical
models [162]. The only AMXP in this short recurrence time group is the highly
intermittent source Aql X-1. By contrast the mildly recycled pulsar IGR J174802466 has the shortest recurrence time between thermonuclear bursts yet recorded, at
3.3 min [221], a possible sign of confined burning.
Rotation and Bursting The AMXPs, which have independent estimates of spin
rate, confirm that bursting can occur for spin rates of up to a few hundred Hz
(although the fastest rotating burster, 4U 1608-522, is a burst oscillation source
not an accretion-powered pulsar, so the spin measure is more indirect [122]). The
mildly-recycled accreting pulsar IGR J17480-2466 is instead the bursting source
with burst oscillations with the lowest measured rotation rate, at 11 Hz. For a
star rotating at ∼10 Hz, the Coriolis force is not dynamically relevant [46, 349],
indicating that Coriolis force induced confinement of the igniting patch is not
essential to the development of X-ray bursts [310] or to very short burst recurrence
times (see previous point).
Cooling as a Signature of Thermonuclear Bursts Cooling during the tails of bursts
(inferred from blackbody fits to the burst spectra) has long been regarded as a
signature trait of thermonuclear Type I (rather than accretion-powered Type II)
bursts. Detailed studies of the bursts from the mildly recycled pulsar IGR J174802466 have now shown that cooling is a sufficient but not necessary condition for
the identification of thermonuclear bursts: under certain circumstances, particularly
at high accretion rates, the cooling signature may not be readily detectable [48, 54,
193, 196]. Bursts from the AMXPs do however show cooling.
Marginally Stable Burning The mildly recycled pulsar IGR J17480-2466 shows
mHz QPOs that are interpreted as marginally stable burning of H and He on the NS
surface [128, 196]. The mHz QPOs are observed when the accretion rate rises and
the source transitions from unstable burning (X-ray bursts) to stable nuclear burning
(no bursts). The only AMXP to have shown mHz QPOs is the highest accretion rate
source, the intermittent pulsar Aql X-1 [286].
Pure Helium Bursts SAX J1808.4-3658 has very bright, short bursts that are
thought to be the only secure example of bursting occurring in a layer of pure