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A. Patruno and A. L. Watts
been identified with ground based observations and in some cases have led to the
discovery of the spectral type of the donor, while radio and infrared observations
have revealed the possible presence of jets.
This chapter is structured as follows:
Section 4.2. An overview of the AMXP family.
Section 4.3. Observations of individual AMXPs
Section 4.4. Coherent timing analysis and accretion torques.
Section 4.5. X-ray pulse properties, their formation and use.
Section 4.6. Long-term evolution of spins and orbital parameters.
Section 4.7. Thermonuclear bursts and burst oscillations.
Section 4.8. Aperiodic phenomena including kilohertz QPOs.
Section 4.9. Future developments and open questions.
4.2 The Accreting Millisecond X-ray Pulsar Family
An AMXP is an accretion powered X-ray pulsar spinning at frequencies ν 50 Hz,
with weak surface magnetic fields (B∼10 8−9 G) and bound in a binary system with
a donor companion of mass M 1 M . All known AMXPs have donor stars that
transfer mass via Roche lobe overflow. This definition excludes binary millisecond
pulsars, which are detached systems with a pulsar powered by its rotational energy,
X-ray pulsars in high mass X-ray binaries, symbiotic X-ray binaries and the slow
X-ray pulsars in LMXBs with ν < 50 Hz. Several LMXBs show millisecond
oscillations during thermonuclear bursts that ignite on their surface. These nuclear
powered X-ray pulsars (NXPs), or burst oscillation sources, are not AMXPs, since
they are powered by nuclear burning rather than channeled accretion (although note
that some AMXPs also show burst oscillations).
Table 4.1 reports the main characteristics of the 23 known pulsating NSs in
LMXBs: 15 AMXPs, 3 slow accreting X-ray pulsars in LMXBs, 1 slow accreting Xray pulsar in an intermediate mass X-ray binary (IMXB), 3 symbiotic X-ray binaries
accreting wind from a K/M giant companion (compatible with being of low mass
type) and 1 mildly recycled accreting pulsar recently discovered in the globular
cluster Terzan 5. This latter source is an 11 Hz X-ray pulsar with a magnetic field of
10 9 –10 10 G and is of particular interest because it might be the only known accreting
pulsar in the process of becoming an AMXP on a short timescale [264]. The other
four slow pulsars are very different systems, in the sense that they have most likely
followed a completely different evolutionary history, have strong magnetic fields
(B∼10 12 G) and may never reach millisecond periods. Symbiotic X-ray binaries
are also different from the AMXPs since they are wide binaries (P b 50 days)
and their NSs have extremely long spin periods caused by a prolonged phase of
spin-down during a wind accretion process.
All of the AMXPs are transient systems, with accretion disks that run through
cycles of outburst and quiescence. X-ray pulsations have never been observed in
A. Patruno and A. L. Watts
been identified with ground based observations and in some cases have led to the
discovery of the spectral type of the donor, while radio and infrared observations
have revealed the possible presence of jets.
This chapter is structured as follows:
Section 4.2. An overview of the AMXP family.
Section 4.3. Observations of individual AMXPs
Section 4.4. Coherent timing analysis and accretion torques.
Section 4.5. X-ray pulse properties, their formation and use.
Section 4.6. Long-term evolution of spins and orbital parameters.
Section 4.7. Thermonuclear bursts and burst oscillations.
Section 4.8. Aperiodic phenomena including kilohertz QPOs.
Section 4.9. Future developments and open questions.
4.2 The Accreting Millisecond X-ray Pulsar Family
An AMXP is an accretion powered X-ray pulsar spinning at frequencies ν 50 Hz,
with weak surface magnetic fields (B∼10 8−9 G) and bound in a binary system with
a donor companion of mass M 1 M . All known AMXPs have donor stars that
transfer mass via Roche lobe overflow. This definition excludes binary millisecond
pulsars, which are detached systems with a pulsar powered by its rotational energy,
X-ray pulsars in high mass X-ray binaries, symbiotic X-ray binaries and the slow
X-ray pulsars in LMXBs with ν < 50 Hz. Several LMXBs show millisecond
oscillations during thermonuclear bursts that ignite on their surface. These nuclear
powered X-ray pulsars (NXPs), or burst oscillation sources, are not AMXPs, since
they are powered by nuclear burning rather than channeled accretion (although note
that some AMXPs also show burst oscillations).
Table 4.1 reports the main characteristics of the 23 known pulsating NSs in
LMXBs: 15 AMXPs, 3 slow accreting X-ray pulsars in LMXBs, 1 slow accreting Xray pulsar in an intermediate mass X-ray binary (IMXB), 3 symbiotic X-ray binaries
accreting wind from a K/M giant companion (compatible with being of low mass
type) and 1 mildly recycled accreting pulsar recently discovered in the globular
cluster Terzan 5. This latter source is an 11 Hz X-ray pulsar with a magnetic field of
10 9 –10 10 G and is of particular interest because it might be the only known accreting
pulsar in the process of becoming an AMXP on a short timescale [264]. The other
four slow pulsars are very different systems, in the sense that they have most likely
followed a completely different evolutionary history, have strong magnetic fields
(B∼10 12 G) and may never reach millisecond periods. Symbiotic X-ray binaries
are also different from the AMXPs since they are wide binaries (P b 50 days)
and their NSs have extremely long spin periods caused by a prolonged phase of
spin-down during a wind accretion process.
All of the AMXPs are transient systems, with accretion disks that run through
cycles of outburst and quiescence. X-ray pulsations have never been observed in