144
A. Patruno and A. L. Watts
4.6 Long Term Evolution and Pulse Formation Process . .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 183
4.6.1 Specific Sources .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 183
4.6.2 The Maximum Spin Frequency of Neutron Stars .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 186
4.6.3 Why Do Most Low Mass X-ray Binaries Not Pulsate? . . ... .. .. .. .. .. .. .. .. .. .. .. .. 187
4.7 Thermonuclear Bursts . . .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 189
4.8 Aperiodic Variability and kHz QPOs . .. .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 194
4.9 Open Problems and Final Remarks . . ... .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 196
References . .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 197
Abstract Accreting Millisecond X-ray Pulsars (AMXPs) are astrophysical laboratories without parallel in the study of extreme physics. In this chapter we review the
past 15 years of discoveries in the field. We summarize the observations of the 15
known AMXPs, with a particular emphasis on the multi-wavelength observations
that have been carried out since the discovery of the first AMXP in 1998. We review
accretion torque theory, the pulse formation process, and how AMXP observations
have changed our view on the interaction of plasma and magnetic fields in strong
gravity. We also explain how the AMXPs have deepened our understanding of the
thermonuclear burst process, in particular the phenomenon of burst oscillations. We
conclude with a discussion of the open problems that remain to be addressed in the
future.
4.1 Introduction
Neutron stars (NSs), amongst the most extreme astrophysical objects in the Universe, allow us to study physics in regimes that cannot be accessed by terrestrial
laboratories. They play a key role in the study of fundamental problems including
the equation of state (EoS) of ultra-dense matter, the production of gravitational
waves, dense matter superfluidity and superconductivity, and the generation and
evolution of ultra-strong magnetic fields. Since the discovery of NSs as radio pulsars
in 1967 [136], many different classes have been discovered including more than
∼130 NSs in low mass X-ray binaries (LMXBs). In LMXBs the NS accretes matter
from a non-collapsed stellar companion (with mass M 1 M ) via an accretion
disk. This chapter focuses on a subgroup of the LMXBs, the accreting millisecond
X-ray pulsars (AMXPs). In the AMXPs, the gas stripped from the companion is
channeled out of the accretion disk and onto the magnetic poles of the rotating
neutron star, giving rise to X-ray pulsations at the spin frequency. We will explore
the details of this process, why it is so rare (the AMXPs are a small class), the
physics of the disk-magnetosphere interaction, and how AMXPs can be used to
probe extreme physics.
Immediately after the discovery of the first millisecond radio pulsar in 1982
[12], LMXBs were identified as possible incubators for millisecond pulsars. It
was suggested that LMXBs might be responsible for the conversion of slow NSs
with high magnetic field (B∼10 12 G), into a rapidly spinning objects with a
A. Patruno and A. L. Watts
4.6 Long Term Evolution and Pulse Formation Process . .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 183
4.6.1 Specific Sources .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 183
4.6.2 The Maximum Spin Frequency of Neutron Stars .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 186
4.6.3 Why Do Most Low Mass X-ray Binaries Not Pulsate? . . ... .. .. .. .. .. .. .. .. .. .. .. .. 187
4.7 Thermonuclear Bursts . . .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 189
4.8 Aperiodic Variability and kHz QPOs . .. .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 194
4.9 Open Problems and Final Remarks . . ... .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 196
References . .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. 197
Abstract Accreting Millisecond X-ray Pulsars (AMXPs) are astrophysical laboratories without parallel in the study of extreme physics. In this chapter we review the
past 15 years of discoveries in the field. We summarize the observations of the 15
known AMXPs, with a particular emphasis on the multi-wavelength observations
that have been carried out since the discovery of the first AMXP in 1998. We review
accretion torque theory, the pulse formation process, and how AMXP observations
have changed our view on the interaction of plasma and magnetic fields in strong
gravity. We also explain how the AMXPs have deepened our understanding of the
thermonuclear burst process, in particular the phenomenon of burst oscillations. We
conclude with a discussion of the open problems that remain to be addressed in the
future.
4.1 Introduction
Neutron stars (NSs), amongst the most extreme astrophysical objects in the Universe, allow us to study physics in regimes that cannot be accessed by terrestrial
laboratories. They play a key role in the study of fundamental problems including
the equation of state (EoS) of ultra-dense matter, the production of gravitational
waves, dense matter superfluidity and superconductivity, and the generation and
evolution of ultra-strong magnetic fields. Since the discovery of NSs as radio pulsars
in 1967 [136], many different classes have been discovered including more than
∼130 NSs in low mass X-ray binaries (LMXBs). In LMXBs the NS accretes matter
from a non-collapsed stellar companion (with mass M 1 M ) via an accretion
disk. This chapter focuses on a subgroup of the LMXBs, the accreting millisecond
X-ray pulsars (AMXPs). In the AMXPs, the gas stripped from the companion is
channeled out of the accretion disk and onto the magnetic poles of the rotating
neutron star, giving rise to X-ray pulsations at the spin frequency. We will explore
the details of this process, why it is so rare (the AMXPs are a small class), the
physics of the disk-magnetosphere interaction, and how AMXPs can be used to
probe extreme physics.
Immediately after the discovery of the first millisecond radio pulsar in 1982
[12], LMXBs were identified as possible incubators for millisecond pulsars. It
was suggested that LMXBs might be responsible for the conversion of slow NSs
with high magnetic field (B∼10 12 G), into a rapidly spinning objects with a