Timing Neutron Stars: Pulsations, Oscillations and Explosions (Astrophysics and Space Science Library, 461)

Neutron stars, whether isolated or in a binary system, display a varied and complex phenomenology, often accompanied by extreme variability of many time scales, which takes the form of pulsations due to the object rotation, quasi-periodicities associated to accretion of matter, and explosions due to matter accreted on the surface or to starquakes of highly magnetized objects. This book gives an overview of the current observational and theoretical standpoint in the research on the physics under the extreme conditions that neutron stars naturally provide. The six chapters explore three physical regions of a neutron star : the space around it, where accretion and pulsar companions allow testing of general relativity ; its surface, where millisecond pulsation and X-ray bursts provide clues about general relativistic effects and the equation of state of neutron matter ; its interior, of course, inaccessible to direct observations, can nevertheless, be probed with all observational parameters related to neutron star variability Preface 6 Contents 7 Contributors 11 1 Astrophysical Constraints on Dense Matter in Neutron Stars 13 Contents 13 1.1 Introduction 14 1.2 Expectations from Nuclear Theory 15 1.2.1 The Basics: Dense Matter and Neutron Stars 15 1.2.2 Models of Matter at High Densities 18 1.2.3 Construction of Neutron Star Models from Microphysics 21 1.3 Constraints on Mass from Binary Observations 22 1.3.1 Newtonian Observations of Binaries 22 1.3.2 Post-Keplerian Measurements of Pulsar Binaries 23 1.3.3 Dynamically Estimated Neutron Star Masses and Future Prospects 25 1.4 Constraints on Radius, and Other Mass Constraints 27 1.4.1 Thermonuclear X-ray Bursts 27 1.4.2 Fits of Thermal Spectra to Cooling Neutron Stars 30 1.4.3 Modeling of Waveforms 34 1.4.4 Maximum Spin Rate 36 1.4.5 Kilohertz QPOs 37 1.4.6 Other Methods to Determine the Radius and Future Prospects 39 1.5 Cooling of Neutron Stars 40 1.5.1 The URCA Processes 41 1.5.2 Additional Neutrino Production Channels and Suppression 42 1.5.3 Photon Luminosity and the Minimal Cooling Model 43 1.5.4 Observations and Systematic Errors 44 1.5.5 Current Status and Future Prospects 45 1.6 Gravitational Waves from Coalescing Binaries 46 1.7 Summary 49 References 51 2 General Relativity Measurements from Pulsars 64 Contents 64 2.1 Why Radio Pulsars 65 2.2 The Many Faces of the Radio Pulsar Zoo 66 2.2.1 Radio Pulsars 66 2.2.2 Intermittent Pulsars 70 2.2.3 Rotating RAdio Transients 72 2.3 Relativistic Binary Pulsars 73 2.3.1 Basic Evolution 73 2.3.1.1 Pulsars with a Neutron Star Companion 75 2.3.1.2 Pulsars with a White Dwarf Companion 77 2.3.1.3 Additional Reading 77 2.3.2 The Current Sample 77 2.4 Pulsar Timing Basics 83 2.4.1 Timing Procedure: Measurement of the ToAs 83 2.4.2 Timing Procedure: Modelling the ToAs 85 2.5 Probing Relativistic Gravity with Pulsars 89 2.5.1 Tests Using PPN Parameters 91 2.5.2 Tests Using PK Parameters 93 2.5.2.1 Double Neutron Star Binaries 94 2.5.2.2 The Unique Case of the Double Pulsar 95 2.5.2.3 Constraints on Tensor-Scalar Theories 97 2.5.3 Future Prospects 99 References 101 3 Magnetars: A Short Review and Some Sparse Considerations 107 3.1 Historical Overview 108 3.2 Observational Characteristics 109 3.2.1 Persistent Emission 109 3.2.1.1 X-Ray Emission 109 3.2.1.2 Hard-X-Ray Emission 111 3.2.1.3 Optical or Infrared Emission 112 3.2.2 Transient Activity 113 3.2.2.1 Giant Flares 113 3.2.2.2 Short Bursts 116 3.2.2.3 Outbursts 118 3.2.3 Magnetar Formation 126 3.2.4 Magnetic Field Evolution and the Neutron Star Bestiary 127 3.2.5 Low-B Magnetars and High-B Pulsars 130 3.2.6 Magnetars in Binary Systems 134 3.3 Final Remarks 136 References 137 4 Accreting Millisecond X-ray Pulsars 153 Contents 153 4.1 Introduction 154 4.2 The Accreting Millisecond X-ray Pulsar Family 156 4.2.1 Intermittency 159 4.3 Observations of the AMXPs 161 4.3.1 SAX J1808.4-3658 161 4.3.2 XTE J1751-305 166 4.3.3 XTE J0929-314 168 4.3.4 XTE J1807-294 168 4.3.5 XTE J1814-338 169 4.3.6 IGR J00291+5934 170 4.3.7 HETE J1900.1-2455 171 4.3.8 Swift J1756.9-2508 172 4.3.9 Aql X-1 173 4.3.10 SAX J1748.9-2021 174 4.3.11 NGC 6440 X-2 175 4.3.12 IGR J17511-3057 175 4.3.13 Swift J1749.4-2807 176 4.3.14 IGR J17498-2921 177 4.3.15 IGR J18245-2452 177 4.4 Accretion Torques 178 4.4.1 Coherent Timing Technique 182 4.4.2 Observations: Accretion Torques in AMXPs 185 4.4.2.1 The Origin of X-ray Timing Noise 188 4.5 Pulse Profiles 190 4.5.1 Pulse Fractional Amplitudes and Phase Lags 190 4.5.2 Pulse Shape Evolution 192 4.6 Long Term Evolution and Pulse Formation Process 193 4.6.1 Specific Sources 193 4.6.2 The Maximum Spin Frequency of Neutron Stars 196 4.6.3 Why Do Most Low Mass X-ray Binaries Not Pulsate? 197 4.7 Thermonuclear Bursts 199 4.8 Aperiodic Variability and kHz QPOs 204 4.9 Open Problems and Final Remarks 206 References 207 5 Thermonuclear X-ray Bursts 219 Contents 219 5.1 Overview 220 5.1.1 Theory of Burst Ignition and Nuclear Burning Regimes 221 5.1.1.1 Fuel Accretion from a Binary Companion Star 221 5.1.1.2 Runaway Thermonuclear Burning in a Thin Shell 222 5.1.1.3 Ignition Conditions and the Stability of Burning 223 5.1.1.4 Burning Regimes as a Function of Accretion Rate 223 5.1.1.5 Base Heating, Rotational Mixing, and Gravitational Separation 225 5.1.2 Status of Burst Observations 227 5.1.2.1 Low (Hard) State Bursts 229 5.1.2.2 High (Soft) State Bursts 230 5.1.2.3 The Rossi X-ray Timing Explorer 230 5.2 X-ray Burst Ignition 231 5.2.1 Thin-Shell Instability and Electron Degeneracy 232 5.2.2 Reignition After a Short Recurrence Time 232 5.2.3 Ignition Latitude 234 5.3 The Burst Spectral Energy Distribution 235 5.3.1 The Continuum Spectrum 235 5.3.2 Discrete Spectral Features 238 5.4 Interaction with the Accretion Environment 240 5.4.1 Reflection by the Accretion Disk 243 5.4.2 Anisotropic Emission 244 5.5 Burst Oscillations and the Neutron Star Spin 245 5.6 mHz Oscillations and Marginally Stable Burning 246 5.6.1 Observations of mHz QPOs 247 5.6.2 Theoretical Interpretation: Marginally Stable Burning 248 5.7 Burst Duration and Fuel Composition 249 5.7.1 Intermediate Duration Bursts 250 5.7.2 Superbursts 252 5.8 Thermonuclear Burst Simulations 254 5.8.1 Single-Zone Models 255 5.8.2 One-Dimensional Multi-Zone Models 255 5.8.3 Multi-Dimensional Models 258 5.9 Nuclear Experimental Physics 259 5.10 Summary and Outlook 261 References 262 6 High-Frequency Variability in Neutron-Star Low-Mass X-ray Binaries 273 Contents 273 6.1 Introduction 274 6.2 History 274 6.3 Basic Frequencies Close to a Neutron Star 276 6.4 Timing Phenomenology: QPOs 101 277 6.5 Linking Observed Frequencies with Theoretical Expectations 292 6.6 QPO Frequency Correlations 296 6.7 Relation Between Properties of the kHz QPOs and Parameters of the Energy Spectrum 297 6.8 Beyond QPO Frequencies 301 6.8.1 The Fractional rms Amplitude of the kHz QPOs 301 6.8.2 The Width of the kHz QPOs 308 6.8.3 The Energy-Dependent Lags and Coherence of the kHz QPOs 315 6.8.4 Other Phenomenology of the kHz QPOs 325 6.9 Probing Neutron-Star Interiors and GR with kHz QPO 329 6.10 Conclusions and Outlook 330 References 331 Index 342