Recurrent Instability in LMXB Accretion Disks: How Strange Is GRS 1915+105?

University of Southampton Faculty of Engineering and Physical Sciences Department of Physics and Astronomy Recurrent Instability in LMXB Accretion Disks: How Strange is GRS 1915+105? Author James Matthew Christopher Court ORCID ID 0000-0002-0873-926X Thesis for the degree of Doctor of Philosophy arXiv:1903.03675v3 [astro-ph.HE] 15 Mar 2019 March 19, 2019 Abstract Low Mass X-Ray Binaries (LMXBs) are systems in which a black hole or neutron star accretes matter from a stellar binary companion. The accreted matter forms a disk of material around the compact object, known as an accretion disk. The X-ray properties of LMXBs show strong variability over timescales ranging from milliseconds to decades. Many of these types of variability are tied to the extreme environment of the inner accretion disk, and hence an understanding of this behaviour is key to understanding how matter behaves in such an environment. GRS 1915+105 and MXB 1730-335 (also known as the Rapid Burster) are two LMXBs which show particularly unusual variability. GRS 1915+105 shows a large number of distinct ‘classes’ of second-to-minute scale variability, consisting of repeated patterns of dips and flares. The Rapid Burster on the other hand shows ‘Type II X-ray Bursts’; second-to-minute scale increases in X-ray intensity with a sudden onset and a slower de- cay. For many years both of these objects were thought to be unique amongst all known LMXBs. More recently, two new objects, IGR J17091-3624 and GRO J1744-28 (also known as the Bursting Pulsar) have been shown to display similar behaviour to those seen in GRS 1915+105 and the Rapid Burster respectively. In this thesis, I first present a new framework with which to classify variability seen in IGR J17091-3624. Using my set of independent variability classes constructed for IGR J17091-3624, I perform a study of the similarities and differences between this source and GRS 1915+105 to better constrain their underlying physics. In GRS 1915, hard X-ray emission lags soft X-ray emission in all variability classes; in IGR J17091, I find that the sign of this lag is different in variability classes. Additionally, while GRS 1915+105 accretes at close to its Eddington Limit, I find that IGR J17091-3624 accretes at only ∼ 5–33% of its Eddington Limit. With these results I rule out any models which require near-Eddington accretion or hard corona reacting to the disk. I also perform a study of the variability seen in the Bursting Pulsar. I find that the flaring behaviour in the Bursting Pulsar is significantly more complex than in the Rapid Burster, consisting of at least 4 separate phenomena which may have separate physical origins. One of these phenomena, ‘Structured Bursting’, con- sists of patterns of flares and dips which are similar to those seen in GRS 1915+105 and IGR J17091-3624. I compare these two types of variability and discuss the possibility that they are caused by the same physical instability. I also present the alternative hypothesis that Structured Bursting is a manifestation of ‘hiccup’ variability; a bimodal flickering of the accretion rate seen in systems approaching the ‘propeller’ regime. Contents List of Figures ix List of Tables xi Dedication xiii Acknowledgements xv Declaration of Authorship xvii 1 Introduction 1 1.1 Anatomy of an X-Ray Binary . .2 1.1.1 Types of X-Ray Binaries: High and Low-Mass . .3 1.1.2 Components of a Low Mass X-Ray Binary . .4 1.2 Low Mass X-Ray Binary Behaviour . .7 1.3 Relativistic Effects . 10 2 The Physics of Accretion 13 2.1 The Shakura-Sunyaev Disk Model . 13 2.1.1 The source of Turbulence . 17 2.2 Accretion Phenomena . 17 2.2.1 The Eddington Limit . 17 2.2.2 The Propeller Effect . 19 2.2.3 Disk Instabilities . 24 2.3 GRS 1915+105 and IGR J17091-3624 . 24 2.3.1 A History of Models of GRS 1915-like Variability . 27 2.4 Type II Burst Sources . 32 2.4.1 A History of Models of Type II Bursts . 34 3 Tools & Methods 35 3.1 Instrumentation . 35 3.1.1 The Rossi X-Ray Timing Experiment ................. 36 3.1.2 The Neil Gehrels Swift Observatory ................. 38 3.1.3 The X-Ray Multi-Mirror Mission ................... 39 i 3.1.4 Chandra ............................... 40 3.1.5 Suzaku ................................ 41 3.1.6 The Nuclear Spectroscopic Telescope Array ............. 42 3.1.7 The International Gamma-Ray Astrophysics Laboratory ...... 43 3.1.8 Dead-time and Pile-up . 43 3.2 Methods & Techniques . 44 3.2.1 Lightcurve Morphology . 44 3.2.2 Timing Analysis . 50 3.2.3 Energy Spectral Analysis . 54 4 Variability in IGR J17091-3624: Classification 57 4.1 Data and Data Analysis . 58 4.1.1 RXTE ................................. 58 4.1.2 Swift ................................. 61 4.1.3 INTEGRAL .............................. 61 4.1.4 XMM-Newton ............................. 61 4.1.5 Chandra ............................... 62 4.1.6 Suzaku ................................ 63 4.2 Results . 63 4.2.1 Outburst Evolution . 63 4.2.2 RXTE ................................. 64 4.2.3 Swift . 88 4.2.4 INTEGRAL . 88 4.2.5 Chandra . 90 4.2.6 XMM-Newton . 91 4.2.7 Suzaku ................................ 91 4.3 Discussion . 93 4.3.1 Variability Classes: IGR J17091 vs. GRS 1915 . 94 4.3.2 General Comparison with GRS 1915+105 . 100 4.3.3 Comparison with the Rapid Burster . 101 4.3.4 Comparison with Altamirano et al., 2011b . 102 4.3.5 New Constraints on Accretion Rate, Mass & Distance . 103 4.3.6 Implications for Models of ‘Heartbeat’ Variability . 104 4.4 Conclusions . 105 5 The Evolution of X-ray Bursts in the ‘Bursting Pulsar’ GRO J1744–28 107 5.1 Data and Data Analysis . 108 5.1.1 RXTE ................................. 108 5.1.2 Swift ................................. 111 5.1.3 INTEGRAL .............................. 112 5.1.4 Chandra ............................... 112 5.1.5 XMM-Newton ............................. 112 ii 5.1.6 Suzaku ................................ 112 5.1.7 NuSTAR ................................ 113 5.2 Results . 113 5.2.1 Outburst Evolution . 113 5.2.2 Categorizing Bursts . 116 5.2.3 Normal Bursts . 118 5.2.4 Minibursts . 132 5.2.5 Mesobursts . 135 5.2.6 Structured ‘Bursts’ . 139 5.3 Discussion . 142 5.3.1 Evolution of Outburst and Bursting Behaviour . 144 5.3.2 Parameter Correlations . 145 5.3.3 Comparison with Previous Studies . 146 5.3.4 Comparison with other objects . 147 5.3.5 Comparison with Models of Type II Bursts . 150 5.4 Conclusions . 153 6 The Bursting Pulsar GRO J1744-28: the Slowest Transitional Pulsar? 155 6.1 Transitional Millisecond Pulsars . 156 6.2 Comparison: TMSPs vs. the Bursting Pulsar . 158 6.3 Discussion . 160 6.3.1 Comparison with other Objects . 164 6.4 Conclusion . 165 7 Discussion 167 7.1 General Observations . 168 7.1.1 Variability Evolution throughout an Ouburst . 168 7.1.2 Criteria for Exotic Variability . 169 7.1.3 Evidence of System Memory . 171 7.2 IGR J17091 vs. the Bursting Pulsar: A Comparison . 172 7.2.1 Variability Classes and Burst Classes . 173 7.2.2 Structured Bursting . 176 7.3 Future Research . 177 8 Conclusions 179 A Model-Independent Classification of each Observation of IGR J17091-3624 183 B List of RXTE Observations of the Bursting Pulsar 189 C Normal Burst Histograms 193 D PANTHEON suite 203 D.1 FITS Genie . 203 iii D.2 Plot Demon . 204 D.3 Spec Angel . 207 D.4 Back Hydra . 208 D.5 PAN Lib . 209 Bibliography 213 Index 247 iv List of Figures 1.1 A cartoon illustrating the basic geometry of a simple X-ray binary. .4 1.2 A series of 5 GHz radio images from Fender et al. (1999) showing a jet being launched from the LMXB GRS 1915+105. .5 1.3 Two simulated, simplified spectra of an LMXB, showing the two main components visible in X-ray: the accretion disk and the corona. .6 1.4 A schematic hardness-intensity diagram adapted from Fender et al. (2004), showing the evolutionary path of a typical black hole LMXB outburst. .8 1.5 Energy spectra of the black hole HMXB Cygnus X-1 in its low/hard and high/soft states, presented as typical spectra of a black hole XRB in these states. .9 1.6 Colour-Colour diagrams from van der Klis (1989b) showing typical evolutionary paths of Atoll-type and Z-type Neutron Star LMXBs. 10 2.1 Diagrams showing the path of an element of gas in a neutron star accretion disk for different arrangements of the corotation and magnetospheric radii. 22 2.2 Typical lightcurves of a selection of variability classes seen in the LMXB GRS 1915+105. 26 2.3 A schematic diagram illustrating the the process described by Neilsen et al. (2011) to describe the ρ variability class in GRS 1915+105. 31 2.4 An RXTE/PCA lightcurve of the Rapid Burster, showing a number of typical Type II X-ray bursts. 33 3.1 A cartoon illustrating the process of folding a periodic lightcurve with a known period. 46 3.2 A cartoon illustrating the procedure of the algorithm described in Section 3.2.1. 48 3.3 A representation of how a continuous variable is convolved with a window- ing function and a sampling function to yield physical data. 51 3.4 Hardness-Intensity diagrams of black bodies with temperatures and normal- isations described by various functional forms. 55 4.1 Lightcurves of IGR J17091-3624, from a number of instruments, during its 2011-2013 outburst.

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