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Accreting neutron stars (NS) and accreting millisecond X-ray pulsars (AMXPs) are one of the most interesting laboratories to study the physics of compact objects. Recently, along with providing highly precise mass and radius measurements, NICER also indicted the requirement of complex magnetic field structures to explain the observed X-ray spectra in some of the isolated millisecond pulsars. Rotation-powered millisecond pulsars are assumed to have undergone significant magnetic field evolution during the accretion phase. However, the details of this process are still poorly understood. We performed a suite of GRMHD simulations of accretion onto neutron stars, contrasting dipolar with non-dipolar field structures. Besides exploring processes in different accretion regimes, the simulations inform on the variability properties of the hotspots which can aid in future pulse-profile modeling of accreting sources.

Disponible preprint en: arXiv:1812.04023v1 [astro-ph.HE] [v1] Mon, 10 Dec 2018 19:00:52 UTC (4,376 KB), In this White Paper we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission for studies related to Observatory Science targets. These include flaring stars, supernova remnants, accreting white dwarfs, low and high mass X-ray binaries, radio quiet and radio loud active galactic nuclei, tidal disruption events, and gamma-ray bursts. eXTP will be excellently suited to study one common aspect of these objects: their often transient nature. Developed by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s.© 2018, Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature., This work was supported by the Royal Society, ERC Starting (Grant No. 639217), the European Union Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Global Fellowship (Grant No. 703916), the National Natural Science Foundation of China (Grant Nos. 11233001, 11773014, 11633007, 11403074, 11333005, 11503008, and 11590781), the National Basic Research Program of China (Grant No. 2015CB857100), NASA (Grant No. NNX13AD28A), an ARC Future Fellowship (Grant No. FT120100363), the National Science Foundation (Grant No. PHY-1430152), the Spanish MINECO (Grant No. AYA2016-76012-C3-1-P), the ICCUB (Unidad de Excelencia 'Maria de Maeztu') (Grant No. MDM-2014-0369), EU's Horizon Programme through a Marie Sklodowska-Curie Fellowship (Grant No. 702638), the Polish National Science Center (Grant Nos. 2015/17/B/ST9/03422, 2015/18/M/ST9/00541, 2013/10/M/ST9/00729, and 2015/18/A/ST9/00746), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA15020100), and the NWO Veni Fellowship (Grant No. 639.041.647).

We present the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), a probe-class mission concept selected for study by NASA. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility. STROBE-X offers an enormous increase in sensitivity for X-ray spectral timing, extending these techniques to extragalactic targets for the first time. It is also an agile mission capable of rapid response to transient events, making it an essential X-ray partner facility in the era of time-domain, multi-wavelength, and multi-messenger astronomy. Optimized for study of the most extreme conditions found in the Universe, its key science objectives include: (1) Robustly measuring mass and spin and mapping inner accretion flows across the black hole mass spectrum, from compact stars to intermediate-mass objects to active galactic nuclei. (2) Mapping out the full mass-radius relation of neutron stars using an ensemble of nearly two dozen rotation-powered pulsars and accreting neutron stars, and hence measuring the equation of state for ultradense matter over a much wider range of densities than explored by NICER. (3) Identifying and studying X-ray counterparts (in the post-Swift era) for multiwavelength and multi-messenger transients in the dynamic sky through cross-correlation with gravitational wave interferometers, neutrino observatories, and high-cadence time-domain surveys in other electromagnetic bands. (4) Continuously surveying the dynamic X-ray sky with a large duty cycle and high time resolution to characterize the behavior of X-ray sources over an unprecedentedly vast range of time scales. STROBE-X's formidable capabilities will also enable a broad portfolio of additional science., 50 pages, Probe class mission concept study report submitted to NASA for Astro2020 Decadal Survey

In this White Paper we present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. The tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. Additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is expected to be launched in the mid 2020s.

SAX J1810.8-2609 is a faint X-ray transient, mostly known for its abnormally low quiescent thermal luminosity, which disagrees with standard cooling models. It is also one of a small sample of stars whose mass and radius have been estimated using spectral modeling of one of its thermonuclear bursts. Here we report the discovery of millisecond oscillations in a type I thermonuclear X-ray burst from SAX J1810.8-2609 observed by RXTE during the 2007 outburst. A strong signal (Leahy-normalized power of 71.5, 4.5e-9 chance of coincidence with a conservative estimate for the number of trials) was present at 531.8 Hz during the decay of one out of six bursts observed. Oscillations were detected for about 6 seconds, during which their frequency increased from 531.4 to 531.9 Hz in a manner similar to other burst oscillation sources. The millisecond oscillations discovered pinpoint the spin frequency of the neutron star, which is important for the spectral modeling, associated mass-radius inference, and understanding the evolutionary status and cooling behavior of the star. As of April 2018 the source is in outburst again, providing a fleeting opportunity to acquire new material for the burst oscillation searches., 4 pages, 2 figures, submitted to ApJ letters

One of the primary science goals of the next generation of hard X-ray timing instruments is to determine the equation of state of the matter at supranuclear densities inside neutron stars, by measuring the radius of neutron stars with different masses to accuracies of a few percent. Three main techniques can be used to achieve this goal. The first involves waveform modelling. The flux we observe from a hotspot on the neutron star surface offset from the rotational pole will be modulated by the star's rotation, giving rise to a pulsation. Information about mass and radius is encoded into the pulse profile via relativistic effects, and tight constraints on mass and radius can be obtained. The second technique involves characterising the spin distribution of accreting neutron stars. The most rapidly rotating stars provide a very clean constraint, since the mass-shedding limit is a function of mass and radius. However the overall spin distribution also provides a guide to the torque mechanisms in operation and the moment of inertia, both of which can depend sensitively on dense matter physics. The third technique is to search for quasi-periodic oscillations in X-ray flux associated with global seismic vibrations of magnetars (the most highly magnetized neutron stars), triggered by magnetic explosions. The vibrational frequencies depend on stellar parameters including the dense matter equation of state. We illustrate how these complementary X-ray timing techniques can be used to constrain the dense matter equation of state, and discuss the results that might be expected from a 10m$^2$ instrument. We also discuss how the results from such a facility would compare to other astronomical investigations of neutron star properties. [Modified for arXiv], Comment: To appear in Reviews of Modern Physics as a Colloquium, 23 pages, 9 figures

Magnetars are the strongest magnets in the present universe and the combination of extreme magnetic field, gravity and density makes them unique laboratories to probe current physical theories (from quantum electrodynamics to general relativity) in the strong field limit. Magnetars are observed as peculiar, burst--active X-ray pulsars, the Anomalous X-ray Pulsars (AXPs) and the Soft Gamma Repeaters (SGRs); the latter emitted also three "giant flares," extremely powerful events during which luminosities can reach up to 10^47 erg/s for about one second. The last five years have witnessed an explosion in magnetar research which has led, among other things, to the discovery of transient, or "outbursting," and "low-field" magnetars. Substantial progress has been made also on the theoretical side. Quite detailed models for explaining the magnetars' persistent X-ray emission, the properties of the bursts, the flux evolution in transient sources have been developed and confronted with observations. New insight on neutron star asteroseismology has been gained through improved models of magnetar oscillations. The long-debated issue of magnetic field decay in neutron stars has been addressed, and its importance recognized in relation to the evolution of magnetars and to the links among magnetars and other families of isolated neutron stars. The aim of this paper is to present a comprehensive overview in which the observational results are discussed in the light of the most up-to-date theoretical models and their implications. This addresses not only the particular case of magnetar sources, but the more fundamental issue of how physics in strong magnetic fields can be constrained by the observations of these unique sources., 81 pages, 24 figures, This is an author-created, un-copyedited version of an article submitted to Reports on Progress in Physics

In order to discern the physical nature of many gamma-ray sources in the sky, we must look not only in spectral and spatial dimensions, but also understand their temporal variability. However, timing analysis of sources with a highly transient nature, such as magnetar bursts, is difficult: standard Fourier techniques developed for long-term variability generally observed, for example, from AGN often do not apply. Here, we present newly developed timing methods applicable to transient events of all kinds, and show their successful application to magnetar bursts observed with Fermi/GBM. Magnetars are a prime subject for timing studies, thanks to the detection of quasi-periodicities in magnetar Giant Flares and their potential to help shed light on the structure of neutron stars. Using state-of-the art statistical techniques, we search for quasi-periodicities (QPOs) in a sample of bursts from Soft Gamma Repeater SGR J0501+4516 observed with Fermi/GBM and provide upper limits for potential QPO detections. Additionally, for the first time, we characterise the broadband variability behaviour of magnetar bursts and highlight how this new information could provide us with another way to probe these mysterious objects., 2012 Fermi Symposium proceedings - eConf C121028

Neutron stars enable us to study both the highest densities and the highest magnetic fields in the known Universe. In this article I review what can be learned about such fundamental physics using magnetar bursts. Both the instability mechanisms that trigger the bursts, and the subsequent dynamical and radiative response of the star, can be used to explore stellar and magnetospheric structure and composition., 6 pages, invited review, to appear in the proceedings of the ERPM conference, Zielona Gora, April 2012

Strong quasi-periodic oscillations in the tails of the giant gamma-ray flares seen in SGR 1806-20 and SGR 1900+14 are thought to be produced by starquakes in the flaring magnetar. However, the large fractional amplitudes (up to ~20%) observed are difficult to reconcile with predicted amplitudes of starquakes. Here we demonstrate that the steeply pulsed emission profile in the tail of the giant flare can enhance the observed amplitude of the underlying oscillation, analogously to a beam of light oscillating in and out of the line of sight. This mechanism will also broaden the feature in the power spectrum and introduce power at harmonics of the oscillation. The observed strength of the oscillation depends on the amplitude of the underlying starquake, the orientation and location of the emission on the surface of the star, and the gradient of the light curve profile. While the amplification of the signal can be significant, we demonstrate that even with uncertainties in the emission geometry, this effect is not sufficient to produce the observed QPOs. This result excludes the direct observation of a starquake, and suggests that the observed variations come from modulations in the intensity of the emission., 5 pages, 4 figures, accepted by ApJL

We present our temporal and spectral analyses of 29 bursts from SGR J0501+4516, detected with the Gamma-ray Burst Monitor onboard the Fermi Gamma-ray Space Telescope during the 13 days of the source activation in 2008 (August 22 to September 3). We find that the T90 durations of the bursts can be fit with a log-normal distribution with a mean value of ~ 123 ms. We also estimate for the first time event durations of Soft Gamma Repeater (SGR) bursts in photon space (i.e., using their deconvolved spectra) and find that these are very similar to the T90s estimated in count space (following a log-normal distribution with a mean value of ~ 124 ms). We fit the time-integrated spectra for each burst and the time-resolved spectra of the five brightest bursts with several models. We find that a single power law with an exponential cutoff model fits all 29 bursts well, while 18 of the events can also be fit with two black body functions. We expand on the physical interpretation of these two models and we compare their parameters and discuss their evolution. We show that the time-integrated and time-resolved spectra reveal that Epeak decreases with energy flux (and fluence) to a minimum of ~30 keV at F=8.7e-6 erg/cm2/s, increasing steadily afterwards. Two more sources exhibit a similar trend: SGRs J1550-5418 and 1806-20. The isotropic luminosity corresponding to these flux values is roughly similar for all sources (0.4-1.5 e40 erg/s)., accepted for publication in ApJ

We present our temporal and spectral analyses of 29 bursts from SGR J0501+4516, detected with the gamma-ray burst monitor on board the Fermi Gamma-ray Space Telescope during 13 days of the source's activation in 2008 (August 22-September 3). We find that the T-90 durations of the bursts can be fit with a log-normal distribution with a mean value of similar to 123 ms. We also estimate for the first time event durations of soft gamma repeater (SGR) bursts in photon space (i.e., using their deconvolved spectra) and find that these are very similar to the T-90 values estimated in count space (following a log-normal distribution with a mean value of similar to 124 ms). We fit the time-integrated spectra for each burst and the time-resolved spectra of the five brightest bursts with several models. We find that a single power law with an exponential cutoff model fits all 29 bursts well, while 18 of the events can also be fit with two blackbody functions. We expand on the physical interpretation of these two models and we compare their parameters and discuss their evolution. We show that the time-integrated and time-resolved spectra reveal that E-peak decreases with energy flux (and fluence) to a minimum of similar to 30 keV at F = 8.7 x 10(-6) erg cm(-2) s(-1), increasing steadily afterward. Two more sources exhibit a similar trend: SGRs J1550-5418 and 1806-20. The isotropic luminosity, L-iso, corresponding to these flux values is roughly similar for all sources (0.4-1.5 x 10(40) erg s(-1)).

The most strongly magnetized neutron stars, the magnetars, have spectacular outbursts of gamma-ray flares powered by decay of the magnetic field. The rapidly changing field is strong enough that it should be able to stress and rupture the star's crust, with potentially interesting seismic consequences. This was confirmed in dramatic fashion a few years ago with the discovery of long-lived seismic vibrations excited by rare giant flares. The starquakes associated with the giant flares are, it seems, so catastrophic that they leave the whole star ringing. These discoveries have opened up the possibility of using asteroseismology to study the extreme conditions of the neutron star interior - the dense matter equation of state in crust and core, and the core magnetic field. This chapter reviews the main observational properties of the oscillations seen in the giant flares, and the state of the art in terms of theoretical models of global seismic vibrations in magnetars. I discuss efforts to extend oscillation searches to smaller magnetar bursts (which are more frequent but less energetic), and the role that crust rupturing might play in triggering magnetar flares and exciting global seismic oscillations., Comment: 16 pages, 2 figures; to appear as a chapter in the book "Neutron Star Crust", edited by C. A. Bertulani and J. Piekarewicz

We perform phase-resolved spectroscopy of the accreting millisecond pulsar, SAX J1808.4-3658, during the slow-decay phase of the 2002 outburst. Simple phenomenological fits to RXTE PCA data reveal a pulsation in the iron line at the spin frequency of the neutron star. However, fitting more complex spectral models reveals a degeneracy between iron-line pulsations and changes in the underlying hotspot blackbody temperature with phase. By comparing with the variations in reflection continuum, which are much weaker than the iron line variations, we infer that the iron-line is not pulsed. The observed spectral variations can be explained by variations in blackbody temperature associated with rotational Doppler shifts at the neutron star surface. By allowing blackbody temperature to vary in this way, we also find a larger phase-shift between the pulsations in the Comptonised and blackbody components than has been seen in previous work. The phase-shift between the pulsation in the blackbody temperature and normalisation is consistent with a simple model where the Doppler shift is maximised at the limb of the neutron star, ~90 degrees prior to maximisation of the hot-spot projected area., 8 pages, 10 figures, 2 tables. Accepted by MNRAS

The shape of the lightcurve during the rising phase of Type I X-ray bursts is determined by many factors including the ignition latitude, the accretion rate, and the rotation rate of the star. We develop a phenomenological model of the burst rise process and show that simple measures of the burst morphology can be robust diagnostics of ignition latitude and burning regime. We apply our results to the large sample of bursts from the Low Mass X-ray Binary 4U 1636-536, and find evidence for off-equatorial ignition for many of the bursts. We argue that such behaviour may be associated with the transition from hydrogen to helium ignition at accretion rates a few percent of Eddington. We show that this model can also explain variations in the detectability of burst oscillations, and discuss the implications for other burst sources., Some additions and clarifications, MNRAS accepted

The recent discovery of high frequency oscillations during giant flares from the Soft Gamma Repeaters SGR 1806-20 and SGR 1900+14 may be the first direct detection of vibrations in a neutron star crust. If this interpretation is correct it offers a novel means of testing the neutron star equation of state, crustal breaking strain, and magnetic field configuration. We review the observational data on the magnetar oscillations, including new timing analysis of the SGR 1806-20 giant flare using data from the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Rossi X-ray Timing Explorer (RXTE). We discuss the implications for the study of neutron star structure and crust thickness, and outline areas for future investigation., Comment: 5 pages, 1 figure, to appear in the proceedings of the conference "Isolated Neutron Stars: from the Interior to the Surface" (April 2006, London), eds. D. Page, R. Turolla, & S. Zane, Astrophysics & Space Science in press

Accreting neutron stars can exhibit high-frequency modulations in their lightcurves during thermonuclear X-ray bursts, known as burst oscillations. These frequencies can be offset from the neutron star spin frequency by several Hz (where known independently) and can drift by 1-3 Hz. One plausible explanation for these observations is that a wave is present in the bursting ocean, the rotating frame frequency of which is the offset from the neutron star spin frequency. As the ocean cools in the aftermath of the burst, the frequency of the wave should decrease (in the rotating frame) hence explaining the drift. A strong candidate to explain this phenomenon is a buoyant r-mode. Before this thesis, models that accounted for the radial structure of these modes neglected the burning of nuclear fuel in the tail of the burst as well as relativistic effects. These previous models over-predicted the frequency drifts and frequency offset. Using burst models that include the burning of nuclear fuel, the evolution of the buoyant r-mode is calculated and the resulting frequency drifts are smaller. Including frame-dragging and gravitational redshift reduces the frequency drift and the rotating frame frequency. This thesis also calculates the frequency of a buoyant r-mode at the ocean-crust interface of a neutron star during a superburst. Such bursts can last hours and are caused by burning carbon unstably. It is shown that the frequency evolution of the r-mode is not consistent with the sole observation of oscillation during a superburst observed on 4U-1636-536.

In this White Paper we present the potential of the Enhanced X-ray Timing and Polarimetry (eXTP) mission for determining the nature of dense matter; neutron star cores host an extreme density regime which cannot be replicated in a terrestrial laboratory. The tightest statistical constraints on the dense matter equation of state will come from pulse profile modelling of accretion-powered pulsars, burst oscillation sources, and rotation-powered pulsars. Additional constraints will derive from spin measurements, burst spectra, and properties of the accretion flows in the vicinity of the neutron star. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Science, the eXTP mission is expected to be launched in the mid 2020s., Comment: Accepted for publication on Sci. China Phys. Mech. Astron. (2019)

Neutron stars are extremely dense objects that harbour extreme physical environments. They consist of a core with a radius of ~10 kilometers and a one-kilometer thick crust. Neutron stars in X-ray binaries accrete matter from a companion star. In transient systems accretion is not continuous, instead accretion outbursts are observed. During an outburst, material accumulates on the surface of the neutron star. This compresses the deeper layers of the crust and induces processes that release heat. Consequently, the neutron star crust heats up and becomes hotter than the core. After the outburst, the crust cools down to restore thermal equilibrium with the core. In this thesis, the thermal evolution of transiently accreting neutron stars is studied. We model the thermal evolution using the code NSCool and compare the results with observations to unravel properties of neutron stars. We improve our modelling methods by taking into account accretion rate variability (Chapter 2). We model the neutron star in KS 1731-260 and find that the constrained properties are strongly affected by variations in accretion rate. In Chapter 3 we model the 20-year outburst history of Aql X-1 and find that the neutron star does not have enough time between its frequent outbursts to restore crust-core equilibrium. Modelling Terzan 5 X-2 reveals that the neutron star in this system might have unusual crust properties (Chapter 4). In Chapter 5, we model how a neutron star reaches an equilibrium state over many outburst cycles and investigate how this equilibrium is affected by various parameters.

We focus on statistical estimation of neutron star parameters, conditional on X-ray data acquired by contemporary and future space telescopes. In particular, we discuss Bayesian inference of: (i) parameters such as masses and radii, which define the spacetime exterior to neutron stars; and (ii) parameters defined by a model of the equation of state of interior dense matter. We tailor to X-ray spectral-timing (also termed pulse-profile or light-curve modelling) with telescopes such as NICER (NASA’s Neutron Star Interior Composition Explorer, installed on the International Space Station). We develop a prototype open-source software package, X-PSI, for neutron star X-ray Pulsation Simulation and Inference. This package is designed to interface native likelihood functionality with open-source posterior sampling software, for use on CPU-nodes of high-performance computing systems. We apply the X-PSI modelling framework to NICER observations of the rotation-powered millisecond pulsar J0030+0451; we jointly constrain its mass and radius, together with the spatial structure of hot surface regions whose X-ray emission signatures are rotationally modulated (or pulsed). The analysis is a proof of concept and acts as a pathfinder for ongoing and future analyses of NICER data. The rotation-powered pulsars targeted by NICER are identified as a logical baseline source-class for community-supported development of an open-source package for neutron star X-ray spectral-timing: specifically, a package that enables the definition and estimation of fundamental parameters pertaining to relativistic stellar structure.

Neutron stars where the output emission is dominated by the decay of an ultra-strong magnetic field, typically in excess of 1014 Gauss, are characterized as magnetars. This decay may occur both gradually and abruptly, which in the latter case results in bright recurrent soft gamma-ray bursts and occasionally giant flares. Here we investigate and develop theoretical models of the trigger and emission mechanisms of these bursts, which involve extreme and fundamental (astro)physical processes. By comparing the model predictions with observed data, we aim to constrain distinct magnetar burst mechanisms. Assuming that magnetar giant flares are triggered by the spontaneous development of a tearing instability in a magnetospheric current sheet, we obtain estimates for the dimension of the corresponding reconnection region. Moreover, we assess inverse Compton and synchrotron emission models for high-energy emission that was observed following the impulsive phase of the giant flare from the magnetar SGR 1806-20 on 2004 December 27. We conclude that the high-energy emission may constitute synchrotron radiation from shock-accelerated particles, whereby the shock is generated between the evaporative outflow from the star and a downward directed reconnection exhaust. We furthermore study the characteristics of the recurrent bursts as manifestations of systems of self-organised critically, such as flares and crust-quakes, and develop a model where we investigate the detectability of a phase-dependence of magnetar burst properties.

A magnetar is a type of neutron star with an extremely strong magnetic field, a hundred million times stronger than the strongest fields made in laboratories. In turn, a neutron star is one of the end products of stellar evolution, and is formed when a massive star has used up all its fuel for nuclear fusion. A neutron star has a mass around twice that of the sun, but a diameter of only twenty km, making it extremely dense. Studying neutron stars, and magnetars in particular, is the only way to study what happens to matter at these extreme densities and magnetic field strengths. Many magnetars are seen through their bursts, which are short bright flashes of radiation on the edge between X-ray and gamma-ray radiation. When radiation interacts with matter, it exerts a small force on that matter. In magnetar bursts, the intensity of the emitted radiation can let this force become as strong as gravity. In Chapters 2 and 3 of this thesis I have studied what happens when this force does overcome gravity, by making numerical models of magnetar atmospheres during a burst. I conclude that during these bursts magnetars will develop highly irregular outflows of matter. In Chapter 4 I have developed a numerical model to test how the atmosphere of a magnetar alters the properties of its emission, in particular to model the observed beaming of magnetar emission, an effect whereby radiation is emitted in a preferred direction. I find that a simple model for the atmosphere of a magnetar can recreate the observed amount of beaming. This makes my model a promising way to use observations of beamed magnetar bursts to learn more about the objects emitting these bursts.

Neutron stars are the most compact known stars; their cores are of higher density than an atomic nucleus. Their rotation rates are generally very predictable, with a slow decrease over time. This spin-down is occasionally interrupted, however, by abrupt 'glitches' when the rotation rate increases slightly. Glitches are attributed to a sudden coupling between a neutron star's crust and an interior superfluid component - and therefore the nature and frequency of glitches encodes important information about physics at densities which cannot be attained in terrestrial laboratories. This thesis contains analysis of both observational and theoretical aspects of the physics of neutron-star glitches. It describes glitch detectability limits and a search algorithm, which together allow for a more systematic analysis of glitch data than was previously possible. These are applied to one of the most closely-observed neutron stars, the Crab pulsar, finding it has an unexpected deficit of small glitches. Next, a multifluid model of the neutron-star interior is used to simulate glitches; it is shown that different kinds of glitch and post-glitch behaviour can be explained within this theoretical approach. One highly-magnetised neutron star with particularly notable glitch properties is PSR J1119-6127. This thesis presents a detailed observational study of its glitches and discusses their implications for theoretical models of the dynamics of the neutron star's crust and exterior magnetic field. Finally, the aftermath of a large glitch from the Vela pulsar is analysed, and the potential to use this data to constrain models of the interior superfluid is discussed.

X-ray bursts originate from accreting neutron stars (NSs) in X-ray binaries (XRBs). They come in two flavours: thermonuclear bursts are due to the sudden runaway burning of the material accreted on the surface; accretion bursts signal a sudden change in the mass accretion rate, leading to enhanced emission in the innermost regions of the accretion flow. While thermonuclear bursts have been observed from 105 NSs as of writing, accretion bursts remain enigmatically confined to only two sources. One of them, the Rapid Burster (RB), is the only one to have shown both types of bursts. My PhD project has dealt with studying this extraordinary source to understand the reason behind its uniqueness and, through that, answer open questions regarding both thermonuclear burning and unstable accretion.

The research in this thesis details ways to characterise and understand variability in short magnetar bursts. Magnetars, neutron stars with exceptionally strong magnetic fields, show variability on many different time scales. Particularly at high X-ray energies, they show bright bursts that are believed to be created by what we call starquakes: a sudden release of energy that triggers global oscillations of the star's solid crust and possibly the interior. These oscillations, observed in the X-ray light curves of the bright giant flares, are of particular interest, because they have to potential to probe the stellar composition and constrain related nuclear physics. Because standard statistical methods fail when applied to magnetar bursts, the research in this thesis aims at developing new methods to search for and characterise oscillations in the light curves of short magnetar bursts, and at un- derstanding variability in these sources as well as the underlying physical processes in general. With this new methodology, several oscillations were found in short bursts from two different magnetars, indicating that similar processes are active in both the short recurrent bursts and the longer, brighter giant flares. Additionally, a new model was developed for the underlying aperiodic variability, which, for the first time, allows a comparison of the X-ray variability in magnetar bursts with toy models of the underlying physics.