Program

Abstracts

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• Johannes in't Zand
  
  Empirical constraints on the cooling and rp-process in X-ray bursts
  
  We discuss the results of modeling the flux decay profile of hundreds of high-quality type-I X-ray bursts from the RXTE-PCA archive with an empirical model consisting of a power law (for the cooling) and a one-sided Gaussian (for the rp burning). It is possible to obtain from this fairly easy constraints on both physical processes.




• Zachary Meisel
  
  Exploring Nuclear Physics Uncertainties in Models of Type-I X-ray Bursts with MESA
  
  We recently developed the capability to model Type-I X-ray bursts with the open-source code MESA. Thus far we have reproduced observed bursts from GS1826-24 and examined the impact of nuclear physics uncertainties on features of the X-ray burst light curve. Results of these studies, comparisons to past work performed with the code KEPLER, and future plans will be presented. Additionally, near-future plans to address some of the most important nuclear physics uncertainties with stable and radioactive ion beams will be presented.




• Andrew Cumming
  
  Winds and mass loss in long X-ray bursts
  
  Strong Eddington-limited X-ray bursts can drive a wind that ejects the outer light element layers, moving the photosphere into the layer of heavy elements produced in the burst. Kajava et al. recently found evidence of heavy elements in the spectrum of an energetic burst from HETE J1900.1-2455. I will discuss the physics of Eddington-driven winds and present new models of radius expansion bursts that take into account mass loss. These models are able to reproduce the timing of the HETE J1900 observations, and make a prediction for what burst properties are needed for heavy elements to be exposed.




• Jerome Chenevez
  
  A search for burst spectral features with NICER
  
  The Neutron star Interior Composition ExploreR (NICER) has been mounted on the International Space Station in summer 2017, and has since observed a number of X-ray bursters with both high timing and high energy resolutions. The soft (0.2-12 keV) energy coverage of NICER is particularly well-suited for X-ray burst investigations. Thermonuclear bursts on the surface of accreting neutron stars reach sometimes luminosities that can temporarily exceed the Eddington limit and drive the photosphere to large radii. Such photospheric radius expansion bursts may eject nuclear burning ashes that are expected to engender absorption features in the burst spectra. Simultaneous detections in NICER spectra of multiple photoionization edges from heavy elements will probe the thermonuclear burning and mixing processes under degenerate conditions. Moreover, the identification of gravitationally redshifted edges would uniquely provide a measure of the neutron star compactness, and thus constitute a probe of the ultra-dense matter equation of state.




• Gry Merete Tveten
  
  From nuclear experiments to astrophysical reaction rates
  
  I could give a short introduction to how nuclear experiments are used to produce the nuclear reaction rates that go into astrophysical models. I would also like to present some recent experimental results on the stable Zn-isotopes that might be relevant to the rp-process.




• Alfredo Estrade
  
  Sensitivity of X-ray bursts to nuclear reaction rates in a single-zone model
  
  We performed X-ray bursts simulations with the ONEZONE single-zone model to evaluate the impact of nuclear reaction rates on the model’s results. We vary individual proton and alpha capture rates in a large nuclear reaction network, and asses their effect on the nucleosynthesis and the lightcurve of the simulated bursts. Our work is an extension of the sensitivity study of Cyburt et al [1] to models with a variety of compositions of the accreted material, with a focus on its hydrogen and helium content. We also consider realistic values for the uncertainty of the reaction rates used. [1] R. H. Cyburt et al, ApJ 830, 55 (2016)




• Hauke Worpel
  
  TBA
  
  TBA




• Anna Watts
  
  Burst oscillations
  
  One very promising technique for measuring the dense matter equation of state (EOS) exploits burst oscillations, hotspots that form on the neutron star surface when material accreted from a companion star undergoes thermonuclear burst. As the star rotates, the hotspot gives rise to a pulsation. Relativistic effects then encode information about the EOS into the pulse profile. However the mechanism that generates burst oscillations remains unknown, 20 years after their discovery. Ignition conditions, flame spread, and the magnetohydrodynamics of the star’s ocean all play a role. I will review the progress that we are making towards cracking this long-standing problem, and establishing burst oscillations as a tool par excellence for probing dense matter. This is a major goal for future large area X-ray telescopes such as eXTP and STROBE-X.




• Zac Johnston
  
  Parameter estimation using large grids of multizone burst models
  
  Modelling of x-ray bursts with multi-zone models has typically focused on individual or small sets of models, and attempts to compare with observations has mostly been limited to using assumed system parameters (such as distance, redshift, anisotropy, etc.). Using Markov chain Monte Carlo (MCMC) methods and the KEPLER code, we explore the large parameter space in order to model multiple burst epochs from GS1826. We address the issue of individual simulations being expensive to run, in order to make an MCMC approach tractable.




• Alex Deibel
  
  Reaction network crust compositions in crust cooling models
  
  A recent work by Lau et al. 2018 (to be submitted) uses a reaction network to follow the compressed ashes of X-ray bursts and superbursts into the inner crust. The authors find that ash compositions with an initially large impurity parameter (Qimp > 40) will ultimately purify to Qimp~10 once they reach the inner crust. As a result, X-ray burst ashes with an initially large impurity parameter and superburst ashes with an initially small impurity result in similar thermal structures for the inner crust. In this talk, I will show how these findings impact crust cooling models for neutron star transients and the effects on the thermal profile of bursting neutron stars.




• Adam Jacobs
  
  X-Ray Burst Reaction Rate Sensitivities
  
  X-ray burst properties can be highly sensitive to the nuclear reaction rates powering the phenomenon. Perhaps the most accessible sensitive property is the lightcurve shape. Much of the curve is powered by rp-process reactions at astrophysical energies involving rare isotopes, meaning many of the rates are uncertain. In this study, I model three reference bursts using the Kepler hydrodynamics code. For each reference a grid of models is generated with varied reaction rates to determine those with the most impact on burst observables and properties, informing nuclear experiment. In this presentation I give an overview of this work and present the key reactions warranting experimental and theoretical constraint.




• Laura Ootes
  
  Constraining shallow heating from crust cooling and superburst ignition
  
  One of the unsolved problems in modelling of neutron star crust cooling is that of the so-called shallow heat. In transiently accreting neutron stars, the crust of the neutron star is heated during an accretion outburst due to deep crustal heating processes involving pycnonuclear reactions, which occur in the inner crust. This causes the crust to become hotter than the core during the outburst, and when the source returns to quiescence, the crust cools down in order to restore thermal equilibrium. X-ray observations allow to track the surface temperature of the neutron star after an outburst and the cooling rates enable us to probe the properties of deeper and deeper layers of the crust with time. Comparison of observed cooling curves with those theoretically calculated have revealed that in many sources an additional, so far not understood heating source must be active during outburst at shallow depths (the shallow heating problem). To gain insight into this problem it is important to constrain the strength and depth of this heating source and we have made significant progress over the last few years in observationally constraining the properties of this mechanism. Shallow heating may be further constrained using the superburst phenomenon. Several sources in which crust cooling is observed have shown superbursts. The ignition depth and temperature required to ignite superbursts are much higher than for ordinary thermonuclear bursts and without shallow heating, it is unclear if the required temperatures can be reached. I will discuss how the shallow heating depth and strength can be constrained further using the requirements from observed cooling curves in quiescence in combination with those from superbursts ignited during outburst.




• Alexander Heger
  
  Modelling of Type I X-Ray bursts
  
  I may show some results from XRB modelling not covered by Monash students. Maybe including the updates we did in modelling for the current models. In the worst case, I may look into some models with the C12+C12 rate in an upcoming nature paper (the rate, no my work).




• Edward Brown
  
  What lies beneath: reaction heating and cooling in the neutron star crust
  
  I will provide updates on our current models of reactive processes in the crust of accreting neutron stars and their influence on X-ray bursts.




• Adelle Goodwin
  
  Neutrino Losses Overestimated in Type I Thermonuclear X-ray Bursts and a New Nuclear Energy Generation Estimate
  
  Type I X-ray bursts are thermonuclear explosions on the surface of accreting neutron stars. Hydrogen rich X-ray bursts burn protons far from the line of stability and can release energy in the form of neutrinos from $\beta$-decays. We have estimated, for the first time, the neutrino fluxes of type I bursts for a range of initial conditions based on the predictions of KEPLER, a 1D implicit hydrodynamics code that calculates the complete nuclear reaction network. We find that neutrino losses are between $6\E{-7}$ for low hydrogen fraction and $0.185$ for high hydrogen fraction, of the total energy per nucleon, \Qnuc. This result is contradictory to the often, but wrongly, adopted value of 35$\,\%$ for neutrino losses during the \textsl{rp} process, in contrast to the original literature. It is only at $\beta$-decays that $\sim35\,\%$ of energy is lost due to neutrino emission, whereas there is no neutrino losses in $(p,\gamma)$ and $(\alpha,p)$ reactions. Using the total measured burst energies from KEPLER for a range of initial conditions, we have determined an approximation formula for the total energy per nucleon released during an X-ray burst, $\Qnuc=0.93+7.41\bar{X}-1.88\bar{X}^2$ $\,\mathrm{MeV}/\mathrm{nucleon}$, where \Xb is the average hydrogen mass fraction of the ignition column, with an RMS error of $0.07\,$MeV/nucleon.




• Yuri cavecchi
  
  X-Ray Burst Rate vs Accretion Rate and Spin Frequency
  
  Nuclear burning and its dependence on the mass accretion rate are fundamental ingredients for describing the complicated observational phenomenology of accreting neutron stars and type I bursts. A long standing puzzle is the decreasing burst rate versus increasing accretion rate experienced by many sources, while theory predicts that the burst rate should constantly increase (nearly) until stabilization. I will show how, by considering different conditions across the surface of the star and including stabilization effects due for example to mixing, it is possible to resolve the apparent contradiction between theory and observations and observe a globally decreasing burst rate even if locally burning keeps increasing with accretion. I will also show how this effect depends on the spin of the star. I will discuss the implications of this scenario for our understanding of nuclear burning in terms both of numerical simulations and observations.




• Douglas Soltesz
  
  Use of (3He,n) Indirect Measurements to Study H and He Burning Reactions of Type-1 X-Ray Bursts
  
  The reaction rate of the 59Cu(p,gamma)60Zn has been identified to have a significant impact on the light curve of X-ray bursts, controlling the reaction flow out of the Ni-Cu cycle impacting the late-time light curve. The 58Ni(3He,n)60Zn indirect measurement can be used to study the 59Cu(p,gamma)60Zn reaction. We are using the neutron evaporation spectrum from 58Ni(3He,n)60Zn in order to extract the level density of 60Zn and constrain the 59Cu(p,gamma)60Zn reaction rate. To agument the (3He,n) technique for lower level-density compound nuclides, a silicon detector array is being developed for use in determining charged-particle decay branching ratios from discrete states. The present status of data analysis and detector development will be discussed, as well as the future plans.




• Matthew Amthor
  
  Coupled Sensitivities in rp-Process Nuclear Reaction Rates
  
  I will present the status of our work on a fully coupled Monte Carlo sensitivity study in the multi-zone burst code Kepler. This work is a next step in understanding the present state of the uncertainty in model-produced x-ray burst observables due to unconstrained (p,g) and (a,p) reaction rates, continuing previous work along this line (Cyburt 2016) that considered individual reaction rate variations in Kepler based on sensitivities identified in the single zone code ONEZONE. Early results indicate some strong higher order effects in reaction rate sensitivities. These are expected due to the strong coupling of reaction flows in the nucleosynthetic network of the rp- and ap-processes. Small scale tests of the run and analysis framework are complete and we plan to present preliminary results of an early set of multi-zone simulations with reaction rate variations sampled in a Monte Carlo approach. The technique will ultimately allow us to extract higher order reaction rate sensitivities that would not have been identified by the individual rate variation study and also allow a first estimation of the true, total uncertainty in burst observables and other model derived values (e.g. mass fractions of burst ashes) due to the fully coupled reaction rate uncertainties.




• Alice Harpole
  
  Multiscale modelling of neutron star oceans
  
  As neutron stars are compact objects, the gravitational field at the surface is very strong. We look at the effects of this, modelling the ocean using general relativistic hydrodynamics. The physics of X-ray bursts acts over a wide range of scales, which introduces numerous when modelling them. We use the multiscale approach to couple together multiple physical models in order to best capture the physics across these various scales. On the smallest scales, the physics is dominated by turbulent burning. The speed of propagation of the burning front is much slower than the acoustic speed, making it difficult to model with conventional numerical schemes. We instead use the low Mach number approximation, which we have derived and implemented for the relativistic fluid equations based on the existing approach developed for the Newtonian case. The large scale propagation of the burning front is believed to be dominated by the Coriolis force. When general relativistic effects are included, frame dragging may also become important. To capture this, we use the relativistic form of the shallow water equations. Finally, we combine the best features of these approximations, extending existing adaptive mesh refinement techniques to include the different physical models at different scales.




• Joonas Nattila
  
  Understanding the nuclear physics of neutron stars with X-ray bursts
  
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• Matthew Caplan
  
  Compositional Domains in Accreted Neutron Star Crusts
  
  In the oceans of accreting neutron stars, hydrogen and helium burns to a produce a mixture of nuclei with a large range of atomic numbers. These mixtures continually freeze out to form new crust, however, recent work suggests that the crust cannot accommodate the entire mixture, and only a limited number of crust compositions can form. I will discuss recent work using molecular dynamics simulations and semianalytic models which suggest that the accreted crust is polycrystalline, formed of domains with distinct compositions. Future work may consider the size of these compositional domains and their impact on crust conductivities, crust heating, and burst phenomenlogy.




• Anna Bilous
  
  Revisiting the fractional amplitudes of type I thermonuclear burst oscillations in the RXTE legacy dataset
  
  Type I thermonuclear X-ray bursts in the low-mass X-ray binaries occur when material accreted from the companion explodes on the surface of a neutron star. For the reasons not entirely understood about 10% of such bursts develop asymmetric brightness patches, which are further observed as modulations of X-ray flux with (approximately) the spin period of a neutron star. These modulations, called burst oscillations, are the unique tools for exploring the nuclear burning in the strong gravity/magnetic fields on the neutron star surface, as well as for probing the state of matter under the most extreme conditions inside the star itself. Thus, describing properties of burst oscillations (in particular, the evolution of oscillation frequency within the burst) is very important. So far, the majority of burst oscillation studies have been performed on the bulk of observations coming from the Rossi X-ray Timing Explorer (RXTE). In our work we re-analyze the archival RXTE data and perform a uniform search for burst oscillations from almost all type I X-ray bursts observed by RXTE. We apply traditional Fourier analysis technique, but rely on more realistic noise models, based on the simulation of photon arrival times for each burst. The simulations take into account the light curve shape, data gaps and dead time influence. In addition, we search for faint signal in stacked power spectra. For the detections, we compute the fractional amplitudes of oscillations and place the upper limits on the factional amplitudes of non-detections. Finally, we compare our findings to the previous works.




• Valery Suleymanov
  
  The influence of accretion on the spectral evolution of X-ray bursting neutron stars.
  
  X-ray bursting neutron stars (NSs) in some low-mass X-ray binaries are attractive sources for determining their masses and radii. We have developed the cooling tail method, which is based on a comparison of the spectral evolution of the cooling NS with the spectra of atmospheric models. It allowed us to obtain strong constraints on the NS radii, which is important for limiting the equation of state of superdense matter in their nuclei. However, in a number of cases the observed spectral evolution of bursts deviates from the theoretical one at luminosities below half of the Eddington one. The higher the persistent accretion luminosity of the system, the greater the magnitude of the deviation. We now have developed a model for NS atmospheres heated by accreted gas and showed that such heating can be the main reason for the observed deviations. At high burst luminosities, the additional heating is inefficient because of particle deceleration by the radiation pressure. Our model describes now the spectral evolution of the X-ray bursts in the ultracompact system 4U 1820--30 at all cooling stages. This allowed us to increase the accuracy of the NS radius determination in this system.




• Jordi Jose
  
  Classical Novae vs X-Ray Bursts: the Physics of Stellar Explosions
  
  At the turn of the 21st Century, new tools and developments, at the crossroads of theoretical and computational astrophysics, observational astronomy, cosmochemistry, and nuclear physics, have revolutionized our understanding of the physics of stellar explosions. The use of space-borne observatories has opened new windows to study the cosmos through multifrequency observations. In parallel to the elemental stellar abundances inferred spectroscopically, cosmochemists are now providing isotopic abundance ratios from micron-sized presolar grains extracted from meteorites. Encapsulated in those grains is pristine information about the suite of nuclear processes that took place in their stellar progenitors. The dawn of supercomputing has also provided astrophysicists withe appropriate tools to study complex physical phenomena that require a multidimensional approach. Last but not least, nuclear physicists have developed new techniques to determine nuclear interactions close to stellar energies. In this talk, a number of breakthroughs from all these different disciplines will be presented, with emphasis on the physical mechanisms that operate during novae and XRBs.




• Emma van der Wateren
  
  Obtaining neutron star mass and radius estimates from the burst oscillations of the accreting MSP J1814-338
  
  Burst oscillations have been observed during thermonuclear X-ray bursts from low mass X-ray binaries. The light coming from the neutron star surface is affected by factors such as general relativistic light deflection, beaming and aberration, which depend on the structure of the star. Modeling their light curve can therefore provide valuable information about the stellar interior. Here we present results which will help constrain the mass and radius of the neutron star. We analyse the light curve of 28 bursts from the accreting millisecond pulsar XTE J1814-338. Using a ray tracing code we fit the folded light curves to a surface pattern model and perform Bayesian inference to estimate the equation of state. Furthermore, we investigate uncertainties in the light curve shape, not addressed by previous studies. This study will serve as a pathfinder for obtaining mass and radius estimates from a larger number of accreting neutron stars, with the overarching goal of constraining the equation of state of dense matter in neutron stars.




• Motoko Serino
  
  MAXI observations of X-ray bursts
  
  Monitor of All-sky X-ray Image (MAXI) observes X-ray sources every 92 minutes. MAXI has two instruments, the Gas Slit Camera (GSC) and the Solid-state Slit Camera. The GSC has larger field of view and effective area, so it is suitable to observe X-ray bursts. Usually the durations of X-ray bursts are not longer than the scan interval. If an X-ray burst event is observed in two or more consecutive scans, we identify it as long X-ray burst which is superburst or intermediate-duration burst. MAXI is useful to monitor the persistent emissions of X-ray bursters. We can study the correlations between the burst activities and the accretion rates. For example we found some superburst occurred when the persistent emissions of the sources were bright. On the other hand some of the long bursts occurred when the accretion rates were low. A summary of these observations will be given in the presentation."




• Frank Chambers
  
  New burning physics and burst oscillations
  
  Accreting neutron stars (NS) can exhibit high-frequency modulations in their lightcurves during thermonuclear X-ray bursts, known as burst oscillations. Their frequencies can be offset from the spin frequency of the NS by several Hz (known independently), and can drift by 1-3 Hz. One plausible explanation of this phenomenon is that a wave is present in the bursting ocean that decreases in frequency (in the rotating frame) as the burst cools, hence explaining the drifts. The strongest candidate is the buoyant r-mode, however, models for the ocean background used in previous studies over-predict frequency drifts by several Hz. Using new background models that were developed to explain the short recurrence times of some bursts (which include shallow heating, and burning in the tail of the burst), the evolution of the buoyant r-mode was calculated. The resulting frequency drifts are smaller, in line with observations.




• Can Gungor
  
  Partial Accretion in the Propeller Stage of Aql X-1
  
  We introduce a unique technique, from the X-ray light curve, to calculate the mass fraction rate transferring onto the neutron star depending on the fastness parameter in the weak propeller stage assuming that the knee, seen in the decay stage of the light curves of low mass X-ray binary transients (LMXBTs), corresponds to the transition from the accretion stage to the propeller stage. We present the outcome of spectral analyses of RXTE/PCA observations of LMXBT Aql X-1 throughout the 2000 and the 2011 outbursts. We applied our technique to a sample of outbursts of Aql X-1 with different maximum flux and duration times. We show that different outbursts follow a similar trend in the parameter space of accreted/inflowing mass flux fraction versus fastness parameter. We plan to introduce preliminary results of Aql X-1 observation obtained by ‘Insight - Hard X-ray Modulation Telescope’ (Insight-HXMT). We discuss the capability of Insight-HXMT observations and the future plan, in a context of having Aql X-1 as an almost unique lab for investigating the outburst, hysteresis, propeller and type-I bursts.




• Celia Sanchez Fernandez
  
  X-ray burst-induced spectral variability in Ginga 1826-24
  
  INTEGRAL has bien observing regularly the clocked burster Ginga 1826-24 over 15 years of operations. We have compiled all the available data and studied the effects of the bursting activity no the system persistent emissiion. We present here the results of this work.




• Gaurava Kumar Jaisawal
  
  NICER views of thermonuclear bursts
  
  A new dawn has begun in the high energy astrophysics after the launch of the Neutron star Interior Composition Explorer (NICER) instrument on the International Space Station in June 2017. NICER has unprecedented timing and spectral sensitivity in the 0.2-12 keV range and is designed to explore the extreme physics around neutron stars. Some of their exotic physics rely on understanding thermonuclear bursts that arise from the unstable burning of accreted hydrogen and/or helium on the neutron star surface. Recently, we have witnessed many exciting X-ray bursts through the NICER’s eye probing towards the extreme burning physics. Some of the results will be presented and discussed during the meeting.




• Khaled Alizai
  
  A multi-instrument catalog of long thermonuclear X-ray bursts.
  
  The International Space Science Institute (ISSI) have previously produced a catalog, based on preliminary data, for long thermonuclear X-ray bursts. The bulk of my PhD studies will be to continue the work done by ISSI and create a catalog of both intermediate – and ‘Superbursts’, based on verified data. The data will be acquired, for starters, from the MINBAR database and will be complemented with new observations for NICER, and other active missions. In this talk, I will introduce my initial thoughts on the structure and content of the catalog. The talk will be based on a recent intermediate burst from the source ‘SAX J1712.6- 3738’, for which a spectral analysis of the burst will be presented. Furthermore, I will discuss the challenges of data quality, giving a superburst observation from ‘SAX J1747.0-2853’, observed on 13 February, 2011 as an example.