saprEMo: a simplified algorithm for predicting detections of electromagnetic transients in surveys

The multi-wavelength detection of GW170817 has inaugurated multi-messenger astronomy. The next step consists in interpreting observations coming from population of gravitational wave sources. We introduce saprEMo, a tool aimed at predicting the number of electromagnetic signals characterised by a specific light curve and spectrum, expected in a particular sky survey. By looking at past surveys, saprEMo allows us to constrain models of electromagnetic emission or event rates. Applying saprEMo to proposed astronomical missions/observing campaigns provides a perspective on their scientific impact and tests the effect of adopting different observational strategies. For our first case study, we adopt a model of spindown-powered X-ray emission predicted for a binary neutron star merger producing a long-lived neutron star. We apply saprEMo on data collected by XMM-Newton and Chandra and during $10^4$ s of observations with the mission concept THESEUS. We demonstrate that our emission model and binary neutron star merger rate imply the presence of some signals in the XMM-Newton catalogs. We also show that the new class of X-ray transients found by Bauer et al. in the Chandra Deep Field-South is marginally consistent with the expected rate. Finally, by studying the mission concept THESEUS, we demonstrate the substantial impact of a much larger field of view in searches of X-ray transients

The formation of compact object binaries through isolated binary evolution

Observations indicate that most stars are in binary or higher multiplicity systems (Preibisch et al., 1999; Sana et al., 2012, 2013; Duchêne and Kraus, 2013; Chini et al., 2013; Sota et al., 2014; Kobulnicky et al., 2014; Dunstall et al., 2015; Moe and Di Stefano, 2017; Sana, 2017). In a binary two stars orbit each other, bound by their mutual gravitational pull. If the orbital separation is short enough the stars interact, drastically altering their evolution. In the past decades, the means to perform complex calculations have drastically improved, giving us the chance to explore the physics of binary evolution in greater detail. This helped explain several observed properties, amongst others, why some stars are more luminous than expected or have peculiar surface abundances. Nonetheless, large uncertainties persist in the field of binary star physics. In 2015, the gravitational waves from two colliding black holes were detected for the first time (Abbott et al., 2016b). For decades it has been hypothesised that two massive stars in an isolated binary could interact without external influences and form a binary black hole system tight enough to merge in a Hubble time (van den Heuvel and De Loore, 1973; Tutukov and Yungelson, 1973). In this dissertation I follow in the footsteps of many other studies and assume the observed gravitational-wave events come from isolated binary evolution, even though other formation channels for the mergers of neutron stars and black holes are also possible. The aim is to study what constraints, if any, properties of gravitational-wave events can place on the evolution of massive stars in binaries. The general approach in of this dissertation is to evolve a population of stars under various model assumptions and estimate the rates and properties of gravitational wave mergers for each model. The predicted distributions enable a quantitative or qualitative assessment of the impact of current uncertainties in binary-star physics on estimates of the rates and masses of gravitational-wave events. Evaluating the effect of uncertainties is crucial to determine whether comparisons between synthetic populations of gravitational-wave sources and observations can place meaningful constraints on binary-star physics. If the uncertainties are large and model-dependent features are not predicted, then the detections of gravitational-wave mergers may only provide marginal constraints. In this dissertation I assess the impact of the following model assumptions. In chapter 4 I investigate the uncertainties in the rate and initial chemical composition with which stars form and the impact of these uncertainties on the predictions of the merger rate of neutron stars and black holes. In chapter 5 I vary the response of stars to mass loss and explore how it alters the interactions that lead to the formation of binary black holes. In chapter 6 I examine whether Cygnus X-1 may evolve into a binary black hole system and if the observed mass of the black hole in Cygnus X-1 provides constraints on the wind mass-loss rates of stars. Chapter 1 and chapter 2 provide introductory material for the reader on stellar evolution and binary interactions. Chapter 3 summarises the theoretical model used in this dissertation to evolve a population of stars. Chapter 7 provides a summary and personal view on the conclusions of this dissertation

Formation of the first three gravitational-wave observations through isolated binary evolution

During its first 4 months of taking data, Advanced LIGO has detected gravitational waves from two binary black hole mergers, GW150914 and GW151226, along with the statistically less significant binary black hole merger candidate LVT151012. We use our rapid binary population synthesis code COMPAS to show that all three events can be explained by a single evolutionary channel -- classical isolated binary evolution via mass transfer including a common envelope phase. We show all three events could have formed in low-metallicity environments (Z = 0.001) from progenitor binaries with typical total masses $\gtrsim 160 M_\odot$, $\gtrsim 60 M_\odot$ and $\gtrsim 90 M_\odot$, for GW150914, GW151226, and LVT151012, respectively.Comment: Published in Nature Communication

Accuracy of inference on the physics of binary evolution from gravitational-wave observations

The properties of the population of merging binary black holes encode some of the uncertain physics of the evolution of massive stars in binaries. The binary black hole merger rate and chirp mass distribution are being measured by ground-based gravitational-wave detectors. We consider isolated binary evolution and explore how accurately the physical model can be constrained with such observations by applying the Fisher information matrix to the merging black hole population simulated with the rapid binary population synthesis code COMPAS. We investigate variations in four COMPAS parameters: common envelope efficiency, kick velocity dispersion, and mass loss rates during the luminous blue variable and Wolf--Rayet stellar evolutionary phases. We find that 1000 observations would constrain these model parameters to a fractional accuracy of a few percent. Given the empirically determined binary black hole merger rate, we can expect gravitational-wave observations alone to place strong constraints on the physics of stellar and binary evolution within a few years.Comment: 12 pages, 9 figures; version accepted by Monthly Notices of the Royal Astronomical Societ

STROOPWAFEL: Simulating rare outcomes from astrophysical populations, with application to gravitational-wave sources

Gravitational-wave observations of double compact object (DCO) mergers are providing new insights into the physics of massive stars and the evolution of binary systems. Making the most of expected near-future observations for understanding stellar physics will rely on comparisons with binary population synthesis models. However, the vast majority of simulated binaries never produce DCOs, which makes calculating such populations computationally inefficient. We present an importance sampling algorithm, STROOPWAFEL, that improves the computational efficiency of population studies of rare events, by focusing the simulation around regions of the initial parameter space found to produce outputs of interest. We implement the algorithm in the binary population synthesis code COMPAS, and compare the efficiency of our implementation to the standard method of Monte Carlo sampling from the birth probability distributions. STROOPWAFEL finds $\sim$25-200 times more DCO mergers than the standard sampling method with the same simulation size, and so speeds up simulations by up to two orders of magnitude. Finding more DCO mergers automatically maps the parameter space with far higher resolution than when using the traditional sampling. This increase in efficiency also leads to a decrease of a factor $\sim$3-10 in statistical sampling uncertainty for the predictions from the simulations. This is particularly notable for the distribution functions of observable quantities such as the black hole and neutron star chirp mass distribution, including in the tails of the distribution functions where predictions using standard sampling can be dominated by sampling noise.Comment: Accepted. Data and scripts to reproduce main results is publicly available. The code for the STROOPWAFEL algorithm will be made publicly available. Early inquiries can be addressed to the lead autho

Common-Envelope Episodes that lead to Double Neutron Star formation

Close double neutron stars have been observed as Galactic radio pulsars, while their mergers have been detected as gamma-ray bursts and gravitational-wave sources. They are believed to have experienced at least one common-envelope episode during their evolution prior to double neutron star formation. In the last decades there have been numerous efforts to understand the details of the common-envelope phase, but its computational modelling remains challenging. We present and discuss the properties of the donor and the binary at the onset of the Roche-lobe overflow leading to these common-envelope episodes as predicted by rapid binary population synthesis models. These properties can be used as initial conditions for detailed simulations of the common-envelope phase. There are three distinctive populations, classified by the evolutionary stage of the donor at the moment of the onset of the Roche-lobe overflow: giant donors with fully-convective envelopes, cool donors with partially-convective envelopes, and hot donors with radiative envelopes. We also estimate that, for standard assumptions, tides would not circularise a large fraction of these systems by the onset of Roche-lobe overflow. This makes the study and understanding of eccentric mass-transferring systems relevant for double neutron star populations.Comment: 26 pages, 10 figures. Includes bug fix. Two new figures and an appendix adde