arXiv subjects

Tim Dietrich

Publications and source records attributed to Tim Dietrich.

At least 91 records · Page 5Linked to original sources

arXiv2023

Using Neural Networks to Perform Rapid High-Dimensional Kilonova Parameter Inference

On the 17th of August, 2017 came the simultaneous detections of GW170817, a gravitational wave that originated from the coalescence of two neutron stars, along with the gamma-ray burst GRB170817A, and the kilonova counterpart AT2017gfo. Since then, there has been much excitement surrounding the study of neutron star mergers, both observationally, using a variety of tools, and theoretically, with the development of complex models describing the gravitational-wave and electromagnetic signals. In this work, we improve upon our pipeline to infer kilonova properties from observed light-curves by employing a Neural-Network framework that reduces execution time and handles much larger simulation sets than previously possible. In particular, we use the radiative transfer code POSSIS to construct 5-dimensional kilonova grids where we employ different functional forms for the angular dependence of the dynamical ejecta component. We find that incorporating an angular dependence improves the fit to the AT2017gfo light-curves by up to ~50% when quantified in terms of the weighted Mean Square Error.

astro-ph.HE↗

arXiv2023

Measuring tidal effects with the Einstein Telescope: A design study

Over the last few years, there has been a large momentum to ensure that the third-generation era of gravitational wave detectors will find its realisation in the next decades, and numerous design studies have been ongoing for some time. Some of the main factors determining the cost of the Einstein Telescope lie in the length of the interferometer arms and its shape: L-shaped detectors versus a single triangular configuration. Both designs are further expected to include a xylophone configuration for improvement on both ends of the frequency bandwidth of the detector. We consider binary neutron star sources in our study, as examples of sources already observed with the current generation detectors and ones which hold most promise given the broader frequency band and higher sensitivity of the third-generation detectors. We estimate parameters of the sources, with different kinds of configurations of the Einstein Telescope detector, varying arm-lengths as well as shapes and alignments. Overall, we find little improvement with respect to changing the shape, or alignment. However, there are noticeable differences in the estimates of some parameters, including tidal deformability, when varying the arm-length of the detectors. In addition, we also study the effect of changing the laser power, and the lower limit of the frequency band in which we perform the analysis.

astro-ph.IM↗

arXiv2023

Second release of the CoRe database of binary neutron star merger waveforms

We present the second data release of gravitational waveforms from binary neutron star merger simulations performed by the Computational Relativity (CoRe) collaboration. The current database consists of 254 different binary neutron star configurations and a total of 590 individual numerical-relativity simulations using various grid resolutions. The released waveform data contain the strain and the Weyl curvature multipoles up to $\ell=m=4$. They span a significant portion of the mass, mass-ratio,spin and eccentricity parameter space and include targeted configurations to the events GW170817 and GW190425. CoRe simulations are performed with 18 different equations of state, seven of which are finite temperature models, and three of which account for non-hadronic degrees of freedom. About half of the released data are computed with high-order hydrodynamics schemes for tens of orbits to merger; the other half is computed with advanced microphysics. We showcase a standard waveform error analysis and discuss the accuracy of the database in terms of faithfulness. We present ready-to-use fitting formulas for equation of state-insensitive relations at merger (e.g. merger frequency), luminosity peak, and post-merger spectrum.

gr-qc↗

arXiv2023

Revealing the strength of three-nucleon interactions with the Einstein Telescope

Three-nucleon forces are crucial for the accurate description of nuclear systems, including dense matter probed in neutron stars. We explore nuclear Hamiltonians that reproduce two-nucleon scattering data and properties of light nuclei, but differ in the three-nucleon interactions among neutrons. While no significantly improved constraints can be obtained from current astrophysical data, we show that observations of neutron star mergers by next-generation detectors like the proposed Einstein Telescope could provide strong evidence to distinguish between these Hamiltonians.

astro-ph.HE↗

arXiv2023

Parameter estimation methods for analyzing overlapping gravitational wave signals in the third-generation detector era

In the coming years, third-generation detectors such as Einstein Telescope and Cosmic Explorer will enter the network of ground-based gravitational-wave detectors. Their current design predicts a significantly improved sensitivity band with a lower minimum frequency than existing detectors. This, combined with the increased arm length, leads to two major effects: the detection of more signals and the detection of longer signals. Both will result in a large number of overlapping signals. It has been shown that such overlapping signals can lead to biases in the recovered parameters, which would adversely affect the science extracted from the observed binary merger signals. In this work, we analyze overlapping binary black hole coalescences with two methods to analyze multi-signal observations: \textit{hierarchical subtraction} and \textit{joint parameter estimation}. We find that these methods enable a reliable parameter extraction in most cases and that joint parameter estimation is usually more precise but comes with higher computational costs.

gr-qc↗

arXiv2023

Synthetic radio images of structured GRB and kilonova afterglows

In this paper, we compute and analyze synthetic radio images of gamma-ray bursts and kilonova afterglows. For modeling the former, we consider GRB170817A-inspired set of parameters, while for the latter, we employ ejecta profiles from numerical-relativity simulations. We find that the kilonova afterglow sky map has a doughnut-like structure at early times that becomes more ring-like at late times. This is caused by the fact that the synchrotron emission from electrons following Maxwellian distribution function dominates the early, beamed, emission while emissions from electrons following power-law distribution is important at late times. For an on-axis observer, the image flux centroid moves on the image plane initially away from the observer. Flux centroid displacement The image sizes, we find, are the largest for equal mass merger simulations with the soft equation of state. The presence of a kilonova afterglow affects the properties inferred from the source sky map even if the gamma-ray burst afterglow dominates the total flux density. The main effect is the reduction of the mean apparent velocity of the source, and an increase in the source size. Thus, neglecting the presence of the kilonova afterglow may lead to systematic errors in the inference of gamma-ray burst properties from the sky map observations. Notably, at the observing angle inferred for GRB170817A the presence of kilonova afterglow would affect the sky map properties only at very late times $t\gtrsim1500\,$days.

astro-ph.HE↗

arXiv2023

Modeling kilonova afterglows: Effects of the thermal electron population and interaction with GRB outflows

Given an increasing number of gamma-ray bursts accompanied by potential kilonovae there is a growing importance to advance modelling of kilonova afterglows. In this work, we investigate how the presence of two electron populations that follow a Maxwellian (thermal) and a power-law (non-thermal) distributions affect kilonova afterglow light curves. We employ semi-analytic afterglow model, $\texttt{PyBlastAfterglow}$. We consider kilonova ejecta profiles from ab-initio numerical relativity binary neutron star merger simulations, targeted to GW170817. We do not perform model selection. We find that the emission from thermal electrons dominates at early times. If the interstellar medium density is high (${\simeq}0.1\,\ccm$) it adds an early time peak to the light curve. As ejecta decelerates the spectral and temporal indexes change in a characteristic way that, if observed, can be used to reconstruct the ejecta velocity distribution. For the low interstellar medium density, inferred for GRB 170817A, the emission from the non-thermal electron population generally dominates. We also assess how kilonova afterglow light curves change if the interstellar medium has been partially removed and pre-accelerated by laterally expanding gamma-ray burst ejecta. We find that the main effect is the emission suppression at early time ${\lesssim}10^{3}\,$days, and at its maximum it reaches ${\sim}40\%$ when the fast tail of the kilonova ejecta moves subsonically through the wake of laterally spreading gamma-ray burst ejecta. The subsequent rebrightening, when these ejecta break through and shocks form, is very mild (${\lesssim}10\%$), and may not be observable.

astro-ph.HE↗

arXiv2023

Reverse phase transitions in binary neutron-star systems with exotic-matter cores

Multi-messenger observations of binary neutron star mergers provide a unique opportunity to constrain the dense-matter equation of state. Although it is known from quantum chromodynamics that hadronic matter will undergo a phase transition to exotic forms of matter, e.g., quark matter, the onset density of such a phase transition cannot be computed from first principles. Hence, it remains an open question if such phase transitions occur inside isolated neutron stars or during binary neutron star mergers, or if they appear at even higher densities that are not realized in the Cosmos. In this article, we perform numerical-relativity simulations of neutron-star mergers and investigate scenarios in which the onset density of such a phase transition is exceeded in at least one inspiralling binary component. Our simulations reveal that shortly before the merger it is possible that such stars undergo a "reverse phase transition", i.e., densities decrease and the quark core inside the star disappears leaving a purely hadronic star at merger. After the merger, when densities increase once more, the phase transition occurs again and leads, in the cases considered in this work, to a rapid formation of a black hole. We compute the gravitational-wave signal and the mass ejection for our simulations of such scenarios and find clear signatures that are related to the post-merger phase transition, e.g., smaller ejecta masses due to the softening of the equation of state through the quark core formation. Unfortunately, we do not find measurable imprints of the reverse phase transition.

gr-qc↗

arXiv2022

How does dark matter affect compact star properties and high density constraints of strongly interacting matter

We study the impact of asymmetric bosonic dark matter on neutron star properties, including possible changes of tidal deformability, maximum mass, radius, and matter distribution inside the star. The conditions at which dark matter particles tend to condensate in the star's core or create an extended halo are presented. We show that dark matter condensed in a core leads to a decrease of the total gravitational mass and tidal deformability compared to a pure baryonic star, which we will perceive as an effective softening of the equation of state. On the other hand, the presence of a dark matter halo increases those observable quantities. Thus, observational data on compact stars could be affected by accumulated dark matter and, consequently, constraints we put on strongly interacting matter at high densities. To confirm the presence of dark matter in the compact star's interior, and to break the degeneracy between the effect of accumulated dark matter and strongly interacting matter properties at high densities, several astrophysical and GW tests are proposed.

astro-ph.HE↗

arXiv2022

Understanding binary neutron star collisions with hypermodels

Gravitational waves from the collision of binary neutron stars provide a unique opportunity to study the behaviour of supranuclear matter, the fundamental properties of gravity, and the cosmic history of our Universe. However, given the complexity of Einstein's Field Equations, theoretical models that enable source-property inference suffer from systematic uncertainties due to simplifying assumptions. We develop a hypermodel approach to compare and measure the uncertainty gravitational-wave approximants. Using state-of-the-art models, we apply this new technique to the binary neutron star observations GW170817 and GW190425 and the sub-threshold candidate GW200311_103121. Our analysis reveals subtle systematic differences between waveform models, and a frequency-dependence study suggests that this is due to the treatment of the tidal sector. This new technique provides a proving ground for model development, and a means to identify waveform-systematics in future observing runs where detector improvements will increase the number and clarity of binary neutron star collisions we observe.

gr-qc↗

arXiv2022

Snowmass 2021 Cosmic Frontier White Paper: The Dense Matter Equation of State and QCD Phase Transitions

Our limited understanding of the physical properties of matter at ultra-high density, high proton/neutron number asymmetry, and low temperature is presently one of the major outstanding problems in physics. As matter in this extreme state is known to only exist stably in the cores of neutron stars (NSs), complementary measurements from electromagnetic and gravitational wave astrophysical observations of NSs, combined with terrestrial laboratory constraints and further theoretical investigations, hold the promise to provide important insight into the properties of matter in a region of the quantum chromodynamics phase space that is otherwise inaccessible. This multidisciplinary endeavor imposes the following requirements for facilities and resources in the upcoming decade and beyond: * A next generation of gravitational wave detectors to uncover more double NS and neutron star-black hole mergers; * Sensitive radio telescopes to find the most massive and fastest spinning NSs; * Large-area, high-time-resolution and/or high angular resolution X-ray telescopes to constrain the NS mass-radius relation; * Suitable laboratory facilities for nuclear physics experiments to constrain the dense matter equation of state; * Funding resources for theoretical studies of matter in this regime; * The availability of modern large-scale high performance computing infrastructure. The same facilities and resources would also enable significant advances in other high-profile fields of inquiry in modern physics such as the nature of dark matter, alternative theories of gravity, nucleon superfluidity and superconductivity, as well as an array of astrophysics, including but not limited to stellar evolution, nucleosynthesis, and primordial black holes.

astro-ph.HE↗

arXiv2022

Long-Term Simulations of Dynamical Ejecta: Homologous Expansion and Kilonova Properties

Accurate numerical-relativity simulations are essential to study the rich phenomenology of binary neutron star systems. In this work, we focus on the material that is dynamically ejected during the merger process and on the kilonova transient it produces. Typically, radiative transfer simulations of kilonova light curves from ejecta make the assumption of homologous expansion, but this condition might not always be met at the end of usually very short numerical-relativity simulations. In this article, we adjust the infrastructure of the BAM code to enable longer simulations of the dynamical ejecta with the aim of investigating when the condition of homologous expansion is satisfied. In fact, we observe that the deviations from a perfect homologous expansion are about 30% at roughly 100ms after the merger. To determine how these deviations might affect the calculation of kilonova light curves, we extract the ejecta data for different reference times and use them as input for radiative transfer simulations. Our results show that the light curves for extraction times later than 80ms after the merger deviate by less than 0.4mag and are mostly consistent with numerical noise. Accordingly, deviations from the homologous expansion for the dynamical ejecta component are negligible for the purpose of kilonova modelling.

astro-ph.HE↗

arXiv2022

Incorporating a radiative hydrodynamics scheme in the numerical-relativity code BAM

To study binary neutron star systems and to interpret observational data such as gravitational-wave and kilonova signals, one needs an accurate description of the processes that take place during the final stages of the coalescence, e.g., through numerical-relativity simulations. In this work, we present an updated version of the numerical-relativity code BAM in order to incorporate nuclear-theory based Equations of State and a simple description of neutrino interactions through a Neutrino Leakage Scheme. Different test simulations, for stars undergoing a neutrino-induced gravitational collapse and for binary neutron stars systems, validate our new implementation. For the binary neutron stars systems, we show that we can evolve stably and accurately distinct microphysical models employing the different equations of state: SFHo, DD2, and the hyperonic BHB$Λϕ$. Overall, our test simulations have good agreement with those reported in the literature.

gr-qc↗

arXiv2022

Numerical Simulations of Dark Matter Admixed Neutron Star Binaries

Multi-messenger observations of compact binary mergers provide a new way to constrain the nature of dark matter that may accumulate in and around neutron stars. In this article, we extend the infrastructure of our numerical-relativity code BAM to enable the simulation of neutron stars that contain an additional mirror dark matter component. We perform single star tests to verify our code and the first binary neutron star simulations of this kind. We find that the presence of dark matter reduces the lifetime of the merger remnant and favors a prompt collapse to a black hole. Furthermore, we find differences in the merger time for systems with the same total mass and mass ratio, but different amounts of dark matter. Finally, we find that electromagnetic signals produced by the merger of binary neutron stars admixed with dark matter are very unlikely to be as bright as their dark matter free counterparts. Given the increasing sensitivity of multi-messenger facilities, our analysis gives a new perspective on how to probe the presence of dark matter.

gr-qc↗

arXiv2022

Multi-messenger constraints on the Hubble constant through combination of gravitational waves, gamma-ray bursts and kilonovae from neutron star mergers

The simultaneous detection of gravitational waves and light from the binary neutron star merger GW170817 led to independent measurements of distance and redshift, providing a direct estimate of the Hubble constant $H_0$ that does not rely on a cosmic distance ladder nor assumes a specific cosmological model. By using gravitational waves as ''standard sirens'', this approach holds promise to arbitrate the existing tension between the $H_0$ value inferred from the cosmic microwave background and those obtained from local measurements. However, the known degeneracy in the gravitational-wave analysis between distance and inclination of the source lead to a $H_0$ value from GW170817 that was not precise enough to resolve the existing tension. In this review, we summarize recent works exploiting the viewing-angle dependence of the electromagnetic signal, namely the associated short gamma-ray burst and kilonova, to constrain the system inclination and improve on $H_0$. We outline the key ingredients of the different methods, summarize the results obtained in the aftermath of GW170817 and discuss the possible systematics introduced by each of these methods.

astro-ph.HE↗

arXiv2022

Elliptica: a new pseudo-spectral code for the construction of initial data

Numerical studies of the dynamics of gravitational systems, e.g., black hole-neutron star systems, require physical and constraint-satisfying initial data. In this article, we present the newly developed pseudo-spectral code Elliptica, an infrastructure for construction of initial data for various binary and single gravitational systems of all kinds. The elliptic equations under consideration are solved on a single spatial hypersurface of the spacetime manifold. Using coordinate maps, the hypersurface is covered by patches whose boundaries can adapt to the surface of the compact objects. To solve elliptic equations with arbitrary boundary condition, Elliptica deploys a Schur complement domain decomposition method with a direct solver. In this version, we use cubed sphere coordinate maps and the fields are expanded using Chebyshev polynomials of the first kind. Here, we explain the building blocks of Elliptica and the initial data construction algorithm for a black hole-neutron star binary system. We perform convergence tests and evolve the data to validate our results. Within our framework, the neutron star can reach spin values close to breakup with arbitrary direction, while the black hole can have arbitrary spin with dimensionless spin magnitude $\sim 0.8$.

gr-qc↗

arXiv2022

Advancing the Landscape of Multimessenger Science in the Next Decade

The last decade has brought about a profound transformation in multimessenger science. Ten years ago, facilities had been built or were under construction that would eventually discover the nature of objects in our universe could be detected through multiple messengers. Nonetheless, multimessenger science was hardly more than a dream. The rewards for our foresight were finally realized through IceCube's discovery of the diffuse astrophysical neutrino flux, the first observation of gravitational waves by LIGO, and the first joint detections in gravitational waves and photons and in neutrinos and photons. Today we live in the dawn of the multimessenger era. The successes of the multimessenger campaigns of the last decade have pushed multimessenger science to the forefront of priority science areas in both the particle physics and the astrophysics communities. Multimessenger science provides new methods of testing fundamental theories about the nature of matter and energy, particularly in conditions that are not reproducible on Earth. This white paper will present the science and facilities that will provide opportunities for the particle physics community renew its commitment and maintain its leadership in multimessenger science.

astro-ph.HE↗

arXiv2022

Quantifying modeling uncertainties when combining multiple gravitational-wave detections from binary neutron star sources

With the increasing sensitivity of gravitational-wave detectors, we expect to observe multiple binary neutron-star systems through gravitational waves in the near future. The combined analysis of these gravitational-wave signals offers the possibility to constrain the neutron-star radius and the equation of state of dense nuclear matter with unprecedented accuracy. However, it is crucial to ensure that uncertainties inherent in the gravitational-wave models will not lead to systematic biases when information from multiple detections are combined. To quantify waveform systematics, we perform an extensive simulation campaign of binary neutron-star sources and analyse them with a set of four different waveform models. Based on our analysis with about 38 simulations, we find that statistical uncertainties in the neutron-star radius decrease to $\pm 250\rm m$ ($2\%$ at $90\%$ credible interval) but that systematic differences between currently employed waveform models can be twice as large. Hence, it will be essential to ensure that systematic biases will not become dominant in inferences of the neutron-star equation of state when capitalizing on future developments.

astro-ph.HE↗