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Andreas Bauswein

Publications and source records attributed to Andreas Bauswein.

At least 37 records · Page 2Linked to original sources
arXiv2023

Exploring thermal effects of the hadron-quark matter transition in neutron star mergers

We study the importance of the thermal behavior of the hadron-quark phase transition in neutron star (NS) mergers. To this end, we devise a new scheme approximating thermal effects to supplement any cold, barotropic hybrid equation of state (EoS) model, i.e. two-phase EoS constructions with a hadronic regime and a phase of deconfined quark matter. The consideration of temperature-dependent phase boundaries turns out to be critical for a quantitative description of quark matter effects in NS mergers, since the coexistence phase can introduce a strong softening of the EoS at finite temperature, which is even more significant than the change of the EoS by the phase transition at T=0. We validate our approach by comparing to existing fully temperature-dependent EoS models and find a very good quantitative agreement of postmerger gravitational-wave (GW) features. Simulations with the commonly-used thermal ideal-gas approach exhibit sizable differences compared to full hybrid models implying that its use in NS merger simulations with quark matter is problematic. Our new scheme provides the means to isolate thermal effects of quark matter from the properties of the cold hybrid EoS and thus allows an assessment of the thermal behavior alone. We show that different shapes of the phase boundaries at finite temperature can have a large impact on the postmerger dynamics and GW signal for the same cold hybrid model. This finding demonstrates that postmerger GW emission contains important complementary information compared to properties extracted from cold stars. We also show by concrete examples that it is even possible for quark matter to only occur and thus be detectable in finite-temperature systems like merger remnants but not in cold NSs (abbreviated).

astro-ph.HE↗
arXiv2023

Measuring the Hubble constant with kilonovae using the Expanding Photosphere Method

While gravitational wave (GW) standard sirens from neutron star (NS) mergers have been proposed to offer good measurements of the Hubble constant, we show in this paper how a variation of the expanding photosphere method (EPM) or spectral-fitting expanding atmosphere method, applied to the kilonovae (KNe) associated with the mergers, can provide an independent distance measurement to individual mergers that is potentially accurate to within a few percent. There are four reasons why the KN-EPM overcomes the major uncertainties commonly associated with this method in supernovae: 1) the early continuum is very well-reproduced by a blackbody spectrum, 2) the dilution effect from electron scattering opacity is likely negligible, 3) the explosion times are exactly known due to the GW detection, and 4) the ejecta geometry is, at least in some cases, highly spherical and can be constrained from line-shape analysis. We provide an analysis of the early VLT/X-shooter spectra AT2017gfo showing how the luminosity distance can be determined, and find a luminosity distance of $D_L = 44.5\pm0.8$ Mpc in agreement with, but more precise than, previous methods. We investigate the dominant systematic uncertainties, but our simple framework, which assumes a blackbody photosphere, does not account for the full time-dependent three-dimensional radiative transfer effects, so this distance should be treated as preliminary. The luminosity distance corresponds to an estimated Hubble constant of $H_0 = 67.0\pm 3.6$ km s$^{-1}$ Mpc$^{-1}$, where the dominant uncertainty is due to the modelling of the host peculiar velocity. We also estimate the expected constraints on $H_0$ from future KN-EPM-analysis with the upcoming O4 and O5 runs of the LIGO collaboration GW-detectors, where five to ten similar KNe would yield 1\% precision cosmological constraints.

astro-ph.CO↗
arXiv2023

Impact of pions on binary neutron star mergers

We study the impact of pions in simulations of neutron star mergers and explore the impact on gravitational-wave observables. We model charged and neutral pions as a non-interacting Boson gas with a chosen, constant effective mass. We add the contributions of pions, which can occur as a condensate or as a thermal population, to existing temperature and composition dependent equations of state. Compared to the models without pions, the presence of a pion condensate decreases the characteristic properties of cold, non-rotating neutron stars such as the maximum mass, the radius and the tidal deformability. We conduct relativistic hydrodynamical simulations of neutron star mergers for these modified equations of state models and compare to the original models, which ignore pions. Generally, the inclusion of pions leads to a softening of the equation of state, which is more pronounced for smaller effective pion masses. We find a shift of the dominant postmerger gravitational-wave frequency by up to 150~Hz to higher frequencies and a reduction of the threshold binary mass for prompt black-hole formation by up to 0.07~$M_\odot$. We evaluate empirical relations between the threshold mass or the dominant postmerger gravitational-wave frequency and stellar parameters of nonrotating neutron stars. These relations are constructed to extract these stellar properties from merger observations and are built based on large sets of equation of state models which do not include pions. Comparing to our calculations with pions, we find that these empirical relations remain valid to good accuracy, which justifies their use although they neglect a possible impact of pions. We also address the mass ejection by neutron star mergers and observe a moderate enhancement of the ejecta mass by a few ten per cent. (abridged)

astro-ph.HE↗
arXiv2023

End-to-end kilonova models of neutron-star mergers with delayed black-hole formation

We investigate the nucleosynthesis and kilonova properties of binary neutron-star (NS) merger models which lead to intermediate remnant lifetimes of ~0.1-1seconds until black-hole (BH) formation and describe all components of material ejected during the dynamical merger phase, NS-remnant evolution, and final viscous disintegration of the BH torus after gravitational collapse. To this end we employ a combination of hydrodynamics, nucleosynthesis, and radiative-transfer tools to achieve a consistent end-to-end modeling of the system and its observables. We adopt a novel version of the Shakura-Sunyaev scheme allowing to vary the approximate turbulent viscosity inside the NS remnant independently of the surrounding disk. We find that asymmetric progenitors lead to shorter remnant lifetimes and enhanced ejecta masses, although the viscosity affects the absolute values of these characteristics. The integrated production of lanthanides and heavier elements in such binary systems is sub-solar, suggesting that the considered scenarios contribute in a sub-dominant fashion to r-process enrichment. One reason is that BH-tori formed after delayed collapse exhibit less neutron-rich conditions than typically found, and often assumed in previous BH-torus models, for early BH formation. The outflows in our models feature strong anisotropy as a result of the lanthanide-poor polar neutrino-driven wind pushing aside lanthanide-rich dynamical ejecta. Considering the complexity of the models, the estimated kilonova light curves show promising agreement with AT2017gfo after times of several days, while the remaining inconsistencies at early times could possibly be overcome in binary configurations with a more dominant neutrino-driven wind relative to the dynamical ejecta.

astro-ph.HE↗
arXiv2023

Compact Dark Objects in Neutron Star Mergers

We estimate the long-lasting gravitational wave (GW) emission of compact dark objects following a binary neutron-star (NS) merger. We consider compact dark objects, which initially reside in the centers of NSs and which may consist of self-interacting dark matter (DM). By approximating the compact dark objects as test particles, we model the merging of NS binaries hosting DM components with three-dimensional relativistic simulations. Our simulation results suggest that the DM components remain gravitationally bound and orbit inside the merger remnant with orbital separations of typically a few km. The subsequent orbital motion of the DM components generates a GW signal with frequencies in the range of a few kHz. When considering a range of different binary masses and high-density equations of state (EoS), we find that the GW frequency of the orbiting DM components scales with the compactness of NSs. Similarly, we find relations between the DM GW frequency and the dominant postmerger GW frequency of the stellar fluid or the tidal deformability, which quantifies EoS effects during the binary inspiral. Hence, a measurement of these quantities can be used to specify the frequency range of the GW emission by DM. Under the assumption that GW back reaction is the only relevant dissipative process, the GW signal may last between seconds and years depending on the mass of the DM component. We estimate the detectability of the GW signals and find that DM components in NS mergers may only be detectable with existing and projected GW instruments if the dark objects are as massive as about 0.01 to 0.1 M_sun. We emphasize that the GW emission is limited by the lifetime of the remnant. A forming black hole will immediately swallow the DM objects because their orbits are smaller than the innermost stable circular orbit of the black hole.

astro-ph.HE↗
arXiv2023

Gravitational wave emission from dynamical stellar interactions

We are witnessing the dawn of gravitational wave (GW) astronomy. With currently available detectors, observations are restricted to GW frequencies in the range between ${\sim} 10\,\mathrm{Hz}$ and $10\,\mathrm{kHz}$, which covers the signals from mergers of compact objects. The launch of the space observatory LISA will open up a new frequency band for the detection of stellar interactions at lower frequencies. In this work, we predict the shape and strength of the GW signals associated with common-envelope interaction and merger events in binary stars, and we discuss their detectability. Previous studies estimated these characteristics based on semi-analytical models. In contrast, we used detailed three-dimensional magnetohydrodynamic simulations to compute the GW signals. We show that for the studied models, the dynamical phase of common-envelope events and mergers between main-sequence stars lies outside of the detectability band of the LISA mission. We find, however, that the final stages of common-envelope interactions leading to mergers of the stellar cores fall into the frequency band in which the sensitivity of LISA peaks, making them promising candidates for detection. These detections can constrain the enigmatic common-envelope dynamics. Furthermore, future decihertz observatories such as DECIGO or BBO would also be able to observe this final stage and the post-merger signal, through which we might be able to detect the formation of Thorne-Żytkow objects.

astro-ph.HE↗
arXiv2023

Needle in a Bayes Stack: a Hierarchical Bayesian Method for Constraining the Neutron Star Equation of State with an Ensemble of Binary Neutron Star Post-merger Remnants

Binary neutron star (BNS) post-merger gravitational-wave emission can occur in the aftermath of a BNS merger -- provided the system avoids prompt collapse to a black hole -- as a quasistable hypermassive remnant experiences quadrupolar oscillations and non-axisymmetric deformations. The post-merger gravitational-wave spectrum possesses a characteristic peak frequency that has been shown to be dependent on the binary chirp mass and the neutron star equation of state (EoS), rendering post-merger gravitational waves a powerful tool for constraining neutron star composition. Unfortunately, the BNS post-merger signal is emitted at high ($\gtrsim 1.5$ kHz) frequencies, where ground-based gravitational wave detectors suffer from reduced sensitivity. It is therefore unlikely that post-merger signals will be detected with sufficient signal-to-noise ratio (SNR) until the advent of next-generation detectors. However, by employing empirical relations derived from numerical relativity simulations, we can combine information across an ensemble of BNS mergers, allowing us to obtain EoS constraints with many low-SNR signals. We present a hierarchical Bayesian method for deriving constraints on $R_{1.6}$, the radius of a 1.6$\mathrm{M_{\odot}}$ neutron star, through an ensemble analysis of binary neutron star mergers. We apply this method to simulations of the next two LIGO-Virgo-KAGRA observing runs, O4 and O5, as well as an extended 4-year run at A+ sensitivity, demonstrating the potential of our approach to yield EoS information from the post-merger signal with current-generation detectors. The A+ 4-year scenario is predicted to improve the constraint on $R_{1.6}$ from the currently available multimessenger-based 95\% credible interval (C.I.) uncertainty of $R_{1.6}=12.07^{+0.98}_{-0.77}$ km to $R_{1.6}=11.91^{+0.80}_{-0.56}$ km, a 22% reduction of the 95% C.I. width.

astro-ph.HE↗
arXiv2023

3D radiative transfer kilonova modelling for binary neutron star merger simulations

The detection of GW170817 and the accompanying electromagnetic counterpart, AT2017gfo, have provided an important set of observational constraints for theoretical models of neutron star mergers, nucleosynthesis, and radiative transfer for kilonovae. We apply the 3D Monte Carlo radiative transfer code ARTIS to produce synthetic light curves of the dynamical ejecta from a neutron star merger, which has been modelled with 3D smooth-particle hydrodynamics (SPH) and included neutrino interactions. Nucleosynthesis calculations provide the energy released from radioactive decays of r-process nuclei, and radiation transport is performed using grey opacities given as functions of the electron fraction. We present line-of-sight dependent bolometric light curves, and find the emission along polar lines of sight to be up to a factor of ~2 brighter than along equatorial lines of sight. Instead of a distinct emission peak, our bolometric light curve exhibits a monotonic decline, characterised by a shoulder at the time when the bulk ejecta becomes optically thin. We show approximate band light curves based on radiation temperatures and compare these to the observations of AT2017gfo. We find that the rapidly declining temperatures lead to a blue to red colour evolution similar to that shown by AT2017gfo. We also investigate the impact of an additional, spherically symmetric secular ejecta component, and we find that the early light curve remains nearly unaffected, while after about 1 day the emission is strongly enhanced and dominated by the secular ejecta, leading to the shift of the shoulder from 1-2 to 6-10 days.

astro-ph.HE↗
arXiv2023

Dynamics and Equation of State Dependencies of Relevance for Nucleosynthesis in Supernovae and Neutron Star Mergers

Neutron stars (NSs) and black holes (BHs) are born when the final collapse of the stellar core terminates the lives of stars more massive than about 9 Msun. This can trigger the powerful ejection of a large fraction of the star's material in a core-collapse supernova (CCSN), whose extreme luminosity is energized by the decay of radioactive isotopes such as 56Ni and 56Co. When evolving in close binary systems, the compact relics of such infernal catastrophes spiral towards each other on orbits gradually decaying by gravitational-wave emission. Ultimately, the violent collision of the two components forms a more massive, rapidly spinning remnant, again accompanied by the ejection of considerable amounts of matter. These merger events can be observed by high-energy bursts of gamma rays with afterglows and electromagnetic transients called kilonovae, which radiate the energy released in radioactive decays of freshly assembled rapid neutron-capture elements. By means of their mass ejection and the nuclear and neutrino reactions taking place in the ejecta, both CCSNe and compact object mergers (COMs) are prominent sites of heavy-element nucleosynthesis and play a central role in the cosmic cycle of matter and the chemical enrichment history of galaxies. The nuclear equation of state (EoS) of NS matter, from neutron-rich to proton-dominated conditions and with temperatures ranging from about zero to ~100 MeV, is a crucial ingredient in these astrophysical phenomena. It determines their dynamical processes, their remnant properties even at the level of deciding between NS or BH, and the properties of the associated emission of neutrinos, whose interactions govern the thermodynamic conditions and the neutron-to-proton ratio for nucleosynthesis reactions in the innermost ejecta. This chapter discusses corresponding EoS dependent effects of relevance in CCSNe as well as COMs. (slightly abridged)

astro-ph.HE↗
arXiv2023

Spherical symmetry in the kilonova AT2017gfo/GW170817

The mergers of neutron stars expel a heavy-element enriched fireball which can be observed as a kilonova. The kilonova's geometry is a key diagnostic of the merger and is dictated by the properties of ultra-dense matter and the energetics of the collapse to a black hole. Current hydrodynamical merger models typically show aspherical ejecta. Previously, Sr$^+$ was identified in the spectrum of the the only well-studied kilonova AT2017gfo, associated with the gravitational wave event GW170817. Here we combine the strong Sr$^+$ P Cygni absorption-emission spectral feature and the blackbody nature of kilonova spectrum, to determine that the kilonova is highly spherical at early epochs. Line shape analysis combined with the known inclination angle of the source also shows the same sphericity independently. We conclude that energy injection by radioactive decay is insufficient to make the ejecta spherical. A magnetar wind or jet from the black hole disk could inject enough energy to induce a more spherical distribution in the overall ejecta, however an additional process seems necessary to make the element distribution uniform

astro-ph.HE↗
arXiv2022

Early deconfinement of asymptotically conformal color-superconducting quark matter in neutron stars

We present a relativistic density functional approach to color superconducting quark matter that mimics quark confinement by a fast growth of the quasiparticle selfenergy in the confining region. The approach is shown to be equivalent to a chiral model of quark matter with medium dependent couplings. While the (pseudo)scalar sector of the model is fitted to the vacuum phenomenology of quantum chromodynamics, the strength of interaction in the vector and diquark channels is varied in order to provide the best agreement with the observational constraints on the mass-radius relation and tidal deformability of neutron stars modelled with our approach. In order to recover the conformal behavior of quark matter at asymptotically high densities we introduce a medium dependence of the vector and diquark couplings motivated by the nonperturbative gluon exchange. Our analysis signals that the onset of deconfinement to color superconducting quark matter is likely to occur in neutron stars with masses below 1.0 $M_\odot$.

nucl-th↗
arXiv2022

Formation and evaporation of strangelets during the merger of two compact stars

We study the partial fragmentation of a strange quark star into strangelets during the process of merger of two strange quark stars. We discuss the fate of the fragments considering their possible evaporation into nucleons. We show that only a rather small amount of large size strangelets, ejected from the spiral arms in the post-merger, survives a total evaporation into nucleons. In this way we demonstrate that: 1) the density of strangelets in the galaxy is too low to trigger the conversion of all neutron stars into strange quark stars and it allows the co-existence of both types of compact objects; 2) the probability of direct detection of a strangelet is negligible and therefore its nondetection is compatible with the strange quark matter hypothesis; 3) most of the matter ejected during and after the merger of two strange quark stars evaporates into nucleons and therefore it can generate a kilonova-like signal.

astro-ph.HE↗
arXiv2022

Early quark deconfinement in compact star astrophysics and heavy-ion collisions

Based on a recently developed relativistic density functional approach to color-superconducting quark matter and a novel quark-hadron transition construction which phenomenologically accounts for the effects of inhomogeneous pasta phases and quark-hadron continuity, we construct a class of hybrid equations of state applicable at the regimes typical for compact star astrophysics and heavy ion collisions. We outline that early quark deconfinement is a notable consequence of strong diquark pairing providing a good agreement with the observational data and driving the trajectories of the matter evolution during the supernovae explosions toward the regimes typical for the compact star mergers and heavy-ion collisions.

nucl-th↗
arXiv2022

Probing neutron stars with the full pre-merger and post-merger gravitational wave signal from binary coalescences

The gravitational wave signal emitted during the coalescence of two neutron stars carries information about the stars' internal structure. During the long inspiral phase the main matter observable is the tidal interaction between the binary components, an effect that can be parametrically modeled with compact-binary solutions to General Relativity. After the binary merger the main observable is frequency modes of the remnant, most commonly giving rise to a short-duration signal accessible only through numerical simulations. The complicated morphology and the decreasing detector sensitivity in the relevant frequencies currently hinder detection of the post-merger signal and motivate separate analyses for the pre-merger and post-merger data. However, planned and ongoing detector improvements could soon put the post-merger signal within reach. In this study we target the whole pre-merger and post-merger signal without an artificial separation at the binary merger. We construct a hybrid analysis that models the inspiral with templates based on analytical calculations and calibrated to numerical relativity and the post-merger signal with a flexible morphology-independent analysis. Applying this analysis to GW170817 we find, as expected, that the post-merger signal remains undetected. We further study simulated signals and find that we can reconstruct the full signal and simultaneously estimate both the pre-merger tidal deformation and the post-merger signal frequency content. Our analysis allows us to study neutron star physics using all the data available and directly test the pre-merger and post-merger signal for consistency thus probing effects such as the onset of the hadron-quark phase transition.

gr-qc↗
arXiv2022

Dynamical ejecta of neutron star mergers with nucleonic weak processes II: Kilonova emission

The majority of existing results for the kilonova (or macronova) emission from material ejected during a neutron-star (NS) merger is based on (quasi-)one-zone models or manually constructed toy-model ejecta configurations. In this study we present a kilonova analysis of the material ejected during the first ~10ms of a NS merger, called dynamical ejecta, using directly the outflow trajectories from general relativistic smoothed-particle hydrodynamics simulations including a sophisticated neutrino treatment and the corresponding nucleosynthesis results, which have been presented in Part I of this study. We employ a multi-dimensional two-moment radiation transport scheme with approximate M1 closure to evolve the photon field and use a heuristic prescription for the opacities found by calibration with atomic-physics based reference results. We find that the photosphere is generically ellipsoidal but augmented with small-scale structure and produces emission that is about 1.5-3 times stronger towards the pole than the equator. The kilonova typically peaks after 0.7-1.5days in the near-infrared frequency regime with luminosities between 3-7x10^40erg/s and at photospheric temperatures of 2.2-2.8x10^3K. A softer equation of state or higher binary-mass asymmetry leads to a longer and brighter signal. Significant variations of the light curve are also obtained for models with artificially modified electron fractions, emphasizing the importance of a reliable neutrino-transport modeling. None of the models investigated here, which only consider dynamical ejecta, produces a transient as bright as AT2017gfo. The near-infrared peak of our models is incompatible with the early blue component of AT2017gfo.

astro-ph.HE↗
arXiv2022

Effects of a strong phase transition on supernova explosions, compact stars and their mergers

We outline a theoretical approach supporting strong phase transitions from normal nuclear matter to the deconfined quark-gluon plasma, in the equation of state (EOS) for compact star matter. Implications of this hypothesis are discussed for astrophysical applications. Special emphasis is devoted to potentially detectable signatures, which can be directly related with the occurrence of a sufficiently strong phase transition. Therefore, simulations of core-collapse supernovae and binary compact star mergers are considered, including the subsequent emission of gravitational waves and, in the case of supernova, in addition the neutrinos play the role of messengers.

nucl-th↗
arXiv2022

Analytic models of the spectral properties of gravitational waves from neutron star merger remnants

We present a new analytic model describing gravitational wave emission in the post-merger phase of binary neutron star mergers. The model is described by a number of physical parameters that are related to various oscillation modes, quasi-linear combination tones or non-linear features that appear in the post-merger phase. The time evolution of the main post-merger frequency peak is taken into account and it is described by a two-segment linear expression. The effectiveness of the model, in terms of the fitting factor or, equivalently, the reduction in the detection rate, is evaluated along a sequence of equal-mass simulations of varying mass. We find that all parameters of the analytic model correlate with the total binary mass of the system. For high masses, we identify new spectral features originating from the non-linear coupling between the quasi-radial oscillation and the antipodal tidal deformation, the inclusion of which significantly improves the fitting factors achieved by the model. We can thus model the post-merger gravitational-wave emission with an analytic model that achieves high fitting factors for a wide range of total binary masses. Our model can be used for the detection and parameter estimation of the post-merger phase in upcoming searches with upgraded second-generation detectors, such as aLIGO+ and aVirgo+, with future, third-generation detectors (Einstein Telescope and Cosmic Explorer) or with dedicated, high-frequency detectors.

astro-ph.HE↗
arXiv2021

Neutrino absorption and other physics dependencies in neutrino-cooled black-hole accretion disks

Black-hole (BH) accretion disks formed in compact-object mergers or collapsars may be major sites of the rapid-neutron-capture (r-)process, but the conditions determining the electron fraction (Y_e) remain uncertain given the complexity of neutrino transfer and angular-momentum transport. After discussing relevant weak-interaction regimes, we study the role of neutrino absorption for shaping Y_e using an extensive set of simulations performed with two-moment neutrino transport and again without neutrino absorption. We vary the torus mass, BH mass and spin, and examine the impact of rest-mass and weak-magnetism corrections in the neutrino rates. We also test the dependence on the angular-momentum transport treatment by comparing axisymmetric models using the standard alpha-viscosity with viscous models assuming constant viscous length scales (l_t) and three-dimensional magnetohydrodynamic (MHD) simulations. Finally, we discuss the nucleosynthesis yields and basic kilonova properties. We find that absorption pushes Y_e towards ~0.5 outside the torus, while inside increasing the equilibrium value Y_e^eq by ~0.05-0.2. Correspondingly, a substantial ejecta fraction is pushed above Y_e=0.25, leading to a reduced lanthanide fraction and a brighter, earlier, and bluer kilonova than without absorption. More compact tori with higher neutrino optical depth, tau, tend to have lower Y_e^eq up to tau~1-10, above which absorption becomes strong enough to reverse this trend. Disk ejecta are less (more) neutron-rich when employing an l_t=const. viscosity (MHD treatment). The solar-like abundance pattern found for our MHD model marginally supports collapsar disks as major r-process sites, although a strong r-process may be limited to phases of high mass-infall rates, Mdot>~ 2 x 10^(-2) Msun/s.

astro-ph.HE↗