arXiv subjects

Stephan Rosswog

Publications and source records attributed to Stephan Rosswog.

At least 19 recordsLinked to original sources

arXiv2026

General relativistic hydrodynamics of stellar tidal disruptions in Kerr spacetime: methods, validation, and first applications

The disruption of a star by the tidal field of a super-massive black hole may provide insights into dormant and otherwise hard-to-study galactic nuclei. State-of-the-art numerical tools have not converged on the importance of the strong relativistic effects in the disruption of stars by potentially spinning black holes. We present a specialised numerical tool to perform global hydrodynamic simulations of stellar tidal disruptions in curved spacetimes. We quantify the role of impact strength and black hole spin onto the stellar structures and mass fallback rates. We adapted the code SPHINCS_BSSN to perform General Relativistic Smoothed-Particle Hydrodynamics (GRSPH) simulations of stellar tidal disruptions in Kerr metric. We coupled the code with a Newtonian self-gravity module, and we added the option to use an entropy evolution formulation to handle numerically challenging situations. Besides describing the implementation and code validation, we present a set of 18 simulations of parabolic tidal disruptions of stellar polytropes to investigate the effect of impact strength and black hole spin. We demonstrate that SPHINCS is capable of performing GRSPH simulations, reproducing benchmark tests to machine precision. Our stellar tidal disruption simulations show that the fallback rates agree with state-of-the-art modelling. Deep events result into structures where self-gravity plays no role, the mass fallback rates peak at lower values, and rise-to-peak timescales decrease with impact strength. In these cases the black hole spin affects noticeable these quantities increasing (decrease) both fallback rate peak and rise-to-peak timescale for prograde (retrograde) spin. Last, fallback rates tend to decay with the characteristic $t^{-5/3}$ on long timescales. The results show that SPHINCS can simulate high-resolution stellar tidal disruptions in Kerr metric at a reasonable computational time.

astro-ph.HE↗

arXiv2026

Interacting stellar winds feeding Sgr A*: from the system of mass-losing stars to the binary IRS 16SW

The discovery of cold structures around Sgr A* has challenged our understanding of the gas dynamics and thermodynamic state of the plasma in its vicinity. This work aims to constrain the conditions for the formation of such structures, namely the cold disc-like structure and the recently discovered G-1-2-3 complex. We conduct hydrodynamic simulations of the observed Wolf-Rayet stars feeding Sgr A*. Our simulations show that the plasma chemical composition is crucial for determining the medium properties. We demonstrate that the formation of a cold disc is possible for chemical compositions that are consistent with observational constraints. However, it is not possible to reproduce all the properties of the observed disc which might suggest the interaction with another structure. Additionally, we present our first results on the hydrodynamic modelling of IRS 16SW as a colliding-wind binary. This is the first step to develop a realistic model on the formation of the G-1-2-3 complex.

astro-ph.GA↗

arXiv2026

Fast and Accurate Prediction of Neutron Star Structure with Deep Neural Networks

Solving the Tolman--Oppenheimer--Volkoff (TOV) equations, together with the tidal perturbation equations, for large numbers of equation-of-state (EOS) samples is a major computational bottleneck in Bayesian inference of the dense-matter EOS, and this will become increasingly limiting as next-generation observatories deliver far larger and more precise datasets. We develop neural-network surrogates for the forward TOV mapping that predict neutron star mass, radius, and tidal deformability simultaneously and directly from the EOS parameters and central density. We train and compare two architectures: a conventional feedforward network and a residual network, the latter of which, to our knowledge, has not previously been explored for TOV surrogate modeling. Trained on a piecewise polytropic EOS parameter space, both networks reproduce the numerical solutions to high accuracy, with the coefficient of determination exceeding 0.999 for all three observables, while accelerating the evaluation of stellar observables by roughly two orders of magnitude relative to direct numerical integration. We find that both architectures achieve excellent predictive accuracy at the network sizes considered here, with the residual network providing a modest improvement in accuracy over the feedforward network at the expense of slightly longer inference times. The overall performance differences remain small, indicating that a feedforward network already has sufficient capacity for this mapping while residual connections offer only incremental gains. Nevertheless, the residual architecture provides a robust baseline for future extensions to richer EOS parameterizations or higher-dimensional regression tasks. The resulting surrogates are well-suited to large-scale Bayesian EOS inference and population studies, where repeated TOV evaluations would otherwise dominate the computational cost.

astro-ph.HE↗

arXiv2026

SPH methods in the modelling of compact objects

We review the current status of compact object simulations that are based on the Smoothed Particle Hydrodynamics (SPH) method. The first section of this review is dedicated to SPH as a numerical method for Newtonian, ideal gas dynamics and it should be fairly self-contained. It begins with the basics of the method, but also describes recent advances including various meshless derivatives or methods for treating shocks. A separate chapter summarizes general relativistic SPH, including its special relativistic limit, and it explains in some detail the recent development of full numerical relativity in SPH where matter is evolved together with a dynamical spacetime. The remainder of the review has an astrophysical focus, here we discuss the status of the simulations of white dwarf--white dwarf, neutron star--neutron star and neutron star--black hole systems. For each type of system the emphasis is on gravitational-wave-driven mergers, but we also briefly summarize dynamical collisions that can occur in locations with large stellar densities.

astro-ph.HE↗

arXiv2026

Nucleosynthesis in the fast ejecta of a neutron star merger

Neutron star mergers are today considered a major production site for rapid neutron capture elements. While the bulk of the matter escapes at fast, but non-relativistic velocities (${\sim} 0.2\,c$), a small amount of the dynamically ejected mass reaches mildly relativistic velocities (${\gtrsim}0.6\,c$). It has been suggested earlier, that in such ejecta parts neutrons may avoid being captured and that their decay could power an early blue precursor to the main kilonova event. Here we study in detail the nucleosynthesis in such fast ejecta with nuclear network calculations along both parametrized and numerical relativity trajectories. We find that the nucleosynthesis can be divided into three channels, in one of which a substantial amount of free neutrons survives when the main r-process has frozen out. We provide a (semi-)analytical model for surviving free neutrons which agrees very well with the network calculations. If the mass fraction of the free neutrons exceeds ${\sim} 0.05$, their $\beta^-$-decay dominates the nuclear heating rate between ${\sim} 100$ and ${\sim} 10^4$ seconds. This dominance leads to a pronounced kilonova precursor that should for plausible ejecta parameters be visible for ULTRASAT out to ${\sim}200\,\rm Mpc$. Since at low electron fractions free neutrons can survive even for moderate velocities, mergers with large tidal ejecta, such as asymmetric neutron star mergers or favorable neutron star black hole mergers, may produce particularly bright blue precursors to their subsequent kilonovae.

astro-ph.HE↗

arXiv2026

Rapid and robust simulation-based inference for kilonovae

With the next generation of both electromagnetic and gravitational wave observatories beginning to come online, rapid analysis methods for kilonova data are becoming increasingly important in astronomy. Traditional Bayesian parameter estimation using Markov chain Monte Carlo (MCMC) is time-consuming and relies on explicit likelihood approximations that can break down when modeling uncertainties are significant. We develop a simulation-based inference (SBI) framework for kilonova parameter estimation using density-estimation likelihood-free inference. The framework uses a Gaussian process emulator trained on $\sim 1300$ POSSIS simulations. We demonstrate that SBI provides a rapid alternative to MCMC that is robust to likelihood misspecification. The standard Gaussian likelihood approximation fails to capture the non-Gaussian, correlated structure of emulator uncertainty; SBI learns this structure directly from forward simulations. Simulation studies show that the SBI method accurately recovers injected parameters, while the MCMC suffers from systematic bias caused by likelihood misspecification. This problem persists when analyzing AT2017gfo, where a subset of the MCMC posteriors pile up at prior boundaries and the SBI posteriors do not. The SBI framework infers a total ejecta mass of $\sim 0.087 M_{\odot}$ dominated by lanthanide-poor ejecta and excludes toroidal and peanut ejecta geometries at the 99th percentile for both components. The SBI framework generates $\sim 2 \times 10^{4}$ posterior samples in seconds.

astro-ph.IM↗

arXiv2026

Binary neutron star mergers with tabulated equations of state in SPHINCS_BSSN

The dynamics and observable signatures of neutron star mergers are governed by physics under the most extreme conditions. They are particularly impacted by the high-density equation of state, which for the most sophisticated models is usually available in the form of tables. Numerical relativity codes usually evolve particularly well-behaved numerical ("conservative") variables, but at the price that the physically interesting ("primitive") variables need to be found at every computational element and at every integration sub-step by means of expensive (and not always successful) root-finding algorithms. We have recently developed the Lagrangian numerical relativity code SPHINCS_BSSN which evolves the spacetime on an adaptive mesh with well tested methods, but the fluid is evolved by means of freely moving particles. Since our evolution equations differ from those of conventional numerical relativity, we need to develop new conservative-to-primitive algorithms if we want to use tabulated equations of state. We present here three such algorithms: a 3D and a 2D Newton-Raphson method and a 1D root-finding algorithm based on Ridders' method. We find the 3D method to be very fast and robust with an average failure fraction in a full-blown neutron star merger simulation (with the DD2 equation of state) well below 1%. While we do not find obvious advantages for the 2D method, the 1D Ridders' method is slow, but essentially fail-safe. Therefore, we choose the 3D Newton-Raphson as default and fall back to the 1D Ridders' method as a safe "parachute".

astro-ph.HE↗

arXiv2026

Quantifying the impact of relativistic precession on tidal disruption event light curves

The tidal field of a black hole can turn a star into a gas stream whose orbit can precess, especially if the a black hole is rapidly spinning. In this work, we investigate the impact of precession on the light curves of tidal disruption events (TDE). To do so, we perform two-dimensional radiation-hydrodynamic simulations of the interaction of the TDE wind and luminosity with the precessed stream wrapped around the black hole. Our results show that in events with black holes of $\sim10^6~\text{M}_{\odot}$ and no orbit-spin inclination, the line of sight has little effect on the light curves, since the stream covers a small fraction of the solid angle as the precession is confined to the orbital plane. In the case of black holes of $\gtrsim10^7~\text{M}_{\odot}$ and high inclination ($i\sim90^{\circ}$), the light curve peaks can be delayed by $\sim$100 days due to presence of the precessed stream blocking the radiation in the early phase of the event. We also discuss our efforts to model self-consistently the hydrodynamic evolution of a tidal stellar stream on curved spacetimes by the presence of a massive black hole.

astro-ph.HE↗

arXiv2026

Binary neutron star mergers with SPHINCS_BSSN: temperature-dependent equations of state and damping of constraint violations

Neutron star mergers hold the key to several grand challenges of contemporary (astro-)physics. In view of the upcoming next generation of ground-based detectors, it is crucial to keep improving theoretical predictions to harvest the full scientific returns from these investments. We introduce here a substantial update of our Lagrangian numerical relativity code SPHINCS_BSSN. Apart from changing our unit system, we add constraint damping terms to the BSSN spacetime evolution equations. We demonstrate that this measure reduces, without noteworthy computational cost, the Hamiltonian constraint violations by more than an order of magnitude. We further implement contributions to thermal energy and pressure that are based on Fermi liquid theory and contain a parametrization of the Dirac effective mass. These terms can be combined with any cold equation of state, and they enhance the physical realism of our simulations and introduce a physics-based concept of a temperature. In a set of merger simulations, we demonstrate good agreement with other temperature-dependent numerical relativity simulations. We find that different parametrizations of the Dirac effective mass can translate into shifts of $\sim 150$ Hz in the dominant post-merger gravitational wave peak frequency.

astro-ph.HE↗

arXiv2025

Tidal phenomena in the Galactic Center: The curious case of X7

Several enigmatic dusty sources have been detected in the central parsec of the Galactic Center. Among them is X7, located at only $\sim$0.02 pc from the central super-massive black hole, Sagittarius A* (Sgr A*). Recent observations have shown that it is becoming elongated due to the tidal forces of Sgr A*. X7 is expected to be fully disrupted during its pericenter passage around 2035 which might impact the accretion rate of Sgr A*. However, its origin and nature are still unknown. We investigated the tidal interaction of X7 with Sgr A* in order to constrain its origin. We tested the hypothesis that X7 was produced by one of the observed stars with constrained dynamical properties in the vicinity of Sgr A*. We employed a set of test-particle simulations to reproduce the observed structure and dynamics of X7. The initial conditions of the models were obtained by extrapolating the observationally constrained orbits of X7 and the known stars into the past, making it possible to find the time and source of origin by minimizing the three-dimensional separation and velocity difference between them. Our results show that ejecta from the star S33/S0-30, launched in $\sim$1950, can to a large extent, replicate the observed dynamics and structure of X7, provided that it is initially elongated with a velocity gradient across it, and with an initial maximum speed of $\sim$600~km~s$^{-1}$. Our results show that a grazing collision between the star S33/S0-30 and a field object such as a stellar mass black hole or a Jupiter-mass object is a viable scenario to explain the origin of X7. Nevertheless, such encounters are rare based on the observed stellar dynamics within the central parsec.

astro-ph.GA↗

arXiv2025

Robustness of Magnetic Field Amplification in Neutron Star Mergers

The dynamics of a binary neutron stars merger is governed by physics under the most extreme conditions, including strong spacetime curvature, ultra-high matter densities, luminous neutrino emission and the rapid amplification of the initial neutron star magnetic fields. Here we systematically explore how sensitive the magnetic field evolution is to the total mass of the merging binary, to the mass ratio of its components, the stellar spins and to the equation of state. For this purpose, we analyze 16 state-of-the-art GRMHD simulations that employ a subgrid-scale model to account for the unresolved small-scale turbulence. We find that strong and rapid amplification of the magnetic field to volume-averaged values of $\sim 10^{16}$~G in the high-density regions is a very robust outcome of a neutron star merger and this result is only marginally impacted by either mass, mass ratio, spin or equation of state.

astro-ph.HE↗

arXiv2025

Prompt Periodicity in the GRB 211211A Precursor: Black-hole or magnetar engine?

The merger origin long GRB 211211A was a class (re-)defining event. A precursor was identified with a $\sim 1$ s separation from the main burst, as well as a claimed candidate quasi-periodic oscillation (QPO) with a frequency $\sim20$ Hz. Here, we explore the implications of the precursor, assuming the quasi-periodicity is real. The precursor variability timescale requires relativistic motion with a Lorentz factor $\Gamma\gtrsim80$, and implies an engine driven jetted outflow. The declining amplitude of the consecutive pulses requires an episodic engine with an `on/off' cycle consistent with the QPO. For a black-hole central engine, the QPO can have its origin in Lense-Thirring precession of the inner disk at $\sim6-9$ $r_g$ (gravitational radii) for a mass $M_\bullet\leq4.5$ $M_{\odot}$, and $\lesssim 7$ $r_g$ for $M_\bullet>4.5$ $M_{\odot}$ and dimensionless spin $\chi\sim 0.3 - 0.9$. Alternatively, at a disk density of $\sim10^{8 - 12}$ g cm$^{-3}$, the required magnetic field strength for a QPO via magnetohydrodynamic effects will be on the order $B\sim10^{12 - 14}$ G. If the central engine is a short lived magnetar or hypermassive neutron star, then a low-frequency QPO can be produced via instabilities within the disk at a radius of $\sim20 - 70$ km, for a disk density $\sim10^{9 - 12}$ g cm$^{-3}$ and magnetic field $\gtrsim10^{13 - 14}$ G. The QPO cannot be coupled to the neutron star spin, as the co-rotation radius is beyond the scale of the disk. Neither engine can be ruled out -- however, we favour an origin for the precursor candidate QPO as early jet-disk coupling for a neutron star -- black hole merger remnant with mass $M_\bullet>4.5$ $M_{\odot}$.

astro-ph.HE↗

arXiv2025

PSR J0952-0607: Probing the Stiffest Equations of State and r-Mode Suppression Mechanisms

We analyze PSR J0952-0607, the most massive and fastest spinning neutron star observed to date, to refine constraints on the neutron star equation of state (EoS) and investigate its robustness against r-mode instabilities. With a mass of $ 2.35 \pm 0.17 \, M_{\odot} $ and a spin frequency of 709.2 Hz, PSR J0952-0607 provides a unique opportunity to examine the effects of rapid rotation on the structure of a neutron star. Using a Bayesian framework, we incorporate the rotationally corrected mass of PSR J0952-0607, alongside PSR J0740+6620's static mass measurement, to constrain the EoS. Our findings demonstrate that neglecting rotational effects leads to biases in the inferred EoS, while including the neutron star spin produces tighter constraints on pressure-density and mass-radius relations. Additionally, we explore the r-mode instability window for PSR J0952-0607 under the assumption of both rigid and elastic crust models and find that a rigid crust allows a higher stable temperature range, whereas an elastic crust places the star within the instability window under certain thermal insulation conditions.

astro-ph.HE↗

arXiv2024

The formation and stability of a cold disc made out of stellar winds in the Galactic Centre

The reported discovery of a cold (~10$^4$ K) disc-like structure within 0.005 pc around the super-massive black hole at the centre of the Milky Way, Sgr A*, has challenged our understanding of the gas dynamics and thermodynamic state of the plasma in its immediate vicinity. State-of-the-art simulations do not agree on whether or not such a disc can be a product of the multiple stellar wind interactions of the mass-losing stars in the region. This study aims to constrain the conditions for the formation of a cold disc as a natural outcome of the system of the mass-losing stars orbiting around Sgr A*, to investigate whether the disc is a transient or long-lasting structure, and to assess the validity of the model through direct comparisons with observations. We performed a set of hydrodynamic simulations of the observed Wolf-Rayet (WR) stars feeding Sgr A* using the finite-volume adaptive mesh refinement code Ramses. We focus, for the first time, on the impact of the chemical composition of the plasma emanating from the WR stars. The simulations show that the chemical composition of the plasma affects the radiative cooling enough to impact the properties of the medium, such as density and temperature, and, as a consequence, the rate at which the material inflows onto Sgr A*. We demonstrate that the formation of a cold disc from the stellar winds is possible for certain chemical compositions that are consistent with the current observational constraints. However, even in such cases, it is not possible to reproduce the reported properties of the observed disc-like structure, namely its inclination and the fluxes of its hydrogen recombination lines. We conclude that the stellar winds alone are not sufficient to form the cold disc around Sgr A* inferred from observations. Either relevant ingredients are still missing in the model, or the interpretation of the observed data needs to be revised.

astro-ph.GA↗

arXiv2024

Simultaneously Constraining the Neutron Star Equation of State and Mass Distribution through Multimessenger Observations and Nuclear Benchmarks

With ongoing advancements in nuclear theory and experimentation, together with a growing body of neutron star (NS) observations, a wealth of information on the equation of state (EOS) for matter at extreme densities has become accessible. Here, we utilize a hybrid EOS formulation that combines an empirical parameterization centered around the nuclear saturation density with a generic three-segment piecewise polytrope model at higher densities. We incorporate data derived from chiral effective field theory ($\chi$EFT), perturbative quantum chromodynamics (pQCD), and from experiments such as PREX-II and CREX. Furthermore, we examine the influence of a total of 129 NS mass measurements up to April 2023, as well as simultaneous mass and radius measurements derived from the X-ray emission from surface hot spots on NSs. Additionally, we consider constraints on tidal properties inferred from the gravitational waves emitted by coalescing NS binaries. To integrate this extensive and varied array of constraints, we utilize a hierarchical Bayesian statistical framework to simultaneously deduce the EOS and the distribution of NS masses. We find that incorporating data from $\chi$EFT significantly tightens the constraints on the EOS of NSs near or below the nuclear saturation density. However, constraints derived from pQCD computations and nuclear experiments such as PREX-II and CREX have minimal impact.

astro-ph.HE↗

arXiv2024

Delayed jet launching in binary neutron star mergers with realistic initial magnetic fields

We analyze a long-lived hyper-massive neutron star merger remnant (post-merger lifetime $>250$ ms) that has been obtained via large eddy simulations with a gradient subgrid-scale model. We find a clear helicoidal magnetic field structure that is governed by the toroidal component of the magnetic field. Although no jet emerges during the simulation time, we observe at late times a significant increase of the poloidal component of the magnetic field at all scales. We also compare with the results of several binary neutron star simulations with moderate resolution of $120$~m, that are evolved up to $50$~ms after the merger, which differ in terms of the initial topology and strength of the magnetic field. We find that the best choice is an isotropic small-scale magnetic field distribution that mimics the turbulent state that generically develops during the merger. This initial configuration reaches a closer agreement with our high-resolution simulation results than the purely dipolar large-scale fields that are commonly employed in these type of simulations. This provides a recipe to perform such simulations avoiding the computationally expensive grids required to faithfully capture the amplification of the magnetic field by Kelvin-Helmholtz instabilities.

astro-ph.HE↗

arXiv2024

Cautionary tales on heating-rate prescriptions in kilonovae

A major ingredient for kilonova lightcurves is the radioactive heating rate and its dependence on the electron fraction and velocity of the ejecta and, in principle, on the nuclear mass formula. Heating-rate formulae commonly used as the basis for kilonova models previously employed in the literature produce substantially different outputs for high electron fractions ($Y_{e} \gtrsim 0.3$) and at late times ($t \gtrsim 1$d) compared to newer prescriptions. Here, we employ standard semi-analytical models for kilonovae with better heating rate prescriptions valid for the full parameter space of kilonova velocities and electron fractions to explore the impact of the heating rate on kilonova lightcurves. We show the dangers of using inappropriate heating rate estimates by simulating realistic observations and inferring the kilonova parameters via a misspecified heating-rate prescription. While providing great fits to the photometry, an incorrect heating-rate prescription fails to recover the input ejecta masses with a bias significantly larger than the typical statistical uncertainty. This bias from an incorrect prescription has significant consequences for interpreting kilonovae, their use as additional components in gamma-ray burst afterglows, and understanding their role in cosmic chemical evolution or for multi-messenger constraints on the nuclear equation of state. We showcase a framework and tool to better determine the impact of different modelling assumptions and uncertainties on inferences into kilonova properties.

astro-ph.HE↗

arXiv2023

Which first order phase transitions to quark matter are possible in neutron stars?

We examine which first order phase transitions are consistent with today's astrophysical constraints. In particular, we explore how a well-constrained mass-radius data point would restrict the admissible parameter space and to this end, we employ the most likely candidates of the recent NICER limits of PSR J0030+0451. To systematically vary the stiffness of the equation of state, we employ a parameterizable relativistic mean field equation of state, which is in compliance with results from chiral effective field theory. We model phase transitions via Maxwell constructions and parameterize them by means of the transitional pressure $p_{\rm trans}$ and the jump in energy density $\Delta\epsilon$. This provides us with a generic setup that allows for rather general conclusions to be drawn. We outline some regions in the $p_{\rm trans}$-$\Delta\epsilon$ parameter space that may allow for a phase transition identification in the near future. We also find that a strongly constrained data point, at either exceptionally large or small radii, would reduce the parameter space to such an extent that mass and radius become insufficient indicators of a phase transition.

nucl-th↗