arXiv subjects

Michail Chabanov

Publications and source records attributed to Michail Chabanov.

8 recordsLinked to original sources

arXiv2026

Detectability of bulk viscosity effects on post-merger gravitational wave signals from binary neutron star mergers

We investigate bulk viscosity (BV) effects on post-merger gravitational wave (PMGW) signals from binary neutron star (BNS) mergers and their detectability using realistic equation of state (EoS) posteriors constrained by recent progress in gravitational-wave (GW) and X-ray observations, as well as neutron skin thickness measurements. A linear fitting formula based on recent BNS simulations with BV is used to estimate the peak frequency shift of the PMGWs caused by BV effects. Subsequently, we evaluate this peak frequency shift for our set of EoS posterior samples. We use the Fisher information matrix and the dataset of simulated observable events from the ET and CE detector network in an optimistic scenario to estimate the measurement accuracy of the peak frequency. We find that BV effects on PMGWs may be detectable only for EoS models with large values of the symmetry energy slope ($L_{\rm sym}$), as favored by neutron-skin thickness measurements. Thus, the detection of BV effects on PMGWs itself can provide abundant information about the symmetry energy and its slope. However, BV effects on PMGWs are generally weak and may be observed only in some extreme and optimistic cases. Therefore, observations of PMGWs and their analyses may not be able to provide very accurate measurements of the symmetry energy through BV effects.

astro-ph.HE↗

arXiv2026

Magnetic Eruption and Nucleosynthesis in GR{\nu}MHD Simulations of Spinning Neutron Star Mergers

We present three-dimensional general relativistic magnetohydrodynamics simulations of equal-mass binary neutron star mergers with varied neutron star spin configurations and second-moment neutrino transport, following the formation and early evolution of long-lived remnants. We compare a fiducial irrotational binary with binaries having spins that are aligned or antialigned with the orbital angular momentum, and examime how spin affects the merger dynamics, magnetic field evolution, outflows, and nucleosynthesis. Compared to the fiducial case, the aligned spin configuration releases more cold, neutron-rich tidal ejecta in the equatorial plane, which enables the development of a more tightly collimated polar outflow erupting from the remnant and inner accretion disk. Conversely, the case with spins antialigned with the orbit experiences a more violent collision at merger, disrupting magnetic amplification, loading the environment with debris, and impeding the propagation of magnetically driven winds. Strong neutrino reprocessing of the polar outflow in the irrotational and aligned spin cases produces $2.4\times 10^{-3}\,M_\odot$ of proton-rich ($Y_e \geq 0.49$) material, resulting in the synthesis of light r-process elements including $^{56}Ni$, whose subsequent decay potentially sends a unique electromagnetic signal from long-lived remnants. However, the outflows remain too dense and slow to be consistent with typical short gamma-ray bursts.

astro-ph.HE↗

arXiv2025

Long-term impact of the magnetic-field strength on the evolution and electromagnetic emission by neutron-star merger remnants

Numerical simulations are essential to understand the complex physics accompanying the merger of binary systems of neutron stars. However, these simulations become computationally challenging when they have to model the merger remnants on timescales over which secular phenomena, such as the launching of magnetically driven outflows, develop. To tackle these challenges, we have recently developed a hybrid approach that combines, via a hand-off transition, a fully general-relativistic code (FIL) with a more efficient code making use of the conformally flat approximation (BHAC+). We here report important additional developments of BHAC+ consisting of the inclusion of gravitational-wave radiation-reaction contributions and of higher-order formulations of the equations of general-relativistic magnetohydrodynamics. Both improvements have allowed us to explore BNS merger remnants with high accuracy and over timescales that would have been computationally prohibitive otherwise. More specifically, we have investigated the impact of the magnetic-field strength on the long-term (i.e., $\sim 200\,{\rm ms}$) and high-resolution (i.e., $150\,{\rm m}$) evolutions of the "magnetar" resulting from the merger of two neutron stars with a realistic equation of state. In this way, and for sufficiently large magnetic fields, we observe the weakening or suppression of differential rotation and the generation of magnetic flares in the outer layers of the remnant. These flares, driven mostly by the Parker instability, are responsible for intense and collimated Poynting flux outbursts and mass ejections. This novel phenomenology offers the possibility of seeking corresponding signatures from the observations of short gamma-ray bursts and hence revealing the existence of a long-lived strongly magnetized remnant.

astro-ph.HE↗

arXiv2025

Relativistic gas accretion onto supermassive black Hole binaries from inspiral through merger

Accreting supermassive black hole binaries are powerful multimessenger sources emitting both gravitational and EM radiation. Understanding the accretion dynamics of these systems and predicting their distinctive EM signals is crucial to informing and guiding upcoming efforts aimed at detecting gravitational waves produced by these binaries. To this end, accurate numerical modeling is required to describe both the spacetime and the magnetized gas around the black holes. In this paper, we present two key advances in this field of research. First, we have developed a novel 3D GRMHD framework that combines multiple numerical codes to simulate the inspiral and merger of supermassive black hole binaries starting from realistic initial data and running all the way through merger. Throughout the evolution, we adopt a simple but functional prescription to account for gas cooling through photon emission. Next, we have applied our new computational method to follow the time evolution of a circular, equal-mass, nonspinning black hole binary for ~200 orbits, starting from a separation of 20r_g and reaching the postmerger evolutionary stage of the system. We have shown how mass continues to flow toward the binary even after the binary "decouples" from its surrounding disk, but the accretion rate onto the black holes diminishes. We have identified how the minidisks orbiting each black hole are slowly drained and eventually dissolve as the binary compresses. We confirm previous findings that the system's luminosity decreases by a factor of a few during inspiral; however, we observe an abrupt increase by ~50% in this quantity at the time of merger, likely accompanied by an equally abrupt change in spectrum. Finally, we have demonstrated that during the inspiral, fluid ram pressure regulates the fraction of the magnetic flux transported to the binary that attaches to the black holes' horizons.

astro-ph.HE↗

arXiv2023

Numerical modelling of bulk viscosity in neutron stars

The early post-merger phase of a binary neutron-star coalescence is shaped by characteristic rotational velocities as well as violent density oscillations and offers the possibility to constrain the properties of neutron star matter by observing the gravitational wave emission. One possibility to do so is the investigation of gravitational wave damping through the bulk viscosity which originates from violations of weak chemical equilibrium. Motivated by these prospects, we present a comprehensive report about the implementation of the self-consistent and second-order formulation of the equations of relativistic hydrodynamics for dissipative fluids proposed by M\"uller, Israel and Stewart. Furthermore, we report on the results of two test problems, namely the viscous damping of linear density oscillations of isolated nonrotating neutron stars and the viscous migration test, both of which confirm our implementation and can be used for future code tests. Finally, we present fully general-relativistic simulations of viscous binary neutron-star mergers. We explore the structural and thermal properties of binary neutron-star mergers with a constant bulk-viscosity prescription and investigate the impact of bulk viscosity on dynamical mass ejection. We find that inverse Reynolds numbers $\sim 1\%$ can be achieved for the highest employed viscosity thereby suppressing the dynamically ejected mass by a factor of $\sim 5$ compared to the inviscid case.

gr-qc↗

arXiv2023

Impact of bulk viscosity on the post-merger gravitational-wave signal from merging neutron stars

In the violent post-merger of binary neutron-star mergers strong oscillations are present that impact the emitted gravitational-wave (GW) signal. The frequencies, temperatures and densities involved in these oscillations allow for violations of the chemical equilibrium promoted by weak-interactions, thus leading to a nonzero bulk viscosity that can impact dynamics and GW signals. We present the first simulations of binary neutron-star mergers employing the self-consistent and second-order formulation of the equations of relativistic hydrodynamics for dissipative fluids proposed by M\"uller, Israel and Stewart. With the spirit of obtaining a first assessment of the impact of bulk viscosity on the structure and radiative efficiency of the merger remnant we adopt a simplified but realistic approach for the viscosity, which we assume to be determined by direct and modified Urca reactions and hence to vary within the stars. At the same time, to compensate for the lack of a precise knowledge about the strength of bulk viscosity, we explore the possible behaviours by considering three different scenarios of low, medium, and high bulk viscosity. In this way, we find that large values of the bulk viscosities damp the collision-and-bounce oscillations that characterize the dynamics of the stellar cores right after the merger. At the same time, large viscosities tend to preserve the $m=2$ deformations in the remnant, thus leading to a comparatively more efficient GW emission and to changes in the post-merger spectrum that can be up to $100\,{\rm Hz}$ in the case of the most extreme configurations. Overall, our self-consistent results indicate that bulk viscosity increases the energy radiated in GWs soon after the merger by $\lesssim 2\%$ in the (realistic) scenario of small viscosity, and by $\lesssim 30\%$ in the (unrealistic) scenario of large viscosity.

gr-qc↗

arXiv2023

General-relativistic hydrodynamics of non-perfect fluids: 3+1 conservative formulation and application to viscous black-hole accretion

We consider the relativistic hydrodynamics of non-perfect fluids with the goal of determining a formulation that is suited for numerical integration in special-relativistic and general-relativistic scenarios. To this end, we review the various formulations of relativistic second-order dissipative hydrodynamics proposed so far and present in detail a particular formulation that is fully general, causal, and can be cast into a 3+1 flux-conservative form, as the one employed in modern numerical-relativity codes. As an example, we employ a variant of this formulation restricted to a relaxation-type equation for the bulk viscosity in the general-relativistic magnetohydrodynamics code $\texttt{BHAC}$. After adopting the formulation for a series of standard and non-standard tests in 1+1-dimensional special-relativistic hydrodynamics, we consider a novel general-relativistic scenario, namely, the stationary, spherically symmetric, viscous accretion onto a black hole. The newly developed solution $-$ which can exhibit even considerable deviations from the inviscid counterpart $-$ can be used as a testbed for numerical codes simulating non-perfect fluids on curved backgrounds.

gr-qc↗

arXiv2022

Crustal magnetic fields do not lead to large magnetic-field amplifications in binary neutron-star mergers

The amplification of magnetic fields plays an important role in explaining numerous astrophysical phenomena associated with binary neutron-star mergers, such as mass ejection and the powering of short gamma-ray bursts. Magnetic fields in isolated neutron stars are often assumed to be confined to a small region near the stellar surface, while they are normally taken to fill the whole stars in the numerical modelling. By performing high-resolution, global, and high-order general-relativistic magnetohydrodynamic simulations we investigate the impact of a purely crustal magnetic field and contrast it with the standard configuration consisting of a dipolar magnetic field with the same magnetic energy but filling the whole star. While the crust-configurations are very effective in generating strong magnetic fields during the Kelvin-Helmholtz-instability stage, they fail to achieve the same level of magnetic-field amplification of the full-star configurations. This is due to the lack of magnetized material in the neutron-star interiors to be used for further turbulent amplification and to the surface losses of highly magnetized matter in the crust-configurations. Hence, the final magnetic energies in the two configurations differ by more than one order of magnitude. We briefly discuss the impact of these results on astrophysical observables and how they can be employed to deduce the magnetic topology in merging binaries.

astro-ph.HE↗