arXiv subjects

Fatemeh Hossein Nouri

Publications and source records attributed to Fatemeh Hossein Nouri.

10 recordsLinked to original sources

arXiv2026

Binary Neutron Star Merger Simulations with Microphysical Equation of State using Spritz

We present the first binary neutron star merger simulations performed with the general-relativistic magnetohydrodynamics (GRMHD) Spritz code employing a finite-temperature tabulated equation of state. Two magnetized binaries with the LS220 equation of state and a GW170817-like chirp mass, differing only in their mass ratios ($q=1$ and $q=0.7$), are evolved from the late inspiral through merger, hypermassive neutron-star (HMNS) formation, and delayed collapse to a black hole (BH). We investigate how the binary mass ratio influences the postmerger evolution, jet-launching conditions, and matter ejection. We find that the equal-mass binary forms a slightly longer-lived HMNS, allowing more efficient magnetic-field amplification during the post-merger evolution. As a result, it accumulates larger magnetic fluxes, develops a magnetically dominated funnel, and launches a collimated magnetically driven polar outflow. In contrast, the unequal-mass binary ejects approximately 50% more mass but exhibits weaker magnetic-field amplification, with the polar region remaining turbulent and baryon loaded throughout our simulation, suggesting that the development of a magnetically dominated funnel is likely delayed.

astro-ph.HE↗

arXiv2025

The impact of disk outflows on the structure of short GRB jets at large scales

Short Gamma-Ray Bursts (GRBs) are known to be associated with binary neutron star (NSNS) or black hole-neutron star (BHNS) mergers. The detection of gravitational wave and its associated electromagnetic counterparts GW/GRB 170817A has shown that interactions between relativistic jets and mildly relativistic ejecta influence observed radiation. Previous studies simulated a uniform jet propagating through a homologously expanding wind, however, jets and disk outflows are launched together during accretion, making the interaction more complex. We investigate how the disk wind impacts jet propagation at distances $r\sim 10^8\,-\,10^{11}~$cm. We are using two-dimensional special relativistic hydrodynamical simulations. As initial conditions, we remap the outflows from general relativistic magnetohydrodynamical simulations of BH accretion disks that represent post-merger NSNS or BHNS remnants. We account for wind stratification and r-process nucleosynthesis, which alter the pressure profile from that of an ideal gas in the initial conditions. We found that a) self-consistent wind pressure leads to significant changes in the jet collimation and cocoon expansion; b) the angular structure of thermal and kinetic energy components in the jets, cocoons, and winds differ with respect to simple homologous models; c) the temporal evolution of the structure reveals conversion of thermal to kinetic energy being different for each component in the system (jet, cocoon, and wind); d) dynamical ejecta alters the interaction between jets and disk winds. Our results show that the jet and cocoon structure is shaped by the accretion disk wind that alters the effect of dynamical ejecta and may have an impact on the observed afterglow emission.

astro-ph.HE↗

arXiv2024

Viscous torque in turbulent magnetized AGN accretion disks and its effects on EMRI's gravitational waves

The merger of supermassive black holes (SMBHs) produces mHz gravitational waves (GW), which are potentially detectable by future Laser Interferometer Space Antenna (LISA). Such binary systems are usually embedded in an accretion disk environment at the centre of the active galactic nucleus (AGN). Recent studies suggest the plasma environment imposes measurable imprints on the GW signal if the mass ratio of the binary is around q $ \sim10^{-4}-10^{-3}$. The effect of the gaseous environment on the GW signal is strongly dependent on the disk's parameters, therefore it is believed that future low-frequency GW detections will provide us with precious information about the physics of AGN accretion disks. We investigate this effect by measuring the viscous torque via modelling the evolution of magnetized tori around the primary massive black hole. Using GRMHD HARM-COOL code, we perform 2D and 3D simulations of weakly-magnetized thin accretion disks, with a possible truncation and transition to advection-dominated accretion flow (ADAF). We study the angular momentum transport and turbulence generated by magnetorotational instability (MRI). We quantify the disk's effective alpha viscosity and its evolution over time. We apply our numerical results to quantify the relativistic viscous torque on a hypothetical low-mass secondary black hole via a 1D analytical approach, and we estimate the GW phase shift due to the gas environment.

astro-ph.HE↗

arXiv2023

$f$-mode oscillations of compact stars with realistic equations of state in dynamical spacetime

In this study, we perform full three dimensional numerical relativity simulations of non-rotating general relativistic stars. Extending the studies for polytropic equation of state, we investigate the accuracy and robustness of numerical scheme on measuring fundamental ($f$)-mode frequency for realistic equations of state (EoS). We use various EoS with varying range of stiffness and numerically evolve perturbed stellar models for several mass configurations (in the range of $1.2 - 2.0$ $M_{\odot}$) for each of these EoS. Using the gravitational waveform obtained from the simulations we extract the $f$-modes of the stars. The obtained results are tested against the pre-existing perturbation methods and find good agreement. Validity and deviation of universal relations have been carried out and are compared with earlier results under Cowling approximation as well as perturbative approaches. We also show that even using perturbed single star simulations can provide good agreement with $f$-modes extracted from inspiral phase of binary neutron star simulations which are computationally more expensive.

gr-qc↗

arXiv2022

Studying Postmerger Outflows from Magnetized Neutrino-cooled Accretion Disks

Neutrino-cooled accretion flow around a spinning black hole, produced by a compact binary merger is a promising scenario for jet formation and launching magnetically-driven outflows. Based on GW170817 gravitational wave detection by LIGO and Virgo observatories followed by electromagnetic counterparts, this model can explain the central engine of the short duration gamma ray bursts (GRB) and kilonova radiations. Using the open-source GRMHD HARM-COOL code, we evolved several 2D magnetized accretion disk-black hole models with realistic equation of state in the fixed curved space-time background. We applied particle tracer technique to measure the properties of the outflows. The disk and black hole's initial parameters are chosen in a way to represent different possible post-merger scenarios of the merging compact objects. Our simulations show a strong correlation between black hole's spin and ejected mass. Generally, mergers producing massive disks and rapidly spinning black holes launch stronger outflows. We observed our models generate winds with moderate velocity ($v/c \sim 0.1-0.2$), and broad range of electron fraction. We use these results to estimate the luminosity and light curves of possible radioactively powered transients emitted by such systems. We found the luminosity peaks within the range of $10^{40}-10^{42}$ erg/s which agrees with previous studies for disk wind outflows.

astro-ph.HE↗

arXiv2022

Nonlinear mode-tide coupling in coalescing binary neutron stars with relativistic corrections

We compute the internal modes of a nonspinning neutron star and its tidal metric perturbation in general relativity, and determine the effect of relativistic corrections to the modes on mode coupling. Claims have been made that a new hydrodynamic instability can occur in a neutron star in a binary neutron star system triggered by the nonlinear coupling of the companion's tidal field to pairs of p-modes and g-modes in it as the binary inspirals toward merger. This "PG" instability may be significant since it can influence the binary's inspiral phase by extracting orbital energy, thereby potentially causing large deviations in their gravitational waveforms from those predicted by theoretical models that do not account for it. This can result in incorrect parameter estimation, at best, or mergers going undetected, at worst, owing to the use of deficient waveform models. On the other hand, better modeling of this instability and its effect on binary orbits can unravel a new phenomenon and shed light on stellar instabilities, via gravitational wave observations. So far, all mode-tide coupling instability studies have been formulated in Newtonian perturbation theory. Neutron stars are compact objects, so relativistic corrections might be important. We present and test a new code to calculate the relativistic eigenmodes of nonrotating relativistic stars. We use these relativistic tide and neutron star eigenmodes to compute the mode-tide coupling strength (MTCS) for a few selected equations of state. The MTCS thus calculated can be at most tens of percent different from its purely Newtonian value, but we confirm the dependencies on orbital separation and equation of state found by Newtonian calculations. For some equations of state, the MTCS is very sensitive to the neutron star crust region, demonstrating the importance of treating this region accurately.

gr-qc↗

arXiv2022

Modeling the GRB jet properties with 3D general relativistic simulations of magnetically arrested accretion flows

We investigate the dependence of the GRB jet structure and its evolution on the properties of the accreting torus in the central engine. Our models numerically evolve the accretion disk around a Kerr black hole using 3D general relativistic magnetohydrodynamic simulations. We use two different analytical hydrodynamical models of the accretion disk, based on the Fishbone-Moncrief and Chakrabarti solutions, as our initial states for the structure of the collapsar disk and the remnant after a binary neutron star merger, respectively. We impose poloidal magnetic fields of two different geometries upon the initial stable solutions. We study the formation and evolution of the magnetically arrested disk state and its effect on the properties of the emitted jet. The jets produced in our models are structured and have a relatively hollow core and reach higher Lorentz factors at an angle $\gtrsim 9{^\circ}$ from the axis. The jet in our short GRB model has an opening angle of up to $\sim 25^{\circ}$ while our long GRB engine produces a narrower jet, of up to $\sim 11^{\circ}$. We also study the time variability of the jets and provide an estimate of the minimum variability timescale in our models. The application of our models to the GRB jets in the binary neutron star post-merger system and to the ultra-relativistic jets launched from collapsing stars are briefly discussed.

astro-ph.HE↗

arXiv2022

Accretion of the magnetized neutrino-cooled torus

Neutrino-cooled accretion flow around a black hole, produced by a compact binary merger, is a promising scenario for jet formation and magnetic-driven winds to explain short duration gamma ray bursts (GRBs) central engine and kilonovae based on GW170817 gravitational wave observation. Magnetorotational instability (MRI) turbulence and Blandford-Znajek (BZ) mechanism are expected to play key roles in the thermal equilibrium of the disk (balancing neutrino cooling) and in driving accretion and creating jets. Using the open-source GRMHD HARM-COOL code, we study the magnetically-driven evolution of an accretion disk with realistic equation of state in the fixed curved space-time background. We identify the effects of the neutrino cooling and the magnetic field, paying particular attention to the dynamical, thermal and composition evolution of the disk and outflows.

astro-ph.HE↗

arXiv2017

Evolution of the Magnetized, Neutrino-Cooled Accretion Disk in the Aftermath of a Black Hole Neutron Star Binary Merger

Black hole-torus systems from compact binary mergers are possible engines for gamma-ray bursts (GRBs). During the early evolution of the post-merger remnant, the state of the torus is determined by a combination of neutrino cooling and magnetically-driven heating processes, so realistic models must include both effects. In this paper, we study the post-merger evolution of a magnetized black hole-neutron star binary system using the Spectral Einstein Code (SpEC) from an initial post-merger state provided by previous numerical relativity simulations. We use a finite-temperature nuclear equation of state and incorporate neutrino effects in a leakage approximation. To achieve the needed accuracy, we introduce improvements to SpEC's implementation of general-relativistic magnetohydrodynamics (MHD), including the use of cubed-sphere multipatch grids and an improved method for dealing with supersonic accretion flows where primitive variable recovery is difficult. We find that a seed magnetic field triggers a sustained source of heating, but its thermal effects are largely cancelled by the accretion and spreading of the torus from MHD-related angular momentum transport. The neutrino luminosity peaks at the start of the simulation, and then drops significantly over the first 20\,ms but in roughly the same way for magnetized and nonmagnetized disks. The heating rate and disk's luminosity decrease much more slowly thereafter. These features of the evolution are insensitive to grid structure and resolution, formulation of the MHD equations, and seed field strength, although turbulent effects are not fully converged

astro-ph.HE↗

arXiv2014

Magnetic effects on the low-T/|W| instability in differentially rotating neutron stars

Dynamical instabilities in protoneutron stars may produce gravitational waves whose observation could shed light on the physics of core-collapse supernovae. When born with sufficient differential rotation, these stars are susceptible to a shear instability (the "low-T/|W| instability"), but such rotation can also amplify magnetic fields to strengths where they have a considerable impact on the dynamics of the stellar matter. Using a new magnetohydrodynamics module for the Spectral Einstein Code, we have simulated a differentially-rotating neutron star in full 3D to study the effects of magnetic fields on this instability. Though strong toroidal fields were predicted to suppress the low-T/|W| instability, we find that they do so only in a small range of field strengths. Below 4e13 G, poloidal seed fields do not wind up fast enough to have an effect before the instability saturates, while above 5e14 G, magnetic instabilities can actually amplify a global quadrupole mode (this threshold may be even lower in reality, as small-scale magnetic instabilities remain difficult to resolve numerically). Thus, the prospects for observing gravitational waves from such systems are not in fact diminished over most of the magnetic parameter space. Additionally, we report that the detailed development of the low-T/|W| instability, including its growth rate, depends strongly on the particular numerical methods used. The high-order methods we employ suggest that growth might be considerably slower than found in some previous simulations.

astro-ph.HE↗