October 2024

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2 Reads

Physical Review D

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October 2024

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2 Reads

Physical Review D

July 2024

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17 Reads

Binary neutron star mergers are among the most energetic events in our Universe, with magnetic fields significantly impacting their dynamics, particularly after the merger. While numerical-relativity simulations that correctly describe the physics are essential to model their rich phenomenology, the inclusion of magnetic fields is crucial for realistic simulations. For this reason, we have extended the BAM code to enable general relativistic magneto-hydrodynamic (GRMHD) simulations employing a hyperbolic `divergence cleaning' scheme. We present a large set of standard GRMHD tests and compare the BAM code to other GRMHD codes, SPRITZ, GRaM-X, and SACRA$_{\rm KK22}$, which employ different schemes for the evolution of the magnetic fields. Overall, we find that the BAM code shows a good performance in simple special-relativistic tests. In addition, we find good agreement and consistent results when comparing GRMHD simulation results between BAM and SACRA$_{\rm KK22}$.

February 2024

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16 Reads

Nature Astronomy

January 2024

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40 Reads

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8 Citations

The Astrophysical Journal Letters

We present a 3D general-relativistic magnetohydrodynamic simulation of a short-lived neutron star remnant formed in the aftermath of a binary neutron star merger. The simulation uses an M1 neutrino transport scheme to track neutrino–matter interactions and is well suited to studying the resulting nucleosynthesis and kilonova emission. A magnetized wind is driven from the remnant and ejects neutron-rich material at a quasi-steady-state rate of 0.8 × 10 ⁻¹ M ⊙ s ⁻¹ . We find that the ejecta in our simulations underproduce r -process abundances beyond the second r -process peak. For sufficiently long-lived remnants, these outflows alone can produce blue kilonovae, including the blue kilonova component observed for AT2017gfo.

November 2023

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39 Reads

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4 Citations

We present a new python/SymPy based code generator for producing executable numerical expressions for partial differential equations in AMReX-based applications. We demonstrate the code generator capabilities for the case of $3+1$ ADM formulations of numerical relativity for the constraint damped, conformal Z4 formulations (Z4c and CCZ4). The generated spacetime solvers are examined for stability and accuracy using a selection of checks from the standard Apples with Apples testbeds for numerical relativity applications. We also explore physically interesting vacuum spacetimes including head-on and inspiraling black hole binary collisions, and investigate the simulated gravitational waveforms from such events with the Newman-Penrose formulation of waveform extraction.

September 2023

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8 Reads

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9 Citations

Monthly Notices of the Royal Astronomical Society

We investigate the influence of parametric magnetic field configurations of a hypermassive neutron star (HMNS) on the outflow properties, nucleosynthesis yields, and kilonova light curves. We perform three-dimensional dynamical space–time general-relativistic magnetohydrodynamic simulations, including a neutrino leakage scheme, microphysical finite-temperature equation of state, and an initial poloidal magnetic field. We find that varying the magnetic field strength and falloff impacts the formation of magnetized winds or mildly relativistic jetted outflows, which in turn has profound effects on the outflow properties. All of the evolved configurations collapse to a black hole ∼38–40 ms after coalescence, where the ones forming jetted outflows seem more effective at redistributing angular momentum, which result in earlier collapse times. Larger mass ejecta rates and radial velocities of unbound material characterize the systems that form jetted outflows. The bolometric light curves of the kilonovae and r-process yields that are produced by the post-merger remnant system change considerably with different magnetic field parameters. We conclude that the magnetic field strength and falloff have robust effects on the outflow properties and electromagnetic observables. This can be particularly important as the total ejecta mass from our simulations (≃10−3 M⊙) makes the ejecta from HMNS a compelling source to power kilonova through radioactive decay of r-process elements.

September 2023

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70 Reads

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14 Citations

We present GRaM-X ( G eneral R elativistic a ccelerated M agnetohydrodynamics on AMRe X ), a new GPU-accelerated dynamical-spacetime general relativistic magnetohydrodynamics (GRMHD) code which extends the GRMHD capability of Einstein Toolkit to GPU-based exascale systems. GRaM-X supports 3D adaptive mesh refinement (AMR) on GPUs via a new AMR driver for the Einstein Toolkit called CarpetX which in turn leverages AMReX, an AMR library developed for use by the United States DOE’s Exascale Computing Project (ECP). We use the Z4c formalism to evolve the equations of GR and the Valencia formulation to evolve the equations of GRMHD. GRaM-X supports both analytic as well as tabulated equations of state. We implement TVD and WENO reconstruction methods as well as the HLLE Riemann solver. We test the accuracy of the code using a range of tests on static spacetime, e.g. 1D MHD shocktubes, the 2D magnetic rotor and a cylindrical explosion, as well as on dynamical spacetimes, i.e. the oscillations of a 3D TOV star. We find excellent agreement with analytic results and results of other codes reported in literature. We also perform scaling tests and find that GRaM-X shows a weak scaling efficiency of ∽ 40-50% on 2304 nodes (13824 NVIDIA V100 GPUs) with respect to single-node performance on OLCF’s supercomputer Summit.

August 2023

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47 Reads

The Astrophysical Journal

The origin of short gamma-ray bursts is associated with outflows powered by the remnant of a binary neutron star merger. This remnant can be either a black hole or a highly magnetized, fast-spinning neutron star, also known as a magnetar. Here we present the results of two relativistic magnetohydrodynamical simulations aimed at investigating the large-scale dynamics and propagation of magnetar collimated outflows through the medium surrounding the remnant. The first simulation evolves a realistic jet by injecting external simulation data, while the second evolves an analytical model jet with similar properties for comparison. We find that both outflows remain collimated and successfully emerge through the static medium surrounding the remnant. However, they fail to attain relativistic velocities and only reach a mean maximum speed of ∼0.7 c for the realistic jet and ∼0.6 c for the analytical jet. We also find that the realistic jet has a much more complex structure. The lack of highly relativistic speeds, which makes these jets unsuitable as short gamma-ray burst sources, is due to numerical limitations and is not general to all possible magnetar outflows. A jet like the one we study, however, could give rise to or augment a blue kilonova component. In addition, it would make the propagation of a relativistic jet easier, should one be launched after the neutron star collapses into a black hole.

May 2023

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10 Reads

We present a 3D general-relativistic magnetohydrodynamic simulation of a short-lived neutron star remnant formed in the aftermath of a binary neutron star merger. The simulation uses an M1 neutrino transport scheme to track neutrino-matter interactions and is well-suited to studying the resulting nucleosynthesis and kilonova emission. We find that the ejecta in our simulations under-produce $r$-process abundances beyond the second $r$-process peak. For sufficiently long-lived remnants, these outflows \textit{alone} can produce blue kilonovae, including the blue kilonova component observed for AT2017gfo.

February 2023

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22 Reads

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6 Citations

The Astrophysical Journal Letters

Long-duration gamma-ray bursts (lGRBs) originate in relativistic collimated outflows—jets—that drill their way out of collapsing massive stars. Accurately modeling this process requires realistic stellar profiles for the jets to propagate through and break out of. Most previous studies have used simple power laws or pre-collapse models for massive stars. However, the relevant stellar profile for lGRB models is in fact that of a star after its core has collapsed to form a compact object. To self-consistently compute such a stellar profile, we use the open-source code GR1D to simulate the core-collapse process for a suite of low-metallicity rotating massive stellar progenitors that have undergone chemically homogeneous evolution. Our models span a range of zero-age main-sequence (ZAMS) masses: M ZAMS = 13, 18, 21, 25, 35, 40, and 45 M ☉ . All of these models, at the onset of core-collapse, feature steep density profiles, ρ ∝ r − α , with α ≈ 2.5, which would result in jets that are inconsistent with lGRB observables. We follow the collapses of four of the seven models until they form black holes (BHs) and the other three models until they form proto-neutron stars (PNSs). We find, across all models, that the density profile outside the newly formed BH or PNS is well represented by a flatter power law with α ≈ 1.35–1.55. Such flat density profiles are conducive to the successful formation and breakout of BH-powered jets and are, in fact, required to reproduce observable properties of lGRBs. Future models of lGRBs should be initialized with shallower post-collapse stellar profiles, like those presented here, instead of the much steeper pre-collapse profiles that are typically used.

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... Whether the metastable remnant of a BNS merger can launch a relativistic, magnetized and collimated outflow has been a matter of debate over the last few years, with different approaches reaching different conclusions. Examples include the development of magnetically-driven outflows for simplified microphysics treatments (e.g., Ciolfi 2020; Pavan et al. 2023;Bamber et al. 2024), or simplified magnetic-field evolutions with more realistic neutrino microphysics (e.g., Mösta et al. 2020;de Haas et al. 2023;Curtis et al. 2024)), or both (Combi & Siegel 2023). Recently, several works have also investigated the importance of dynamo processes (Most & Quataert 2023;Most 2023;Kiuchi 2024) as a prerequisite for field breakout and the formation of a collimated outflow. ...

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January 2024

The Astrophysical Journal Letters

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September 2023

Monthly Notices of the Royal Astronomical Society

... The BSSNOK formulation arises naturally as a mixed first and second order (in space derivatives) PDE system, and as such, central finite difference numerical schemes have always been the most natural choice. This is in fact the case for a wide class of numerical codes like Einstein-Toolkit [17], LazEv [18,19], BAM [20,21], McLachlan [22], GRChombo [23], AMReX [24], SACRA [25,26], MHDueT [27], ExaGRyPE [28]. ...

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- Publisher preview available
November 2023

... AthenaK is not the only, nor the first, effort to leverage GPUs for numerical relativity applications. For instance, AsterX and GRaM-X are notable examples Kalinani et al. (2024); Shankar et al. (2023). Both of these codes solve the GRMHD equations coupled with the Einstein equations in the Z4c formulation, using the CarpetX AMR driver Schnetter et al. (2004) within the Einstein Toolkit Brandt et al. (2024), which is built on AMReX Zhang et al. (2019Zhang et al. ( , 2021. ...

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September 2023

... However, in our simulations, t = 0 marks the time of BH formation, which is preceded by a few seconds-long proto-NS phase (e.g., . During this phase, the innermost shells exhibit a free-fall radial density profile, α p = 1.5, which we adopt in the models pBs, pBw, pB0 (Halevi et al. 2023). To maintain the same total mass in the star, these models feature higher density values at the center than those with α p = 0. ...

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February 2023

The Astrophysical Journal Letters

... The radial grid extends from just inside the event horizon to 6×10 11 cm with numerical resolution at the base AMR level of N r × N θ × N φ = 384 × 96 × 192 cells, in the r-, θ-, and φ-directions, respectively. We apply a refinement criterion following Gottlieb et al. (2022b) -At each radius r, the jet and cocoon halfopening angles are measured based on the specific entropy of the fluid. If either half-opening angle contains less than the desired number of cells, ∆N θ = 96 or ∆N φ = 192, the grid refines to the next AMR level, until it reaches the desired number of cells across each dimension, up to one level of refinement. ...

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June 2022

The Astrophysical Journal Letters

... For instance, in recent years, various GRMHD simulations of BNS and black hole-neutron star (BHNS) mergers have demonstrated that magnetic fields play a critical role in launching relativistic jets, which can act as a pre-cursor or potentially power short gamma-ray bursts (SGRBs) [23][24][25][26][27][28][29][30]. Additionally, they can have a profound impact on matter ejection, contributing to the kilonova production [31][32][33][34]. On the other hand, for accurate modelling of the gas dynamics in SMBBH merger simulations, magnetic fields prove to be fundamental in driving turbulence and accretion processes from the circumbinary disks (CBD) [35][36][37] to the minidisks surrounding each SMBH [38][39][40][41][42], as well as in jet generation [41,[43][44][45]. ...

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November 2022

Monthly Notices of the Royal Astronomical Society

... The magnitude and time dependence of the accretion onto the BH depend on the stellar mass and density profile, respectively. 1D core-collapse simulations find that density profiles from stellar evolution models, ρ(r) ∝ r −2.5 , flatten prior to the BH formation to ρ(r) ∝ r −1.5 (Halevi et al. 2022). For freefall of a typical stellar envelope mass, numerical and analytic results show that this power-law leads to a steady BH accretion ofṁ 10 −2 M s −1 (Gottlieb et al. 2022b(Gottlieb et al. , 2023. ...

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November 2022

... The level of accuracy plays a critical role, in particular for future detectors such as LISA [2] and ET [3]. In order to benefit the most from the anticipated observational reach [2][3][4][5][6][7][8][9], the modeling of GW sources must therefore continue to develop-both through analytic methodologies [10][11][12][13][14][15][16][17][18] and numerical simulations [19][20][21]-in parallel with the expected increase in sensitivity with next-generation GW interferometers. ...

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October 2013

... over a sphere at radius, r ∼ 300 km, for ∼ 20 ms after the merger. We only include mass fluxes if the matter is unbound (i.e., can escape to infinity), according to u 0 < −1 (e.g., [123,124]) . In Table I, we report the values of the masses ejected for each simulation. ...

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December 2021

Physical Review D