Binary neutron star mergers

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Mergers of a pair of neutron stars for different equations of state (EoS) starting from a single polytropic (first simulation) to Quark-Hadron Crossover EoS from Baym et al. (2019). Simulations performed by forming initial data using LORENE and evolution using Einstein Toolkit.
Plot specifications: Timescale is in "Cactus Units", where 1000 CU ≈ 5 milliseconds. The axes are normalized such that units 20 units ≈ 30kms.

Binary neutron stars of Masses = 1.2 Msun merging to form a hypermassive neutron star. EoS used is a single polytropic with Γ = 2. Here brighter means denser regions.

Top-down view (cross-section of XY plane). M = 1.5 Msun pair starting at 45km separation. EoS = piecewise polytropic QHC19B. Here redder regions are denser regions, bluer regions are less dense. Merger at t ≈ 1400 CU, Black hole formation at about t ≈ 2600 CU.

Equatorial view (cross-section of XZ plane). M = 1.5 Msun pair starting at 45km separation. EoS = piecewise polytropic QHC19B.

Notice QHC 19B (above) and 19C (below) undergo significantly different evolution histories. For EoS = 19B, the hypermassive neutron star collapses to being a black hole very quickly in comparison to 19C, which persists for much longer.

Top-down view. M = 1.5 Msun pair starting at 45km separation. EoS = piecewise polytropic QHC19C. Merger at t ≈ 1400 CU, Black hole formation at about t ≈ 7700 CU.

Equatorial view. M = 1.5 Msun pair starting at 45km separation. EoS = piecewise polytropic QHC19C.
Evolution of maximum rest-mass density vs time for several equations of state employed. The blue band indicates QHC-crossover densities (note: SLy does not have a crossover: however, indicating the crossover density is suggestive of the need to account for quark matter in hadronic EOSs). In all these cases, the NSs start in the crossover density range (2--5 $n_0$) followed by a rise in density, leading to a collapse to a black hole (in all except the bottom-right panel). The bottom-right case (QHC19D 1.399) does not form a black hole within the simulation time.
Gravitational wave signature for the binary. The elongated "ringdown" phase after the merger for 19C makes it the perfect fit for observations with future interferometer detections.

Parfiles for this work can be found at:










References:
[2] AK, H.I. Kim, I.-S. Suh, G.J. Mathews, Binary neutron star mergers as a probe of quark-hadron crossover equations of state, Phys. Rev. D 106, 103027 (2022).
[3] S.R. Brandt, B. Brendal*, W. E. Gabella*, R. Haas*, B. Karakas*, AK*, S.G. Rosofsky*, A.P. Schaffarczyk*, et al. (2020, May 30). The Einstein Toolkit (Version The "Turing" release, ET_2020_05). (*=co-second authors)

Einstein Toolkit contributions

The Einstein Toolkit is an international community-driven software platform of core computational tools to advance and support research in relativistic astrophysics and gravitational physics. As an active member of the Einstein Toolkit community, I perform tests on the binary black hole and the binary neutron star gallery examples.

LORENE contributions

Additionally, I have summarized the framework and workflow for construction of binary neutron star initial data using LORENE in a few different documents available below.
PhD Thesis
Google doc
A tutorial video from 2021 North American Einstein Toolkit meeting.