Quasi-universal Behavior of the Threshold Mass in Unequal-mass, Spinning Binary Neutron Star Mergers

Abstract The lifetime of the remnant produced by the merger of two neutron stars can provide a wealth of information on the equation of state of nuclear matter and on the processes leading to the electromagnetic counterpart. Hence, it is essential to determine when this lifetime is the shortest, corresponding to when the remnant has a mass equal to the threshold mass, M th, to prompt collapse to a black hole. We report on the results of more than 360 simulations of merging neutron-star binaries covering 40 different configurations differing in mass ratio and spin of the primary. Using this data, we have derived a quasi-universal relation for M th and expressed its dependence on the mass ratio and spin of the binary. The new expression recovers the results of Koeppel et al. for equal-mass, irrotational binaries and reveals that M th can increase (decrease) by 5% (10%) for binaries that have spins aligned (antialigned) with the orbital angular momentum and provides evidence for a nonmonotonic dependence of M th on the mass asymmetry in the system. Finally, we extend to unequal masses and spinning binaries the lower limits that can be set on the stellar radii once a neutron star binary is detected, illustrating how the merger of an unequal-mass, rapidly spinning binary can significantly constrain the allowed values of the stellar radii.

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3D magnetized jet break-out from neutron-star binary merger ejecta: afterglow emission from the jet and the ejecta

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab115 ◽

2021 ◽

Vol 502 (2) ◽

pp. 1843-1855

Author(s):

Antonios Nathanail ◽

Ramandeep Gill ◽

Oliver Porth ◽

Christian M Fromm ◽

Luciano Rezzolla

Keyword(s):

Neutron Star ◽

Thermal Emission ◽

Opening Angle ◽

Hollow Core ◽

Binary Neutron Star ◽

Vlbi Observations ◽

Superluminal Motion ◽

General Relativistic ◽

Star Binary ◽

Magnetohydrodynamic Simulations

ABSTRACT We perform 3D general-relativistic magnetohydrodynamic simulations to model the jet break-out from the ejecta expected to be produced in a binary neutron-star merger. The structure of the relativistic outflow from the 3D simulation confirms our previous results from 2D simulations, namely, that a relativistic magnetized outflow breaking out from the merger ejecta exhibits a hollow core of θcore ≈ 4°, an opening angle of θjet ≳ 10°, and is accompanied by a wind of ejected matter that will contribute to the kilonova emission. We also compute the non-thermal afterglow emission of the relativistic outflow and fit it to the panchromatic afterglow from GRB170817A, together with the superluminal motion reported from VLBI observations. In this way, we deduce an observer angle of $\theta _{\rm obs}= 35.7^{\circ \, \, +1.8}_{\phantom{\circ \, \, }-2.2}$. We further compute the afterglow emission from the ejected matter and constrain the parameter space for a scenario in which the matter responsible for the thermal kilonova emission will also lead to a non-thermal emission yet to be observed.

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A More Stringent Constraint on the Mass Ratio of Binary Neutron Star Merger GW170817

The Astrophysical Journal ◽

10.3847/2041-8213/aaa0c6 ◽

2017 ◽

Vol 851 (2) ◽

pp. L45 ◽

Cited By ~ 18

Author(s):

He Gao ◽

Zhoujian Cao ◽

Shunke Ai ◽

Bing Zhang

Keyword(s):

Neutron Star ◽

Mass Ratio ◽

Binary Neutron Star ◽

Stringent Constraint

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On the opening angle of magnetized jets from neutron-star mergers: the case of GRB170817A

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1454 ◽

2020 ◽

Vol 495 (4) ◽

pp. 3780-3787 ◽

Cited By ~ 1

Author(s):

Antonios Nathanail ◽

Ramandeep Gill ◽

Oliver Porth ◽

Christian M Fromm ◽

Luciano Rezzolla

Keyword(s):

Neutron Star ◽

Gamma Ray ◽

Initial Conditions ◽

Observation Angle ◽

Opening Angle ◽

Binary Neutron Star ◽

Best Fitting ◽

Standard Picture ◽

General Relativistic ◽

Star Binary

ABSTRACT The observations of GW170817/GRB170817A have confirmed that the coalescence of a neutron-star binary is the progenitor of a short gamma-ray burst (GRB). In the standard picture of a short GRB, a collimated highly relativistic outflow is launched after merger and it successfully breaks out from the surrounding ejected matter. Using initial conditions inspired from numerical-relativity binary neutron-star merger simulations, we have performed general-relativistic hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations in which the jet is launched and propagates self-consistently. The complete set of simulations suggests that: (i) MHD jets have an intrinsic energy and velocity polar structure with a ‘hollow core’ subtending an angle θcore ≈ 4°–5° and an opening angle of θjet > ≳ 10°; (ii) MHD jets eject significant amounts of matter and two orders of magnitude more than HD jets; (iii) the energy stratification in MHD jets naturally yields the power-law energy scaling E(> Γβ) ∝ (Γβ)−4.5; (iv) MHD jets provide fits to the afterglow data from GRB170817A that are comparatively better than those of the HD jets and without free parameters; and (v) finally, both of the best-fitting HD/MHD models suggest an observation angle θobs ≃ 21° for GRB170817A.

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The heavier the better: how to constrain mass ratios and spins of high-mass neutron star mergers

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slaa079 ◽

2020 ◽

Vol 496 (1) ◽

pp. L16-L21 ◽

Cited By ~ 1

Author(s):

Elias R Most ◽

Lukas R Weih ◽

Luciano Rezzolla

Keyword(s):

Black Hole ◽

Neutron Star ◽

Lower Bound ◽

Equations Of State ◽

High Mass ◽

Maximum Mass ◽

Mass Ratio ◽

Effective Spin ◽

Binary Neutron Star ◽

Merger Event

ABSTRACT The first binary neutron star merger event, GW170817, and its bright electromagnetic counterpart have provided a remarkable amount of information. By contrast, the second event, GW190425, with $M_{\rm tot}=3.4^{+0.3}_{-0.1}\, \mathrm{ M}_{\odot }$ and the lack of an electromagnetic counterpart, has hardly improved our understanding of neutron star physics. While GW190425 is compatible with a scenario in which the merger has led to a prompt collapse to a black hole and little ejected matter to power a counterpart, determining the mass ratio and the effective spin $\tilde{\chi }$ of the binary remains difficult. This is because gravitational waveforms cannot yet well constrain the component spins of the binary. However, since the mass of GW190425 is significantly larger than the maximum mass for non-rotating neutron stars, $M_{_{\rm TOV}}$, the mass ratio q cannot be too small, as the heavier star would not be gravitationally stable. Making use of universal relations and a large number of equations of state, we provide limits in the $(\tilde{\chi },q)$ plane for GW190425, namely qmin ≥ 0.38 and $\tilde{\chi }_{\rm max}\le 0.20$, assuming $M_\mathrm{tot} \simeq 3.4\, \mathrm{ M}_\odot$. Finally, we show how future observations of high-mass binaries can provide a lower bound on $M_{_{\rm TOV}}$.

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