ASTROPHYSICS

Introduction

Gravitational wave detectors such as LIGO, VIRGO, and KAGRA, have brought about an era of multi-messenger astronomy that has given new insights into the merger of binary compact objects.  In all cases, the ability to constrain the characteristics of the compact objects is very limited, especially in the absence of an electro-magnetic (EM) counterpart.  Gravitational wave events such as GW190425, however, present a very unique opportunity to study the mass gap regime where the binary could consist of either a black hole and a neutron star (BHNS) or a highly asymmetric neutron star binary (BNS).

The objectives of this project are to extend our knowledge and learn fundamental lessons about the stability of hyper-massive neutron star remnants from the merger of highly asymmetric binary neutron star mergers. 

This includes binaries where the neutron stars are not rotating as well as scenarios where the primary can have up to extremal spins near the mass shedding limit.  Additionally, we have looked into BHNS and BNS systems in the mass gap region and compared them against GW190425 to ascertain the phenomenology that distinguishes these two scenarios.  Finally, we have performed the first fully general relativistic magneto-hydrodynamic (GRMHD) evolution from inspiral to ~300ms post-merger using accurate microphysics to characterize the post-merger observables, the remnant disk properties, and determine the possibility of a jet.

Such surveys are the first of their kind due to the computational demand, but more importantly due to a lack of a publicly available initial data solver that is capable of generating the initial conditions that satisfy Einstein’s field equations for these configurations. 

To achieve our goals we use a combination of state-of-the-art numerical-relativity tools, mainly developed in the astrophysics group at Goethe University and have developed the FUKA[2] initial data solvers to generate constraint satisfying initial data  to obtain the results in [3-5].

Results and Methods

The development of FUKA has relied significantly on the pn56bi project to obtain the successful results detailed in [2].  FUKA is the first publicly available initial data solver that provides the ability to construct eccentricity-reduced initial data for binary black holes, BHNS, and BNS binaries to include mass asymmetry and spin. 

The merger simulations proposed in this project have been carried out using FIL, which is a high-order extension of the publicly available general-relativisticMHD code ILGRMHD. The basic algorithms in ILGRMHD have been developed by the Illinois Relativity Group, but the publicly available version is particularly compact and efficient, providing a speedup factor of almost two when compared with other GRMHD codes. Among the numerous new developments that the FIL code implements, we would like to highlight the fully high-order flux update, the capability for finite-temperature dependent EOSs, an improved primitive-variable recovery, the inclusion of weak-interactions, and a simplified neutrino leakage scheme. 

Finally, FIL is built on the Einstein Toolkit evolution framework.  FIL utilizes Carpet, a fixed-mesh box-in-box refinement such that each refinement level doubles in resolution.  This allows for flexibility in setting up our evolutions as we can adapt the resolution to the physics we are interested in resolving.

As detailed in [3], we have performed systematic surveys into the post-merger stability of the hyper-massive neutron star remnant from a BNS merger to determine the influence of mass asymmetry and spin.  This work has also thoroughly examined the resulting remnant disk masses and the ejected mass to include analyzing the average distribution of the thermodynamic quantities of the ejected mass, the resulting luminosity light curves and the time of peak emission which are critical when comparing to astrophysical observations (Fig. 1).

Furthermore, we have performed a thorough study in the mass gap region specifically targeting GW190425 to determine the ability to distinguish a BHNS merger from a BNS merger. As a result, we have found that fast mass ejecta might be the only distinguishing feature, being accompanied by a distinct class of electromagnetic  transients [4].

Finally, we have performed the first GRMHD simulation with accurate microphysics of a BHNS coalescence up to ~350ms post-merger where the NS has an initial magnetic field confined to its interior [5].  We have performed an extensive analysis on the accretion disk properties to facilitate the study of accretion disks in other codes such that the initial conditions can reflect a realistic merger remnant and disk configuration.  In addition, we have analyzed the resulting phenomenology to estimate the likelihood of a relativistic jet forming at a later time.  Even though a jet is not observed in our simulation, the expected characteristics are present such as funnel clearing and a steady growth in the magnetic field energy (Fig. 2).

Ongoing Research / Outlook

It is of particular interest to our group to further ascertain the impact of spin from the secondary neutron star on the merger and post-merger dynamics of a BNS.  This includes the physics contributing to remnant stability as well as the threshold mass to prompt collapse.

Additionally, we have developed a novel description of nuclear matter using the holographic model which includes a consistent description of nuclear matter in the high density and high temperature regime that is in agreement with QCD.  This is of particular importance as this would provide a first insight into a realistic equation of state that incorporates a first order phase transition to quark matter.

Acknowledgements

The PI acknowledges funding from the Hessian Research Cluster ``ELEMENTS'' and from the ERC Advanced Grant ``JETSET'' (Grant No. 884631).} Dr. Most acknowledges support from the Princeton Center for Theoretical Science, the Princeton Gravity Initiative, and the Institute for Advanced.

References and Links

[1] https://relastro.uni-frankfurt.de/research/

[2] L. J. Papenfort et al., 10.1103/PhysRevD.104.024057

[3] L. J. Papenfort et al., arXiv:2201.03632

[4] E.R. Most et al., Astrophys.J. 912 (2021) 1, 80

[5] E.R. Most et al., Mon.Not.Roy.Astron.Soc. 506 (2021) 3, 3511-3526

Scientific Contact

Prof. Dr. Luciano Rezzolla
Institute for Theoretical Physics
Max-von-Laue-Str. 1,
Room 2.143
D-60438 Frankfurt am Main, Germany
Tel/Fax:  +49-69-79847871/47879
rezzolla@itp.uni-frankfurt.de