From inspiral to kilonova

Project Information

INSPIRATION

Grant agreement ID: 101053985

DOI

10.3030/101053985

EC signature date 14 July 2022

Start date 1 October 2022

End date 30 September 2028

Funded under

• European Research Council (ERC)

Total cost

€ 2 499 675,00

EU contribution

€ 2 499 675,00

Coordinated by

UNIVERSITY OF HAMBURG
Germany

Periodic Reporting for period 1 - INSPIRATION (From inspiral to kilonova)

Reporting period: 2022-10-01 to 2025-03-31

Summary of the context and overall objectives of the project

Recent years have seen the blossoming of what is called “multi-messenger astrophysics”. Here the aim is to observe different “messengers” from the same astrophysical event, for example photons, neutrinos and gravitational waves. Having the different stories that they tell at hand, is very efficient in ruling out incorrect interpretations of what has been observed. This approach is particularly promising when it comes to violent encounters between some of the Universe’s most enigmatic objects, namely neutron stars and black holes. The first observation of the merger of two neutron stars in 2017, both in gravitational and electromagnetic waves, has enabled huge leaps forward for science and was therefore celebrated as "2017 Breakthrough of the Year". Multi-messenger astrophysics has an enormous potential to solve many longstanding puzzles such as the origin of the heaviest elements in the cosmos, the source of the brightest cosmic explosions or the nature of the densest matter in the Universe. All of this promise, however, hinges on our understanding of which signals are produced by collisions of compact stellar objects and exactly this is the topic of the ERC Advanced Grant “INSPIRATION: from inspiral to kilonova”.

The INSPIRATION project aims at predicting within a coherent framework the various potentially observable signals that are emitted when two neutron stars collide with each other. Key to this coherent framework is the world-wide unique numerical relativity code SPHINCS_BSSN. This code solves the full set of Einstein’s General Relativity simulation on an adaptive mesh and can so dynamically evolve the curved space-time around two neutron stars. Differently from standard numerical relativity approaches, it solves the evolution of matter by means of freely moving particles. This has the advantage that one can accurately follow the matter that is ejected into space in the merger of two neutron stars. It is this matter that produces all the electromagnetic emission from a neutron star merger and in particular the phenomenon called “kilonova” whose emission is powered by the radioactive decay of freshly synthesized unstable heavy elements.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

• Major upgrade of SPHINCS_BSSN:
- We have developed a much more accurate mapping of particle properties (simulating matter) to our adaptive mesh (simulating space-time). Our new methodology (“Local Regression Estimate”, LRE) calculates mapping kernels by minimizing in a sophisticated way error functionals. Our LRE approach has substantially increased the accuracy of our code, which we will from throughout the project.
- We have further improved our particle-mesh initial data. Both of these topic are described in Rosswog et al., Frontiers in Applied Mathematics and Statistics, Vol. 9 (2023).
• Improving the accuracy of particle hydrodynamics. Standard Smoothed Particle Hydrodynamics (SPH) does not enforce some consistency relations that are desirable for high-accuracy approximations. I have developed a new particle hydrodynamics method in which these relations are fulfilled by construction. A corresponding method paper has just been submitted (Rosswog,2024). Soon to be adapted for our code SPHINCS_BSSN.
• Together with my former Postdoc Oleg Korobkin, I have summarized the current understanding of “Heavy elements and electromagnetic transients from neutron star mergers” in a recent review article.
• Together with INSPIRATION-funded Postdoc Jan-Erik Christian I have explored which first order phase transitions to quark matter are possible in neutron stars, see Christian et al. (2024).
• We have realized that a non-negligible fraction of neutron stars contain a single rapidly spinning neutron star together with a non-spinning companion. We have predicted different multi-messenger signatures of such mergers, see Rosswog et al. 2024a.
• Together with Nikhil Sarin, Stockholm University, I have explored the impact of the nuclear heating rate on the appearance of kilonovae and the inference of their parameters from a neutron star merger observation, see Sarin & Rosswog 2024.
• Via general relativistic magnetohydrodynamic simulations we have investigated magnetic field evolution in a neutron star merger and find that starting from realistic initial B-fields leads to a substantial delay in the jet formation, see Aguilera-Miret et al. (2024). A further paper on the magnetic field evolution in neutron star mergers is about to be submitted.
• A number of previous simulations had seen a small, but observationally very important fast ejecta component (v>0.5c) whose origin was unclear. Using SPHINCS_BSSN, we have identified two ejection mechanisms for this matter and we predicted its observational signatures, see Rosswog et al. 2024b.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

• I believe that our LRE mapping method (Rosswog et al.2023) is a major advance well beyond the current state of the art. It has, at the same resolution, made our simulations much more accurate and therefore has a major impact on the INSPIRATION project. The method itself is very general and can be used for a broad range of other applications, e.g. for the “hand shake” between simulations with different codes, or, for visualization purposes.
• The development of particle hydrodynamics methods that enforce consistency relations by construction to machine precision, is in my opinion a very crucial step forward and will be the future of astrophysical particle hydrodynamics. Many variants that will be explored, one of which is a general relativistic version that will enter in SPHINCS_BSSN.
• The final identification of the ejection mechanism of fast ejecta, see Rosswog et al. 2024a, is a major progress. This matter had been seen in simulations for more than a decade without a clear understanding how it is launched. We have identified two ejection mechanisms and made prediction for its electromagnetic emission. Currently new instruments are built (ULTRASAT satellite) to detect the electromagnetic emission from such fast ejecta. These signatures carry the imprint of the dense matter equation of state and of the binary parameters.

Merger between a rapidly spinning and a non-spinning neutron star, at contact

Merger between a rapidly spinning and a non-spinning neutron star, a few milliseconds later