Capabilities

The code is designed for numerical relativity simulations of exotic compact objects, and in particular spherically symmetric boson stars (BSs). ExoZvezda is capable of evolving both single and binary spacetimes and contains a range of built-in diagnostic tools like the Apparent Horizon Finder, star tracking, flux calculations and many more.

The code features an in-built BS solver developed here that is able to compute single 1D spherically-symmetric BS solutions in isotropic gauge. This BS solution is then used and interpolated onto the 3D grid. The procedure is done once or twice, depending if one evolves a single BS or a binary system.

Capabilities of Evolving Binary Boson-Star Systems

• Equal-mass binaries. Our previous works studied many possible head-on and grazing collisions, see e.g. arXiv:2108.11995 and arXiv:2207.05690. The key ingredient in facilitating successful binary evolution in our code is a new initial data superposition for equal-mass binaries, which we call the equal-mass fix. Our code also allows for evolutions with plain superposition often used in the literature, but we advise against it, due to its inherent spurious features. Please refer to arXiv:2108.11995 to find out about why we use the equal-mass fix instead of plain superposition.
• Unequal-mass binaries. Our work on equal-mass binaries has also been extended in arXiv:2212.08023 to unequal-mass binaries, where we use a newly developed unequal-mass fix for initial data construction.
• High-precision quasi-circular inpirals, first presented in arXiv:2406.02715. Our repository includes one of the parameter files from the paper.
• Star tracking using non-linear least-squares fitting (i.e. you can track the position of the BSs through the evolution).
• Apparent Horizon Finder, also available in the main GRChombo code.
• Calculation of various fluxes as per arXiv:2203.13845.

Future Capabilities/Our wish-list

• Addition of Proca stars (vector cousins of boson stars).
• Addition of mixed binaries.
• Full elliptic initial condition solver for boson star binaries.