Head-on collisions of $\ell$-boson stars

Abstract

Fully non-linear numerical evolutions of the Einstein-(multi)-Klein-Gordon equations are performed to study head-on collisions of -boson stars. Despite being spherically symmetric, -boson stars have a (hidden) frame of reference, used in defining their individual multipolar fields. To assess the impact of their relative orientation, we perform simulations with different angles between the axes of the two colliding stars. Additionally, two scenarios are considered for the colliding stars: that they are composites of either the same or different scalar fields. Despite some model-specific behaviours, the simulations generically indicate that: 1) the collision of two sufficiently (and equally) massive stars leads to black hole formation; 2) below a certain mass threshold the end result of the evolution is a bound state of the composite scalar fields, that neither disperses nor collapses into a black hole within the simulation time; 3) this end product (generically) deviates from spherical symmetry and the equipartition of the number of bosonic particles between the different scalar fields composing the initial boson stars is lost, albeit not dramatically. This last observation indicates that the end result of these collisions belongs to the previously reported larger family of equilibrium multi-field boson stars, generically non-spherical, and of which -boson stars are a symmetry enhanced point. We also extract and discuss the waveforms from the collisions studied.

The piercing of a boson star by a black hole

“…This means that these initial data do not satisfy the constraint equations ( 20), (21). While not ideal, the constraint violation incurred is small for large initial distances, and such a construction has been standard practice in studies of BS binaries [61][62][63].…”

The gravitational afterglow of boson stars

“…Their potentially high compactness implies that mergers can generate GWs detectable with present GW observatories. Most present work in the literature on BSs focusses on the GW signatures generated during the pre-merger infall or inspiral [69][70][71][72][73] and during the merger phase itself [21,28,[74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89]; these are, of course, the regimes of most notable interest in the GW observation of neutron-star and black-hole binary coalescences. The main focus of our work, however, is the long-lived post-merger GW emission or afterglow resulting from the merger of two BSs into a single compact but horizon-free remnant; for first explorations of BS coalescences including the relaxation into a non-rotating BS or a hairy BH see [75,79,82].…”

Proca-Higgs balls and stars in a UV completion for Proca self-interactions

“…A key aspect of boson stars is that they are dynamically robust, for some models and in parts of the parameter space [55][56][57][58][59][60][61][62][63][64][65][66], allowing dynamical strong gravity studies, such as collisions and mergers of individual stars, to test their stability and obtain to gravitational wave templates -see e.g. [67][68][69][70][71][72][73][74][75][76]. Scalar boson stars have flat spacetime counterparts if appropriate scalar self-interactions are included in the model; the corresponding flat spacetime solitons go by the name of Q-balls [77].…”