Collisions of dark matter axion stars with astrophysical sources

Abstract

If QCD axions form a large fraction of the total mass of dark matter, then axion stars could be very abundant in galaxies. As a result, collisions with each other, and with other astrophysical bodies, can occur. We calculate the rate and analyze the consequences of three classes of collisions, those occurring between a dilute axion star and: another dilute axion star, an ordinary star, or a neutron star. In all cases we attempt to quantify the most important astrophysical uncertainties; we also pay particular attention to scenarios in which collisions lead to collapse of otherwise stable axion stars, and possible subsequent decay through number changing interactions. Collisions between two axion stars can occur with a high total rate, but the low relative velocity required for collapse to occur leads to a very low total rate of collapses. On the other hand, collisions between an axion star and an ordinary star have a large rate, Γ ∼ 3000 collisions/year/galaxy, and for sufficiently heavy axion stars, it is plausible that most or all such collisions lead to collapse. We identify in this case a parameter space which has a stable region and a region in which collision triggers collapse, which depend on the axion number (N ) in the axion star, and a ratio of mass to radius cubed characterizing the ordinary star (M s /R 3 s ). Finally, we revisit the calculation of collision rates between axion stars and neutron stars, improving on previous estimates by taking cylindrical symmetry of the neutron star distribution into account. Collapse and subsequent decay through collision processes, if occurring with a significant rate, can affect dark matter phenomenology and the axion star mass distribution.

Early seeds of axion miniclusters

“…Most discussed lately is the issue of dilute axion-stars [20]-gravitationally bound solitons [31][32][33][34][35][36] of the axion field which can easily appear in this scenario (for instance in the cores of miniclusters [37,38], but not only). More compact objects can lead to more pronounced [39][40][41] and even coherent effects, see [42]. Recently, a new branch of "dense" axion stars (energy density ∼ V QCD ) was suggested [43], with even more spectacular consequences [44,45].…”

“…where | Φ(k)| are picked from an exponential distribution 41) and the average value is exponentially suppressed above a certain critical momentum k cr b = exp(−k 2 /k 2 cr ) .…”

Global view of QCD axion stars

“…As such, their phenomenological effects can be searched for in the dark matter halo. Searches for effects of dilute axion stars include: collisions with neutron stars giving rise to high-intensity ra-dio photon emission [42,43]; microlensing [44]; transient effects from rare encounters of an axion star with Earth [22,45]; or possible capture in the solar system leading to high-density subhalos [46]. This field continues to attract increasing interest and new ideas for how to probe dilute axion stars in the halo.…”

“…A recent surge in studies of boson stars [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] stems, in part, from the renewed interest in determining whether dark matter (DM) could consist of condensates of ax-ions or other axion like particles. A particularly wellmotivated scalar DM candidate is the QCD axion, parametrized by a decay constant f = 6 × 10 11 GeV and particle mass m = 10 −5 eV; 2 as a result, bound states of QCD axions (which we will call QCD axion stars) have received special attention.…”

The fate of dense scalar stars

“…In some scenarios, long-lived (pseudo-)solitons can survive till the current time, either providing a natural candidate for dark matter [25][26][27][28] or a source of distinct signatures if a fraction of the dark matter is in the form of compact objects, see e.g. [29][30][31][32][33][34].…”