Abstract
A fraction of light scalar dark matter, especially axions, may organize into Bose-Einstein condensates, gravitationally bound clumps, "boson stars", and be present in large number in galactic halos today. We compute the expected number of gravitational microlensing events of clumps composed of the ordinary QCD axion and axion-like-particles and derive microlensing constraints from the EROS-2 survey and the Subaru Hyper Suprime-Cam observation. We perform a detailed lensing calculation, including the finite lens and source size effects in our analysis. We constrain the axion mass in terms of the fraction of dark matter collapsed into clumps, the individual clump densities, and the axion self-coupling. We also consider and constrain clumps composed of a generic scalar dark matter candidate with repulsive self-interactions. Our analysis opens up a new window for the potential discovery of dark matter.
Highlights
Several astrophysical observations, such as galactic rotation curves, cosmic microwave background, and large scale structure, are well explained by cold dark matter [1]
The axion motivated by the solution to the strong CP problem [2,3,4] of quantum chromodynamics (QCD), and axionlike particles whose existence are predicted in string theory [5], are prominent cold dark matter candidates
We found that the finite lens size effect can be parametrized by one parameter wE, which is defined by the ratio of the pointlike Einstein ring radius to the typical size of a clump
Summary
Several astrophysical observations, such as galactic rotation curves, cosmic microwave background, and large scale structure, are well explained by cold dark matter [1]. The relaxation rate is given by [40] Γkin ∼ nφσgrvφN , where σgr is the gravitational scattering cross section [while the contribution from self-interactions arises from the replacement σgr → σsi (the cross section of the self-interactions), which is normally negligible], N is the occupancy number associated with the Bose enhancement, nφ is the axion number density, and vφ is the typical speed of axions in minihalos In both scenarios, there is a constraint on the PQ symmetry breaking scale Fa. In the postinflationary scenario, the decay of topological defects critically affects the axion abundance leading to the so-called domain wall problem. In the Appendix, we derive the lens equation including the finite lens size effect
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