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Abstract
When axion stars fly through an astrophysical magnetic background, the axion-to-photon conversion may generate a large electromagnetic radiation power. After including the interference effects of the spacially-extended axion-star source and the macroscopic medium effects, we estimate the radiation power when an axion star meets a neutron star. For a dense axion star with 10−13M⊙, the radiated power is at the order of 1011W×(100μeV/ma)4(B/1010Gauss)2 with ma as the axion particle mass and B the strength of the neutron star magnetic field. For axion stars occupy a large fraction of dark matter energy density, this encounter event with a transient O(0.1s) radio signal may happen in our galaxy with the averaged source distance of one kiloparsec. The predicted spectral flux density is at the order of μJy for a neutron star with B∼1013 Gauss. The existing Arecibo, GBT, JVLA and FAST and the ongoing SKA radio telescopes have excellent discovery potential of dense axion stars.
Highlights
The QCD axion, as a byproduct of a solution to the strong CP problem [1,2,3,4,5,6,7], can serve as a cold dark matter of the universe [8]
We have found that the power is not strong enough to explain the fast radio burst (FRB) [35], contrary to the claim in Refs. [36,37,38], where the above effects are not included
We have concentrated on the radiation power from a dense axion star
Summary
The QCD axion, as a byproduct of a solution to the strong CP problem [1,2,3,4,5,6,7], can serve as a cold dark matter of the universe [8]. We first estimate the radiated power from the homogeneous axion component interacting with the magnetic field in our galaxy/neutron star, which turns out to be too small to be detected by on-going and future radio telescope [32,33,34]. Afterwards, we estimate the radiation power of the axion star in the static magnetic field, 1010–1015 Gauss, associated with the neutron star, which can be potentially large enough to be detected in the radio telescopes. On the other hand and for a neutron star with a large magnetic field, the radiation power could be large enough to be detected by the on-going or future radio telescopes such as Arecibo [39], GBT [40], JVLA [41], FAST [42] and SKA [43].
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