◉ According to Iwazaki and colleagues, FRBs arise from the collisions between axion stars and neutron stars.
◉ Axions are one of most promising candidates of dark matter. A prominent feature of axions is that they are converted to radiations under strong magnetic fields. The axions form axion stars known as oscillaton made of axions bounded gravitationally.
◉ According to them, when the axion stars collide with neutron stars having strong magnetic fields, the electric fields are generated, which make electrons in atmospheres of neutron stars coherently oscillate. Thus, the electrons emit coherent radiations with the frequency given by the axion mass. Since the electrons are much dense in the atmospheres, the large amount of radiations with the frequency ma/2π ≃ 2.4 GHz can be produced in the collisions. The total amount of the energy of the radiations is given by 10-¹²M (10km/10²km)² ∼10⁴³GeV, where the radii of the neutron stars and the axion stars are supposed to be 10km and 10²km, respectively. This is their production mechanism of FRBs.
◉ They also proposed that such radiations are also produced by the collision of white dwarfs and axion stars. But, have wider bandwidths to that of neutron stars. These features can be observable only if the white dwarfs have very strong magnetic fields ≥ 10^9G.
Fast Radio Bursts have recently been discovered at around 1.4 GHz frequency. The durations of the bursts are typically a few milliseconds. The origin of the bursts has been suggested to be extra-galactic owing to their large dispersion measures. This suggests that the large amount of the energies ∼ 10⁴³GeV/s is produced at the radio frequencies. The event rate of the burst is estimated to be ∼ 10-³per year in a galaxy. Furthermore, no gamma or X ray bursts associated with the bursts have been detected. Follow up observations of FRBs do not find any signals from the direction of the FRB. To find progenitors of the bursts, several models have been proposed. They ascribe FRBs to traditional sources such as neutron star-neutron star mergers, magnetors, black holes, etc..
Now, Iwazaki and colleagues model ascribes FRBs to axions, which are one of most promising candidates of dark matter. A prominent feature of axions is that they are converted to radiations under strong magnetic fields. The axions form axion stars known as oscillaton made of axions bounded gravitationally. The axion stars are condensed objects of axion miniclusters, which have been shown to be produced after the QCD phase transition and to form the dominant component of dark matter in the Universe. Furthermore, the axion miniclusters have been shown to form the axion stars by gravitationally losing their kinetic energies. Thus, the axion stars are the dominant component of dark matter. Iwazaki and colleagues in their recent work, proposed a progenitor of the FRBs that the FRBs arise from the collisions between the axion stars and neutron stars.
In their paper, they presented the details of their models. First of all, they derived the solutions of the axion stars by expanding the axion field and the gravitational fields in terms of eigen modes cos(nωt) with n integers, where the eigen value ω can be determined by solving axion field equation representing gravitationally bounded axions. It is given such that ω = ma – k²/2ma where k²/2ma( ≪ ma ) denotes a gravitational binding energy of an axion bounded by the axion star; k ≃ G×Ma×(m_a)², where G and Ma are the gravitational constant and the mass of the axion star, respectively. Then, they found that the mass Ma of the axion star is given in terms of the radius Ra = 1/k of the axion stars such that Ma = 1/(G × (ma)² × Ra). They obtained the value of the mass Ma by the comparison of the theoretical and observational even rate of FRBs in their production mechanism of the FRBs. They have found that the masses and radii of the axion stars are given by Ma ∼ 10-¹²M and Ra ∼ 10²km, respectively.
They also showed that the oscillating electric fields are generated on the axion stars under external magnetic fields. Thus, when the axion stars collide with neutron stars with strong magnetic fields, the electric fields are generated, which make electrons in atmospheres of neutron stars coherently oscillate. Thus, the electrons emit coherent radiations with the frequency given by the axion mass. Since the electrons are much dense in the atmospheres, the large amount of radiations with the frequency ma/2π ≃ 2.4 GHz can be produced in the collisions. The total amount of the energy of the radiations is given by 10-¹²M (10km/10²km)² ∼10⁴³GeV, where the radii of the neutron stars and the axion stars are supposed to be 10km and 10²km, respectively. This is their production mechanism of FRBs.
According to the mechanism, we can explain naturally the durations ( ∼ ms ) and amount of the energies ( 10^ 40erg ) of the bursts.— said Iwazaki
They also discussed the optical properties of the atmospheres of neutron stars. The geometrical depth of the hydrogen atmospheres of old neutron stars is of the order of 0.1cm. The radiations emitted in the atmospheres can pass through them because they are shown to be optically thin for the radiations with the frequency (ma/2π) ≃ 2.4 GHz. They can also pass through magnetospheres of neutron stars. After they pass the magnetospheres, the radiations are circularly polarized owing to the absorption of the radiations with either right or left handed polarization. The circular polarization of a FRB has recently been observed.
Although the radiations produced by their mechanism are monochromatic having the frequency given by ω =ma/2π, the observed radiations have bandwidths at least wider than the range 1.2GHz∼ 1.6GHz used by actual observations. They showed that the bandwidths ωth ( ω ± ωth ) of FRBs arise from thermal fluctuations of electrons. Thus, the bandwidths are narrow. The presence of such narrow bandwidths owing to the thermal fluctuations is a distinctive feature of their model and can be tested observationally.
We have shown that the bandwidths are caused by the thermal fluctuations of electrons emitting the radiations— said Iwazaki
While, in the actual collisions, the tidal forces of the neutron stars distort the formation of the axion stars. When the axion stars are close to the neutron stars, the gravitational forces of the neutron stars are stronger than those of axion stars binding themselves. Then, the axions freely fall to the neutron stars. But the coherence of the axions is kept because the number density of the axions in the volume (ma)-³ is quite large.
But guys, there are many studies which claimed that FRBs could be produced by the collision of axion stars and white dwarfs. For such cases, Iwazaki and colleagues found that the duration of the bursts is of the order of 0.1second and that the radiations have wider bandwidths than those of the radiations from neutron stars. These features can be observable only if the white dwarfs have very strong magnetic fields ≥ 10^9G. Although the number of such white dwarfs in a galaxy is still unknown, the production rate of the bursts is sufficiently large for them to be detectable if their number is larger than 10^6 in a galaxy.
The number of the white dwarfs with strong magnetic fields ≥ 109G in a galaxy is not known and the estimation of the number is difficult. Although we know the presence of such white dwarfs, the number of them could be very few. Thus, the event rate of the FRBs associated with the white dwarfs could be much small so that the FRBs are undetectable.— said Iwazaki
If our production mechanism of FRBs is true, we can reach a significant conclusion that the axions are the dominant component of dark matter and their mass is about 10^-5 eV, which is in the window allowed by observational and cosmological constraints.— said Prof. J. Arafune
Reference: Aiichi Iwazaki, J. Arafune, “Fast Radio Bursts from Axion Stars”, ArXiv, pp. 1-13, 2015. https://arxiv.org/abs/1412.7825
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