Radio line properties of axion dark matter conversion in neutron stars

Abstract

Axions are well-motivated candidates for dark matter. Recently, much interest has focused on the detection of photons produced by the resonant conversion of axion dark matter in neutron star magnetospheres. Various groups have begun to obtain radio data to search for the signal, however, more work is needed to obtain a robust theory prediction for the corresponding radio lines. In this work we derive detailed properties for the signal, obtaining both the line shape and time-dependence. The principal physical effects are from refraction in the plasma as well as from gravitation which together lead to substantial lensing which varies over the pulse period. The time-dependence from the co-rotation of the plasma with the pulsar distorts the frequencies leading to a Doppler broadened signal whose width varies in time. For our predictions, we trace curvilinear rays to the line of sight using the full set of equations from Hamiltonian optics for a dispersive medium in curved spacetime. Thus, for the first time, we describe the detailed shape of the line signal as well as its time dependence, which is more pronounced compared to earlier results. Our prediction of the features of the signal will be essential for this kind of dark matter search.

Astroparticle Physics with Compact Objects

“…Renewed interest in this method of axion searches led to fast progress in our understanding of the details of the process [86,[109][110][111][112][113][114]. The conversion process is slightly different for non-relativistic axions: governing linearized equations are altered [86], and conversion occurs in a resonant mode, in a stark contrast to conversion of 'hot' axions, as discussed above.…”

“…The conversion process is slightly different for non-relativistic axions: governing linearized equations are altered [86], and conversion occurs in a resonant mode, in a stark contrast to conversion of 'hot' axions, as discussed above. In addition, plasma effects that influence propagation of EM waves are much more important for the conversion of non-relativistic axions [113,114].…”

Astroparticle physics with compact objects

“…Renewed interest to this method of axion searches lead to fast progress in our understanding of the details of the process [86,[109][110][111][112][113][114]. Conversion process is slightly different for non-relativistic axions: governing linearized equations are altered [86], and conversion occurs in a resonant mode, in a stark contrast to conversion of 'hot' axions, as discussed above.…”

Extraterrestrial Axion Search with the Breakthrough Listen Galactic Center Survey

“…Axion masses spanning from 10 − 100 µeV constitute a particularly compelling range of parameter space, as the DM abundance is arguably achieved most naturally for these candidates [8][9][10][11][12][13][14]. Recent work has shown that QCD axions may efficiently convert into photons in the magnetospheres of neutron stars (NSs), generating powerful spectral lines that may be observable using near-future radio telescopes [21][22][23][24][25][26][27][28][29][30][31][32][33]. Axion-like particles (ALPs), arising ubiquitously in String Theory from the compactification of extra dimensions [34,35] and having a comparable phenomenology to the QCD axion, represent a compelling alternative DM candidate with the potential to be observed by radio telescopes today.…”

“…The axion-photon conversion process in realistic NS magnetospheres (including photon refraction, photon absorption, plasma broadening, the anisotropic response of the medium, General Relativistic effects, etc.) has been described and simulated with increasing complexity in recent years [24,26,[30][31][32]. The signal appears as a narrow radio line at the frequency corresponding to the axion mass.…”