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 timedependence 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.

Diluted Axion Star Collisions with Neutron Stars

“…For instance, Ref. [26] has pointed out that the Doppler shift effects dominate the signal frequency broadening over the one from dark matter velocity dispersion. For the diluted axion star considered here, the axion particles in the initial axion star state are in a coherent BEC state.…”

“…The resonant conversions of the axion particles for free axion particles and axion particles inside axion mini-clusters are discussed in Refs. [22][23][24][25][26] and [27,28], respectively.…”

Scanning the landscape of axion dark matter detectors: applying gradient descent to experimental design

“…Axions [1][2][3][4][5] represent one of the leading candidates for dark matter [6][7][8] and are predicted by a range of theories beyond the standard model [9,10]. One possibility to detect axions is via their astrophysical signatures, for instance via galactic halo axions decaying into two photons [11][12][13][14][15][16][17][18][19][20][21][22] or through observations of neutron stars [14,[23][24][25][26][27][28][29][30][31][32]. It is also of vital importance to complement these efforts through laboratory searches and there are a number of current and proposed experiments aiming to detect axion dark matter here on earth.…”

Towards Robust Constraints on Axion Dark Matter using PSR J1745-2900

“…Recent work [18,19] has shown that this assumption is not sufficiently accurate to predict the signal due to the effects of the plasma and gravity which modify the ray tracing techniques first explored for straight line trajectories in [13]. These techniques allow calculation of the angular, frequency and time-dependence of the signal resulting from the propagation of photons through the magnetosphere.…”