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

Abstract

We apply novel, recently developed plasma ray-tracing techniques to model the propagation of radio photons produced by axion dark matter in neutron star magnetospheres and combine this with both archival and new data for the galactic centre magnetar PSR J1745-2900. The emission direction to the observer and the magnetic orientation are not constrained for this object leading to parametric uncertainty. Our analysis reveals that ray-tracing greatly reduces the signal sensitivity to this uncertainty, contrary to previous calculations where there was no emission at all in some directions. Based on a Goldreich-Julian model for the magnetosphere and a Navarro-Frank-White model for axion density in the galactic centre, we obtain the most robust limits on the axion-photon coupling, to date. These are comparable to those from the CAST solar axion experiment in the mass range ∼ 4.2 − 60 µeV. If the dark matter density is larger, as might predicted by a "spike" model, the limits could be much stronger. The dark matter density in the region of the galactic centre is now the biggest uncertainty in these calculations.

Extraterrestrial Axion Search with the Breakthrough Listen Galactic Center Survey

“…These constraints seem to suggest that the GC magnetar allows for the resonant conversion of axions with masses O(150)µeV, however the maximum plasma frequency achievable in this model is closer to ∼ 64µeV. This error appears to have only been corrected in the latest result [33].…”

“…For reference, we also recast the VLA bound on the GC magnetar (black line, left), using our forward modeling which includes both the effect of axion-photon de-phasing as well as the adiabatic correction to the conversion probability. The flux limit for the VLA result was taken from [28] (and has been applied more recently in [33]), and the analysis was performed using the quoted bandwidth, given by ∆f = 8.3MHz × m a 4.1µeV (S38)…”

“…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.…”

“…A search for axions from the GC NS population was previously performed using the Effelsberg 100-m telescope [27] in the L-band (m a ∈ (5.2, 6.0) µeV) and S-band (m a ∈ (9.8, 11.0) µeV); relative to [27], our present search covers a broader mass range (m a ∈ (15,35) µeV), makes use of more exposure time (∼280 min as opposed to ∼80 min in [27]), uses improved NS population models, and incorporates state-ofthe-art simulations for the axion-photon conversion process at the level of the individual NS [30]. Observations from the Very Large Array (VLA) of the GC magnetar SGR J1745-2900 have also been interpreted in the context of axion-photon conversion [28,29,33] -a search first suggested in [23]. Our present search includes the GC magnetar within the field of view and has a stronger radio flux sensitivity, thus offering a notable improvement over the VLA analysis in the mass range studied.…”

“…It is crucial to work beyond leading order in the axionphoton conversion probability, which has not been done in previous radio searches (e.g., [27][28][29]33]). Those analyses naively applied the leading-order perturbative re-sults for the conversion probabilities in the NS magnetospheres such that at their upper limit values for g aγγ the conversion probabilities evaluate to well beyond unity.…”