Updated bounds on axion-like particles from X-ray observations

Abstract

In this work we revisit five different point sources within or behind galaxy clusters in order to constrain the coupling constant between axion-like particles (ALPs) and photons. We use three distinct machine learning (ML) techniques and compare our results with a standard χ2 analysis. For the first time we apply approximate Bayesian computation to searches for ALPs and find consistently good performance across ML classifiers. Further, we apply more realistic 3D magnetic field simulations of galaxy clusters and compare our results with previously used 1D simulations. We find constraints on the ALP-photon coupling at the level of state-of-the-art bounds with gaγγ ≲ 0.6 × 10−12 GeV−1, hence improving on previous constraints obtained from the same observations.

TASI Lectures on the Particle Physics and Astrophysics of Dark Matter

“…The intensity modulation is difficult to observe unless there are reasons that the change in intensity varies rapidly with frequency. Such spectral modulations are indeed expected from a variety of extragalactic 𝑋-ray sources (see, e.g., [201][202][203][204] and references therein).…”

“…This is not a realistic picture of what 𝑃 𝛾→𝛾 may look like, since it approximates the cluster as a single domain, but it illustrates the oscillatory features that arise due to the sin term in (114). With multiple domains, the oscillations at keV frequencies can become even more frequent (see, e.g., [201][202][203][204]). The axion-photon coupling 𝑔 𝑎𝛾𝛾 controls the strength of the oscillations.…”

“…The intracluster medium is often modeled as a series of disconnected domains. For example, for the Coma cluster the domain sizes are estimated as 𝐿 ∼ 10 kpc (the cluster size is around a Mpc), the free electron density is 𝑛 𝑒 ∼ 10 −3 cm −3 , and the characteristic magnetic field strengths in the domains are estimated as 𝐵 0 ∼ 5 𝜇G (see [204] for more precise values). The magnetic field changes direction and orientation by an amount O (1) from domain to domain, which suppresses the conversion probability relative to what one would assume taking a single domain of the same total length.…”

“…The issue, though, is that we can only make statistical predictions for 𝑃 𝛾→𝛾 (𝜔, 𝑔 𝑎𝛾𝛾 ) because of the random domains; a given realization will have a unique modulation pattern. See [201][202][203][204] for methods that have been developed to deal with this complication, with the most up-to-date limits constraining 𝑔 𝑎𝛾𝛾 few × 10 −13 GeV −1 for 𝑚 𝑎 10 −12 eV, though these constraints are subject to systematic uncertainties on e.g. magnetic field assumptions.…”

Physics Beyond the Standard Model with Future X-Ray Observatories: Projected Constraints on Very-light Axion-like Particles with Athena and AXIS

“…This will be the subject of a future publication. We highlight that training the CNN on residuals from fitting synthetic data-rather than on spectra-could also help improve the CNN's performance (see Schallmoser et al 2021).…”