X-ray Searches for Axions from Super Star Clusters (2008.03305v2)

Abstract: Axions may be produced in abundance inside stellar cores and then convert into observable X-rays in the Galactic magnetic fields. We focus on the Quintuplet and Westerlund 1 super star clusters, which host large numbers of hot, young stars including Wolf-Rayet stars; these stars produce axions efficiently through the axion-photon coupling. We use Galactic magnetic field models to calculate the expected X-ray flux locally from axions emitted from these clusters. We then combine the axion model predictions with archival Nuclear Spectroscopic Telescope Array (NuSTAR) data from 10 - 80 keV to search for evidence of axions. We find no significant evidence for axions and constrain the axion-photon coupling $g_{a\gamma\gamma} \lesssim 3.6 \times 10^{-12}$ GeV$^{-1}$ for masses $m_a \lesssim 5 \times 10^{-11}$ eV at 95\% confidence.

X-ray Searches for Axions from Super Star Clusters

The paper presents a detailed paper aimed at investigating the possible presence of axions, a class of theoretical particles that provide extensions to the Standard Model, by examining X-ray emissions from super star clusters (SSCs). Axions, and axion-like particles, have been hypothesized to solve the strong CP problem in quantum chromodynamics (QCD) and might be produced inside stellar cores. This paper specifically focuses on their production in two large star clusters: the Quintuplet and Westerlund 1, both of which contain numerous high-mass stars such as Wolf-Rayet stars.

Stellar Axion Production and Galactic Conversion

The authors model axion production within SSCs through the Primakoff process, which is facilitated by the axion-photon coupling, denoted as $g_{a\gamma\gamma}$. Throughout the analysis, numerical simulations were performed using the stellar evolution code, MESA, for various initial conditions including metallicity and rotational velocity, as outlined in the paper. The paper indicates efficient axion production, particularly from Wolf-Rayet stars due to their high mass and temperatures.

Once produced, axions could travel through the Galaxy and encounter the Galactic magnetic field where they might be converted into X-ray photons. The conversion probability varies with different components of the magnetic field, including localized filament structures like the Galactic Center radio arc. Models of the magnetic fields and electron density profiles were employed to compute the expected X-ray flux from this conversion, incorporating the JF12 and PTKN11 field models.

X-ray Data Analysis

The authors conducted an extensive analysis of archival NuSTAR data, seeking signatures of axions converting to X-rays. Data reduction was meticulously conducted, ensuring calibration for each observation. Observations from the Quintuplet cluster and Westerlund 1 were analyzed independently and collectively, while also controlling for potential contamination sources such as ghost-ray contamination in Westerlund 1 and molecular cloud emission near the Arches cluster.

Through template fitting and energy bin analyses, no significant evidence of axion signals was detected across the examined energies. The paper established an upper bound on the axion-photon coupling, constraining $g_{a\gamma\gamma} \lesssim 3.6 \times 10^{-12}$ GeV$^{-1}$ for low axion masses, based on combined data from Quintuplet and Westerlund 1.

Implications and Future Directions

The constraints derived from this paper are among the most stringent limits on low-mass axions from astrophysical sources and demonstrate the potential of using stellar environments as laboratories for particle physics research. Unlike terrestrial experiments such as the CAST experiment, which observe axions produced in the solar plasma, this research taps into stellar phenomena as sources of potential axion emission.

Further exploration of magnetic field configurations could refine these astrophysical limits. Understanding localized magnetic enhancements in the inner Galaxy could potentially lead to stronger constraints. Additionally, other promising targets for future analysis, such as nearby supergiant stars like Betelgeuse or young neutron stars like Cassiopeia A, might offer new insights.

This paper contributes meaningfully to the ongoing development of indirect detection strategies for axions, offering a robust framework for integrating astrophysical observations with theoretical particle physics models. Its results underscore the intricate interplay between celestial dynamics and fundamental physics, maintaining the pursuit for understanding such elusive particles.