Potential solar axion signatures in X-ray observations with the XMM-Newton observatory (1403.2436v2)

Abstract: The soft X-ray flux produced by solar axions in the Earth's magnetic field is evaluated in the context of ESA's XMM-Newton observatory. Recent calculations of the scattering of axion-conversion X-rays suggest that the sunward magnetosphere could be an observable source of 0.2-10 keV photons. For XMM-Newton, any conversion X-ray intensity will be seasonally modulated by virtue of the changing visibility of the sunward magnetic field region. A simple model of the geomagnetic field is combined with the ephemeris of XMM-Newton to predict the seasonal variation of the conversion X-ray intensity. This model is compared with stacked XMM-Newton blank sky datasets from which point sources have been systematically removed. Remarkably, a seasonally varying X-ray background signal is observed. The EPIC count rates are in the ratio of their X-ray grasps, indicating a non-instrumental, external photon origin, with significances of 11(pn), 4(MOS1) and 5(MOS2) sigma. After examining the constituent observations spatially, temporally and in terms of the cosmic X-ray background, we conclude that this variable signal is consistent with the conversion of solar axions in the Earth's magnetic field. The spectrum is consistent with a solar axion spectrum dominated by bremsstrahlung- and Compton-like processes, i.e. axion-electron coupling dominates over axion-photon coupling and the peak of the axion spectrum is below 1 keV. A value of 2.2e-22 /GeV is derived for the product of the axion-photon and axion-electron coupling constants, for an axion mass in the micro-eV range. Comparisons with limits derived from white dwarf cooling may not be applicable, as these refer to axions in the 0.01 eV range. Preliminary results are given of a search for axion-conversion X-ray lines, in particular the predicted features due to silicon, sulphur and iron in the solar core, and the 14.4 keV transition line from 57Fe.

• The paper models solar axion conversion to X-rays in the Earth's magnetic field and compares predictions to XMM-Newton blank-sky observations, identifying a statistically significant, seasonally variable X-ray signal.
• The observed variable X-ray spectrum is consistent with bremsstrahlung and Compton-like processes from axion-photon and axion-electron interactions and suggests a micro-eV range axion mass.
• These findings imply solar axions could contribute to the cosmic X-ray background and provide constraints for future high-sensitivity experiments seeking axionic dark matter.

Analysis of Solar Axion Signatures in X-Ray Observations with XMM-Newton

The paper "Potential Solar Axion Signatures in X-ray Observations with the XMM-Newton Observatory" postulates the existence of solar axions and examines their potential to produce observable X-ray flux in the Earth's magnetic field, utilizing the capabilities of the XMM-Newton observatory. The research investigates the expected X-ray signal from axions converted via their interaction with the Earth's magnetic field and compares this with the observational data from XMM-Newton, aiming to explore whether solar axions could contribute to the cosmic X-ray background (CXB).

Core Methodology and Findings

The authors employ a model combining the geomagnetic field with the ephemeris of XMM-Newton to predict the seasonal visibility of axion-converted X-rays. This model is validated against stacked datasets from XMM-Newton's observations of blank skies, with point sources meticulously removed. Notably, they identify a seasonally variable X-ray background, which they argue is consistent with axion-photon and axion-electron photonic interactions leading to X-ray production. This variability is significant, with statistical counts showing 11σ significance for the pn camera, 4σ for MOS1, and 5σ for MOS2, indicating a substantial external, non-instrumental source for these X-rays.

The derived X-ray spectrum suggests dominance by bremsstrahlung and Compton-like processes over the Primakoff effect, with these spectra peaking below 1 keV. Importantly, the paper indicates an axion mass in the micro-eV range, leading to a derived value of 2.2 x 10^-22 GeV for the product of the axion-photon and axion-electron coupling constants. The authors note that comparisons with other studies, such as those focusing on white dwarf cooling, are limited due to differences in axion masses involved.

Implications and Speculation on Future Research

The implication of axions as producers of observable X-ray flux, if substantiated, could provide insights into the nature of dark matter, specifically in the form of axionic dark matter candidates. The acknowledgment of axions potentially comprising components of the CXB could enhance our understanding of both astrophysical phenomena and the background radiation in the universe. Moreover, this investigation may propel further experimental initiatives focusing on high-sensitivity X-ray observations, particularly those combining other observational modalities like gamma-ray telescopes or polarimetric studies.

Given these findings, further exploration could focus on refining the models of geomagnetic axion conversion, considering other celestial observatories such as Chandra, with its contrasting effective area and field of view, or X-ray polarimetry instruments like the proposed XIPE mission. Additionally, examining the X-ray spectrum for line features from other elements potentially impacted by axions could provide more definitive evidence and insights into the axion's role and properties.

Conclusion

This research contributes to the ongoing investigation into axions as viable candidates for dark matter. By proposing a feasibility model that axions generated in the solar core could contribute to the cosmic X-ray background, the paper drives the conversation forward in axion research. As observational techniques evolve and datasets expand, the methodology and findings outlined here will serve as a pivotal reference point for future studies in the experimental pursuit of dark matter. The seasonal variation and specific spectral characteristics provide critical constraints and hypotheses that are ripe for further investigation and validation.