Potential solar axion signatures in X-ray observations with the XMM-Newton observatory

The soft X-ray flux produced by solar axions in the Earth's magnetic field is evaluated in the context of the European Space Agency'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 European Photon Imaging Camera count rates are in the ratio of their X-ray grasps, indicating a non-instrumental, external photon origin, with significances of 11σ (pn), 4σ (MOS1) and 5σ (MOS2). After examining the distribution of the constituent observations spatially, temporally and in terms of the accepted representation 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, assuming the resultant photons are not strictly forward-directed, and enter the field of view of XMM–Newton. The spectrum is consistent with a solar axion spectrum dominated by bremsstrahlung- and Compton-like processes, distinct from a Primakoff spectrum, i.e. axion–electron coupling dominates over axion–photon coupling and the peak of the axion spectrum is below 1 keV. A value of 2.2 × 10−22 GeV−1 is derived for the product of the axion–photon and axion–electron coupling constants, for an axion mass in the μeV range. Comparisons, e.g., 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 narrow features due to silicon, sulphur and iron in the solar core, and the 14.4 keV transition line from 57Fe.