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The search for diffuse non-thermal, inverse Compton (IC) emission from galaxyclusters at hard X-ray energies has been underway for many years, with mostdetections being either of low significance or controversial. In this work, weinvestigate 14-195 keV spectra from the Swift Burst Alert Telescope (BAT)all-sky survey for evidence of non-thermal excess emission above theexponentially decreasing tail of thermal emission in the flux-limited HIFLUGCSsample. To account for the thermal contribution at BAT energies, XMM-NewtonEPIC spectra are extracted from coincident spatial regions so that both thermaland non-thermal spectral components can be determined simultaneously. We findmarginally significant IC components in six clusters, though after closerinspection and consideration of systematic errors we are unable to claim aclear detection in any of them. The spectra of all clusters are also summed toenhance a cumulative non-thermal signal not quite detectable in individualclusters. After constructing a model based on single-temperature fits to theXMM-Newton data alone, we see no significant excess emission above thatpredicted by the thermal model determined at soft energies. This result alsoholds for the summed spectra of various subgroups, except for the subsample ofclusters with diffuse radio emission. For clusters hosting a diffuse radiohalo, a relic, or a mini-halo, non-thermal emission is initially detected atthe \sim5-sigma confidence level - driven by clusters with mini-halos - butmodeling and systematic uncertainties ultimately degrade this significance. Inindividual clusters, the non-thermal pressure of relativistic electrons islimited to \sim10% of the thermal electron pressure, with stricter limits forthe more massive clusters, indicating that these electrons are likely notdynamically important in the central regions of clusters.

The Swift BAT Perspective on Non-thermal Emission in HIFLUGCS Galaxy Clusters