Searching for Evidence for the Merger-Minihalo Connection in the Ophiuchus Cluster

Galaxy clusters are the largest gravitationally bound systems in the observable universe. Measurements of their masses are often used to determine cosmological parameters, which in turn tell us how the universe has evolved through time. However, methods for measuring these masses are very sensitive to disturbances in the intracluster medium (ICM), so clusters with relaxed "cool cores" (CCs) are preferred, as that signals that the cluster has not been disrupted by merger activity recently. However, these CCs tend to be accompanied by radio structures known as minihalos with no apparent source. One theory for the formation of these minihalos is that turbulence induced by a merging subcluster accelerates an existing population of relativistic electrons, producing synchrotron emission at radio wavelengths. Previous observations of the Ophiuchus Cluster have detected a radio minihalo near its CC, and a recent observation of the cluster by Werner et al. (2016) revealed a discontinuity in the surface brightness that they attributed to a merger. However, their observation was taken using the Chandra X-ray Observatory, whose energy bands are sensitive to absorption from our own Galaxy, making it difficult to measure temperature variations that are the telltale sign of merger activity. This study, therefore, used the Nuclear Spectroscopic Telescope Array (NuSTAR), which operates at higher X-ray energies unaffected by absorption, to search for evidence of a merger, which would support there being a connection between radio minihalos and minor merger activity. Spectra from regions around and inside the discontinuity detected by Chandra were fit to a standard emission spectrum to determine the temperature distribution of the region. We find strong evidence of hotter gas in front of the potential merging subcluster, consistent with a shock front being driven through the ICM. We will also present a combined analysis of these data with data from XMM-Newton to help disentangle effects of stray light that contaminate parts of the NuSTAR observation.