Detection of Excess Hard Emission Associated with Shocks and Radio Structures in the Heart of the Virgo Cluster with NuSTAR

NuSTAR’s imaging capabilities have been underutilized in the analysis of diffuse, extended objects such as galaxy clusters due to the relatively large PSF — especially its wings — which for bright point sources can scatter photons across the field of view The development of the publicly available nucrossarf code seeks to remedy this situation, allowing cross-contamination from one region to another to be accurately modeled. We apply this technique to archival observations of the center of the Virgo cluster, originally targeted to study the AGN at the heart of M87. Global spectra reveal extended, hard excess emission above the coincident thermal and scattered AGN photons, well in excess of the 2.5% systematic uncertainty in the reconstruction of the wings of the PSF. Combined images above 10 keV reveal a slightly different morphology of the extended low energy X-ray emission, more consistent with the morphology of the extended halo in the radio. Features in the hard band images correspond to the locations of shocks seen with Chandra and regions with the flattest radio spectral index seen with LOFAR, consistent with inverse Compton scattering by shock-accelerated relativistic electrons that produce the radio emission. Accounting for point-like emission from the AGN, we characterize these features in detail as well as the overall temperature structure of the ICM. We estimate the average magnetic field strength in this region, assuming the hard excess is the result of inverse Compton emission from the diffuse radio halo, and discuss the implications for future inverse Compton searches in lower mass systems as well as detailed, spatially-resolved ICM studies with NuSTAR in general.