NuSTAR has recently spent more than two days observing one of the most significant galaxy cluster interactions in the local Universe – the early stage merger of the massive systems Abell 401 and Abell 399. This merger is one of the most energetic events in the Universe, and has profound effects on the Intra-Cluster Medium (ICM); the hot, diffuse, gas that fills the volume between cluster member galaxies, which emits strongly in soft X-rays, and is responsible for a significant percentage of the thermal pressure support that balances a cluster’s gravitational collapse. The ICM of each cluster is undergoing shocks caused by the interaction, becoming even hotter (increasing its thermal pressure), and producing hard X-ray photons that NuSTAR is sensitive to. Without NuSTAR observations, temperature measurements will be biased toward lower temperatures. This shock heating will preferentially occur at the interface between the galaxy clusters, and the high-energy spatial resolution of NuSTAR images is indispensable to being able to make multiple temperature measurements in different spatial regions around the clusters. These NuSTAR observations are also being used to searching for another, more elusive, source of hard X-ray emission from galaxy clusters – Cosmic Microwave Background photons (the leftover microwave radiation from the Big Bang) that have been inverse-compton scattered to X-ray energies by populations of cosmic rays in the ICM. It is extremely hard to identify this emission, but in combination with radio observations, they can both provide information about the cluster magnetic field strength, and the contributions of the magnetic field and the cosmic ray population to the pressure support of a galaxy cluster. In an era where it is now possible to constrain the other known source of ICM pressure (turbulence, using observations from the recently launched JAXA/NASA/ESA mission XRISM), NuSTAR will help build a full picture of the ongoing astrophysical processes in galaxy clusters.

Authors: David Turner (Research Associate, Michigan State University Astronomy Group), Karl Forster (NuSTAR Science Operations Manager)