During the past week, NuSTAR observed the Perseus Cluster, the brightest galaxy cluster in the sky in the X-rays, in coordination with the JAXA-NASA-ESA mission XRISM. Perseus is a calibration source for the wide-field Xtend imager on XRISM, but it also provides extremely valuable science for the primary XRISM instrument, Resolve. Resolve is the first high-spectral-resolution X-ray imaging spectrometer to fly an extended mission, replacing the similar instrument lost on the Hitomi mission. The XRISM science team has studied earlier observations of Perseus to try to resolve Doppler motions of the super-heated intra-cluster medium (ICM) gas, groundbreaking studies that will help us understand how these enormous galaxy clusters formed and evolved. However, the supermassive black hole at the center of the Perseus cluster also emits copious X-rays, which need to be disentangled from the ICM X-ray signal to properly isolate and measure the cluster gas motions. Simultaneous observations by NuSTAR are in a higher energy band than XRISM where the data is dominated by X-rays from the black hole. This will allow the XRISM calibration team to account for the contribution from the black hole in the XRISM observations, and precisely measure not only the gas motion but also the abundances of key elements and the temperature structure of the ICM. Since this is a calibration target observed twice a year by XRISM, a very deep total exposure will be obtained, and the vital collaboration with NuSTAR will enable a transformative view into the astrophysics of galaxy clusters.

Authors: Eric D. Miller (XRISM In-Flight Calibration Lead, MIT Kavli Institute for Astrophysics and Space Research)