Electron Heating at Galaxy Cluster Shocks: Measuring the Temperature of the Bullet Cluster Shock with NuSTAR

The Bullet cluster is famous for driving a shock into an oncoming subcluster's intracluster medium with its cool core (the "bullet"). Chandra data suggest a very high electron temperature right at the front (>30 keV), implying that electrons are directly heated by the passing shock contrary to expectations. However, Chandra's sensitivity to such high temperatures is low, given that its effective area declines swiftly above ∼4 keV. NuSTAR, the first focusing hard (>10 keV) X-ray observatory, is much better matched to the emission from gas with high temperatures, assuming its much poorer spatial resolution can be appropriately modeled. We present a demonstration of this technique with joint Chandra-NuSTAR imaging spectroscopy of the Bullet cluster and its shock. On average across its entire length, the shock temperature is in line with both the expectations of no direct heating by the shock (only increased temperature from adiabatic compression) and direct heating; both predictions overlap due to the lower Mach number farther away from the cool core. To compare directly with the Chandra-only measurement, we also constrain the shock temperature immediately ahead of the cool core, possibly to confirm this exciting example of direct electron heating driven by a weak shock. The prospects for future measurements in other clusters with NuSTAR will also be discussed.