Simulations of two-temperature jets in galaxy clusters: II. X-ray property of forward shock

TL;DR

This study uses two-temperature MHD simulations to analyze how projection effects and temperature non-equilibration influence X-ray measurements of forward shocks in galaxy cluster jets, revealing underestimations of Mach numbers in observations.

Contribution

It systematically investigates factors affecting Mach number estimates in powerful radio jets using realistic simulations and mock X-ray observations, clarifying observational discrepancies.

Findings

Density jump measurements underestimate Mach numbers at low viewing angles.

Projection effects significantly reduce observed post-shock temperatures.

Temperature jump remains near unity regardless of electron-proton equilibration.

Abstract

Forward shocks by radio jets, driven into the intracluster medium, are one of the indicators that can be used to evaluate the power of the jet. Meanwhile high-angular-resolution X-ray observations show the Mach numbers of powerful radio jets are smaller compared to that of theoretical and numerical studies, $\mathcal{M_{\rm obs}} < 2$. Our aim is to systematically investigate various factors, such as projection effects and temperature non-equilibration between protons and electrons, that influence the Mach number estimate in a powerful jet. Using two-temperature magnetohydrodynamic simulation data for the Cygnus A radio jets, whose Mach number is approximately 6, we construct mock X-ray maps of simulated jets from various viewing angles. Further, we evaluate the shock Mach number from density/temperature jump using the same method of X-ray observations. Our results demonstrate that measurements from density jump significantly underestimate the Mach numbers, $\mathcal{M} < 2$, around the jet head at a low viewing angle, $\lessapprox 50^{\circ}$. The observed post-shock temperature is strongly reduced by the projection effect, as our jet is in the cluster center where the gas density is high. On the other hand, the temperature jump is almost unity, even if thermal electrons are in instant equilibration with protons. Upon comparison, we find that shock property of our model at viewing angle of $<$ $55^{\circ}$ is in a good agreement with that of Cygnus A observations.