Exploring a cosmic ray inverse-Compton origin to the SZ-to-X-ray pressure deficit in the cool core cluster ZwCl 3146

Abstract

We explore the possibility that inverse-Compton (IC) scattering of cosmic microwave background photons by $\sim$GeV cosmic rays (CRs) injected by the central active galactic nucleus (AGN) in cool core (CC) clusters produces a non-negligible continuum-like X-ray signal that is easily misinterpreted as intracluster medium (ICM) thermal bremsstrahlung continuum. This is particularly relevant to the cooling flow problem--the lack of star formation relative to X-ray-inferred ICM cooling rates. Using ZwCl 3146, a relaxed CC system at $z = 0.291$, we compare pressure profiles derived via X-rays and the thermal Sunyaev-Zel'dovich (SZ) effect. While SZ measurements probe only thermal ICM electrons, additional CR-IC emission would appear to boost the X-ray-inferred pressure. Relative to unity, we measure a $\simeq30\%$ decrement in $P_{SZ}/P_X$ within 100 kpc of the ZwCl 3146 center at a statistical significance of $\simeq 3.3\sigma$, consistent with predicted deficits from CR-IC contamination in reasonable models of central AGN-driven CR injection. X-ray spectral fits of a two-component model with thermal ICM and CR-IC emission are consistent with CR-IC as the cause of this deficit. We test alternative explanations and systematics that could drive such a decrement, with the leading order systematics associated with halo triaxiality. Collectively, these systematics are unlikely to produce a $P_{SZ}/P_X$ decrement $\gtrsim10\%$. While our results establish that non-negligible CR-IC emission is plausible in ZwCl 3146, we stress that more detailed studies of larger cluster samples are required to robustly assess whether CR-IC is relevant to the cooling flow problem.