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

TL;DR

This paper tests whether inverse-Compton emission from GeV-scale CRs accelerated by a cluster-center AGN can bias X-ray inferences of the ICM in cool-core clusters. By comparing the SZ-derived thermal pressure $P_{SZ}$ with X-ray-derived pressure $P_X$ in ZwCl 3146, the authors find a central deficit $P_{def} = 0.72 \pm 0.08$ within $\lesssim100$ kpc, at ~3.3$\\sigma$, consistent with CR-IC contamination predicted by AGN CR-heated halo models. A joint X-ray spectral analysis supports a CR-IC component in the core, with $P_{th}/P_X = 0.45^{+0.11}_{-0.11}$, aligning with the SZ/X-ray deficit within uncertainties. Systematics from halo triaxiality, helium sedimentation, clumping, and instrument calibration are quantified and found unlikely to explain more than about 10% of the deficit, suggesting CR-IC contamination as a plausible contributor to the cooling flow problem in this system. Nevertheless, the authors call for larger samples and deeper, joint spectral-spatial analyses to robustly establish the role of CR-IC emission across cool-core clusters.

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 $\simeq 3.3σ$, 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.

Figures (4)

• Figure 1: Left:$0.5-7$ keV (background-subtracted) Chandra X-ray counts map of ZwCl 3146 with annular bins defining pressure profile extraction regions. Middle: MUSTANG-2 SZ map of ZwCl 3146 for visualization. Note that the pressure profile is not derived from this map, but rather the time-ordered data. Right: SZ- and X-ray-derived pressure profiles. Uncertainties are $1\sigma$.
• Figure 2: Ratios of SZ-to-X-ray derived pressure profiles (black data points; $1\sigma$ uncertainties) with predictions from Hopkins2025CRs.CC overplotted for various CR injection luminosities. The predicted deficit in $P_{\text{SZ}} / P_X$ at radii within $\simeq100$ kpc of the cluster center becomes more extreme for higher CR injection luminosities. Our data from MUSTANG-2 and Chandra indicate the presence of a deficit in $P_{\text{SZ}} / P_X$ at $\simeq3.3\sigma$ significance.
• Figure 3: Census of possible contributions to the SZ-to-X-ray pressure deficit within 100 kpc of the ZwCl 3146 center.
• Figure 4: Left: spectral fit of the innermost annular bin in the ZwCl 3146 core, with contributions from the fitted tbabs $\times$ (apec + nlapec) model for ICM thermal and CR-IC emission indicated. Right: same, for the fitted tbabs $\times$ apec model for ICM thermal emission exclusively (no CR-IC component). This check confirms that the thermal pressure inferred from X-ray spectroscopy and SZ are consistent for the innermost ZwCl 3146 bin ($\simeq33$ kpc) in the simple case where the CR-IC spectral shape is similar to the thermal ICM continuum shape.