Observational Implications of Cosmic Ray-Inverse Compton 'Boosted' Cool Cores in Clusters

TL;DR

This paper investigates how cosmic ray inverse-Compton emission in galaxy cluster cool cores can mimic thermal X-ray signatures, potentially resolving the cooling flow problem and explaining observed correlations without requiring actual rapid cooling.

Contribution

It introduces the idea that CR-IC emission significantly impacts X-ray observations of CCs, offering an alternative explanation for their properties and related correlations.

Findings

CR-IC can produce thermal-like X-ray spectra in CC centers.

CR-IC explains observed correlations between radio, X-ray, and AGN properties.

CR-IC affects pressure estimates and abundance measurements in CCs.

Abstract

X-ray luminous cool-core (CC) galaxy clusters contain powerful cosmic ray (CR) sources. High-energy CRs powering GHz synchrotron lose energy rapidly, but long-lived (~Gyr-old) populations of 0.1-1 GeV CRs persist, propagating to ~100 kpc distances and radiating via inverse-Compton (IC) scattering of CMB photons. We explore observable consequences of such CR-IC emission. This produces remarkably thermal X-ray spectra, which could contribute significantly to emission in CC centers. These naturally connect to ultra-steep radio sources and radio mini-halos at younger ages, but become undetectable in most radio, hard-X-ray, and $\gamma$-ray searches (though future imaging may detect them), while reproducing apparent density, temperature, entropy, and mass deposition rates of CCs. This would provide an alternative resolution of the cooling flow problem: clusters may appear as strong CCs because of strong CR-IC, while not actually cooling so rapidly. This predicts many observed correlations between AGN/jet properties, radio galaxy and minihalo properties, cooling radii, cavity radii and apparent X-ray cooling luminosity $L_{\rm X,cool}$. Since $L_{\rm X,cool}$ is actually from CR-IC, the observed radio-X-ray ($L_{\rm radio}-L_{\rm X,cool}$), apparent cavity power ($P_{\rm cav}-L_{\rm X,cool}-L_{\rm radio}$), and strong CC-AGN correlations are predicted without free parameters. Since CR-IC leads to X-ray overestimates of thermal pressure, the ratio of SZ to X-ray pressures should drop in CC centers. CR-IC also suppresses abundances inferred from X-ray relative to optical/UV measurements in CC centers. Both of these appear to be seen in sufficiently-resolved CCs. Effects on cluster cosmology, hydrostatic mass estimation, and non-thermal pressure/turbulence estimators are small. Redshift evolution of CC surface brightness profiles could provide strong constraints or imply CR-IC at high-$z$.