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

TL;DR

The paper proposes that ancient cosmic ray halos (ACRHs) powered by central AGN CR injection produce inverse-Compton X-ray emission that can dominate the apparent soft X-ray cooling luminosity in cluster cores. This CR-IC emission yields a continuum with a thermal-like shape that mimics CC profiles in density, temperature, and entropy without requiring rapid actual cooling, offering a natural resolution to the classical cooling-flow problem. The framework makes concrete, parameter-light predictions for SZ pressure deficits, metallicity dilution in X-rays, and correlations between radio and X-ray properties, and it connects CCs to younger radio populations and ultra-steep spectra through an evolutionary sequence. Multi-wavelength tests (hard X-rays, gamma rays, optical/UV line signatures, and especially high-resolution SZ) can distinguish CR-IC from purely thermal CCs, with current data broadly consistent with the CR-IC scenario while preserving AGN feedback as a driving mechanism. Overall, CR-IC provides a unifying interpretation of CC phenomena, explains several observed correlations, and suggests new observational probes to test the prevalence and impact of CR halos in clusters.

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 $γ$-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-$z$.

Figures (31)

• Figure 1: Predicted mono-age CR lepton spectra for ACRHs at different radii and/or ages. First, the "injection zone": the region very near CR acceleration in space and time (CR age $\Delta t_{\rm Gyr} \equiv \Delta t / {\rm Gyr} \rightarrow 0$, and spatial extent/radius $R \rightarrow 0$), where the injection can be thought of as volume-filling. A plausible quasi-steady-state injection-region spectrum (injection balancing losses in the region) is the local ISM (LISM) spectrum, corresponding to synchrotron index $\alpha \sim 1$. For reference, we also compare a very shallow "pure power-law" (PPL) injection spectrum (1D $dN_{\rm cr}/dp_{\rm cr} \propto p_{\rm cr}^{-\delta}$ with $\delta=-2.2$, or $\alpha \sim 0.5$) with no escape or losses. We show the spectra evolved to age $\Delta t_{\rm Gyr}\gtrsim 1$, or equivalently for the CRs that propagate to radii $R \sim 100\,{\rm kpc}\,\Delta t_{\rm Gyr}\,v_{100}$, including Coulomb, bremsstrahlung, and IC+synchrotron losses. At $\gtrsim 10^{8}$ yr (or $R \gg$ kpc, for constant streaming speeds in a homogeneous medium), the CR spectra become broadly similar independent of injection spectrum (slightly softer if we assume the PPL spectrum in the inner region), dominated by $\sim 0.1-1$ GeV leptons.
• Figure 2: Continuum X-ray spectra predicted for CR leptons in an ACRH IC scattering cosmic background light, for CRs with a near-source/injection-zone LISM-like (thick) or pure power-law-like (thin) spectrum (Fig. \ref{['fig:cr.spectra']}). We evolve to different ages (accounting for Coulomb, bremsstrahlung, and IC losses) $\Delta t = \Delta t_{\rm Gyr}\,$Gyr, corresponding also to emission at a distance $R \sim 100\,{\rm kpc}\,\Delta t_{\rm Gyr}\,v_{100}$ from the injection site, for a mono-age population in a homogeneous background. We compare APEC thermal spectra for primordial gas with different temperatures. At $\gg 10^{7}\,$yr or equivalently $\gtrsim$ kpc from the injection sites, continuum CR-IC spectra become almost indistinguishable from thermal X-ray spectra with $T \sim$ keV (independent of the injection spectrum).
• Figure 3: All-wavelength surface brightness predicted for ACRHs leptons, for different ages where most of the radiation emerges ($\sim$ Gyr), or equivalently integrating over all radii out to $\sim 100\,$kpc for a constant (steady-state) injection, for a representative toy-model cluster at redshift $z=0.05$ with X-ray IC luminosity $L_{\rm X,\,IC} \approx \dot{E}_{\rm cr,\,\ell} \sim 10^{43}\,{\rm erg\,s^{-1}}$. We compare the meta-galactic background light compiled in cooray:2016.extragalactic.background.light.compilations.reviewkhaire:2019.extragalactic.background.light.spectratompkins:2023.cosmic.radio.background. The spectra are qualitatively similar over a wide range of ages/sizes and injection spectra. There are three peaks: most of the emission emerges in soft X-rays with a very thermal-like peak (CR-IC scattering CMB photons); there is a secondary $\sim 100\,\times$ less-luminous peak at $\sim$ MeV soft $\gamma$-rays from relativistic bremsstrahlung, and a much less-luminous peak at low-frequency $\sim$ MHz radio from synchrotron.
• Figure 4: Effective X-ray spectral temperature $T_{\rm X,\,eff}^{\rm IC}$ fit to pure CR-IC emission (as Fig. \ref{['fig:spectrum.xr']}), as a function of the age $\Delta t$ (time since escaping injection zone) of the CRs for different mono-age CR populations (for an LISM-like injection spectrum). We generate this for each age, at each radius, in several hundred mock clusters (§ \ref{['sec:thermal']}) varying cluster mass, density, temperature, metallicity, and magnetic field profiles through which CRs are propagated (all at redshift $z\sim0$). Different (energy-dependent) loss rates lead to somewhat different CR spectra at a given radius and age, producing different $T_{\rm X,\,eff}^{\rm IC}$. Shaded regions show the $1-3\sigma$ inclusion regions at each $\Delta t$, lines show some approximate scalings, e.g. $T_{\rm X,\,eff}^{\rm IC} \sim 4\,{\rm keV}\,\exp{(-\Delta t_{\rm Gyr}})$. The values at young $\Delta t\rightarrow 0$ depend on the assumed injection spectrum, while those at $\gtrsim 10^{8}\,$yr are more shaped by losses, and those at $\gg 10^{9}$ yr ($\ll$ keV) always correspond to regimes where most CR energy has already been lost (emission is minimal).
• Figure 5: Hard X-ray ACRH spectrum for a luminous cluster as Fig. \ref{['fig:spectrum.allband']}, compared to the extragalactic background and to the most stringent limits possible at present from standard deep hard X-ray searches at $20-80$ keV for IC signatures (shaded shows the range of upper limits and known detections; see § \ref{['sec:hardxray']}). The shape of the shaded region shows the "standard" IC spectral shape being searched for, which assumes a zero-age pure-power-law CR distribution ($E_{\rm ph}^{2} dN/dE_{\rm ph} \propto E_{\rm ph}^{+1/2}$). The ACHRs are factor $\sim 100$ less-luminous than detectable limits and much softer (owing to losses from high-energy CRs). The hard X-ray emission that is present comes primarily from $\sim 0.1-1\,$GeV leptons IC scattering cosmic infrared background photons.