Exploring a Cosmic-Ray Inverse-Compton Origin to the SZ-to-X-Ray Pressure Deficit in the Cool Core Cluster ZwCl 3146
4 affiliations
- 1California Institute of Technology
- 2University of Pennsylvania
- 3National Radio Astronomy Observatory
- 4University of Virginia
Abstract
We explore the possibility that inverse-Compton (IC) scattering of cosmic microwave background photons by ∼GeV cosmic rays (CRs) injected by the central active galactic nucleus (AGN) in cool core (CC) clusters produces a nonnegligible 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 25% decrement in P SZ / P X within 100 kpc of the ZwCl 3146 center at a statistical significance ≃3 σ , 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 leading-order systematics associated with halo triaxiality and AGN-driven cavities. On average, these systematics could bias P SZ / P X low by ≃5%, a factor of 2 lower than our measurement uncertainty, with an object-to-object variation ≃5%. While our results establish that nonnegligible CR–IC emission is plausible in ZwCl 3146, we stress that detailed studies of larger cluster samples are required to robustly assess whether CR–IC is relevant to the cooling flow problem.
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BibTeX
@article{silich2026exploring,
title = {{Exploring a Cosmic-Ray Inverse-Compton Origin to the SZ-to-X-Ray Pressure Deficit in the Cool Core Cluster ZwCl 3146}},
author = {Emily M. Silich and Jack Sayers and Philip F. Hopkins and Charles Romero and Brian Mason and John Orlowski-Scherer and Craig L. Sarazin},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aea299},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea299},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Exploring a Cosmic-Ray Inverse-Compton Origin to the SZ-to-X-Ray Pressure Deficit in the Cool Core Cluster ZwCl 3146
AU - Emily M. Silich
AU - Jack Sayers
AU - Philip F. Hopkins
AU - Charles Romero
AU - Brian Mason
AU - John Orlowski-Scherer
AU - Craig L. Sarazin
PY - 2026
DA - 2026/10/05
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aea299
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aea299
AB - We explore the possibility that inverse-Compton (IC) scattering of cosmic microwave background photons by ∼GeV cosmic rays (CRs) injected by the central active galactic nucleus (AGN) in cool core (CC) clusters produces a nonnegligible 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 25% decrement in P SZ / P X within 100 kpc of the ZwCl 3146 center at a statistical significance ≃3 σ , 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 leading-order systematics associated with halo triaxiality and AGN-driven cavities. On average, these systematics could bias P SZ / P X low by ≃5%, a factor of 2 lower than our measurement uncertainty, with an object-to-object variation ≃5%. While our results establish that nonnegligible CR–IC emission is plausible in ZwCl 3146, we stress that detailed studies of larger cluster samples are required to robustly assess whether CR–IC is relevant to the cooling flow problem.
SN - 0004-637X
SN - 1538-4357
ER -