Galaxy-cluster-stacked Fermi-LAT II: extended central hadronic signal
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arXiv
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| Format: | Preprint |
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2025
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| _version_ | 1866914136205557760 |
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| author | Keshet, Uri |
| author_facet | Keshet, Uri |
| contents | Faint $γ$-ray signatures emerge in Fermi-LAT data stacked scaled to the characteristic $θ_{500}$ angles of MCXC galaxy clusters. After Paper I of this series thus discovered virial shocks, later supported in other bands, this second paper focuses on cluster cores. Stacking $1$-$100$ GeV source-masked data around clusters shows a significant ($4.7σ$ for 75 clusters) and extended central excess, inconsistent with central point sources. The resolved signal is best fit ($3.7σ$ TS-test) as hadronic emission from a cosmic-ray ion (CRI) distribution that is flat both spectrally ($p\equiv1-d\ln u/d\ln E=2.0\pm0.3$) and spatially (CRI-to-gas index $σ=0.1\pm0.4$), carrying an energy density $du(0.1θ_{500})/d\ln E=10^{-13.6\pm0.5}$ erg cm$^{-3}$ at $E=100$ GeV energy; insufficient resolution would raise $p$ and $σ$. Such CRI match the long-predicted distribution needed to power diffuse intracluster radio emission in its various forms (mini-halos, giant halos, standard relics, their transitional forms, and mega-halos), disfavoring models that invoke electron (re)acceleration in weak shocks or turbulence. Stringent upper limits on residual $γ$-ray emission, e.g. from dark-matter annihilation, are imposed. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2502_19494 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Galaxy-cluster-stacked Fermi-LAT II: extended central hadronic signal Keshet, Uri High Energy Astrophysical Phenomena Faint $γ$-ray signatures emerge in Fermi-LAT data stacked scaled to the characteristic $θ_{500}$ angles of MCXC galaxy clusters. After Paper I of this series thus discovered virial shocks, later supported in other bands, this second paper focuses on cluster cores. Stacking $1$-$100$ GeV source-masked data around clusters shows a significant ($4.7σ$ for 75 clusters) and extended central excess, inconsistent with central point sources. The resolved signal is best fit ($3.7σ$ TS-test) as hadronic emission from a cosmic-ray ion (CRI) distribution that is flat both spectrally ($p\equiv1-d\ln u/d\ln E=2.0\pm0.3$) and spatially (CRI-to-gas index $σ=0.1\pm0.4$), carrying an energy density $du(0.1θ_{500})/d\ln E=10^{-13.6\pm0.5}$ erg cm$^{-3}$ at $E=100$ GeV energy; insufficient resolution would raise $p$ and $σ$. Such CRI match the long-predicted distribution needed to power diffuse intracluster radio emission in its various forms (mini-halos, giant halos, standard relics, their transitional forms, and mega-halos), disfavoring models that invoke electron (re)acceleration in weak shocks or turbulence. Stringent upper limits on residual $γ$-ray emission, e.g. from dark-matter annihilation, are imposed. |
| title | Galaxy-cluster-stacked Fermi-LAT II: extended central hadronic signal |
| topic | High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2502.19494 |