Synchrotron emission from virial shocks around stacked OVRO-LWA galaxy clusters. Issue 4 (17th March 2023)

• Record Type: Journal Article
• Authors: Hou, Kuan-Chou; Hallinan, Gregg; Keshet, Uri
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 521:Issue 4(2023)
• Page Start: 5786
• Page End: 5809
• Publication Date: 2023-03-17
• Subjects: magnetic fields -- galaxies: clusters: general -- galaxies: clusters: intracluster medium -- intergalactic medium -- radio continuum: general; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stad785
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: Galaxy clusters accrete mass through large-scale, strong, structure-formation shocks. Such a virial shock is thought to deposit fractions ξ e and ξ B of the thermal energy in cosmic-ray electrons (CREs) and magnetic fields, respectively, thus generating a leptonic virial ring. However, the expected synchrotron signal was not convincingly established until now. We stack low-frequency radio data from the OVRO-LWA around the 44 most massive, high latitude, extended MCXC clusters, enhancing the ring sensitivity by rescaling clusters to their characteristic, R 500 radii. Both high (73 MHz) and co-added low (36–68 MHz) frequency channels separately indicate a significant (4–5σ) excess peaked at (2.4–2.6) R 500, coincident with a previously stacked Fermi γ-ray signal interpreted as inverse-Compton emission from virial-shock CREs. The stacked radio signal is well fit (TS-test: 4–6σ at high frequency, 4–8σ at low frequencies, and 8–10σ joint) by virial-shock synchrotron emission from the more massive clusters, with $\dot{m}\xi _e\xi _B\simeq (1\!-\!4)\times 10^{-4}$, where $\dot{m}\equiv \dot{M}/(MH)$ is the dimensionless accretion rate for a cluster of mass M and a Hubble constant H . The inferred CRE spectral index is flat, p ≃ 2.0 ± 0.2, consistent with acceleration in a strong shock. Assuming equipartition or using $\dot{m}\xi _e\sim 0.6~{{\ \rm per\ cent}}$ inferred from the Fermi signal yields $\xi _B\simeq (2\!-\!9)~{{\ \rm per\ cent}}$, corresponding to B ≃ (0.1–0.3) $\mu$ G magnetic fields downstream of typical virial shocks. Preliminary evidence suggests non-spherical shocks, with factor 2–3 elongations.