MeerKAT reveals an ultra-steep spectrum radio halo in Abell 370

Hubble Frontier Field cluster Abell 370 (z = 0.375) has been the target of a suite of legacy multi-wavelength observational campaigns and is a prime candidate to host a diffuse radio halo, given its high mass (~ 10¹⁵\, M_) and non-relaxed dynamical state. Previous detections of its radio halo were of low significance and imaging fidelity, leading to an uncertain characterisation of its nature. We use deep MeerKAT Ultra-High Frequency observations (580-1015\, MHz) to robustly map the halo’s spatial extent and spectral properties. We find a radio emission size of ~ 1.1\, Mpc, significantly larger than previously reported. We measure a MeerKAT UHF in-band integrated spectral index of α = −1.54 ± 0.05. This result is consistent with an ultra-steep spectrum radio halo (α ≲ −1.5). We find a strong, sub-linear correlation (k = 0.60 ± 0.08) between the radio and X-ray surface brightness. The radio halo’s position on the P1.4\, GHz-M₅₀₀ scaling relation is underluminous, consistent with the known USSRH population. The combined evidence strongly supports that the diffuse central radio emission in Abell 370 is an Ultra-Steep Spectrum Radio Halo (USSRH), refining its previous classification and providing further support for the turbulent re-acceleration model.