Odd radio circle J1555+2726: Observational evidence for shock re-acceleration of fossil plasma

Context. Odd radio circles (ORCs) are peculiar extragalactic sources recently discovered. They are characterised by one or more bright rings extending on scales of hundreds of kiloparsecs, and are filled with fainter emission. A massive elliptical galaxy lies at the centre of ORCs. While their formation mechanism is debated, a shock is plausibly involved. Aims. We aim to constrain the nature of ORC J1555+2726 (ORC4) through the analysis of its morphological and spectral properties. Methods. We utilised Low Frequency ARray (LOFAR), upgraded Giant Metrewave Radio Telescope (uGMRT), and MeerKAT radio data at 50-1284 MHz to obtain images and spectral index maps at high resolution and sensitivity. Results. Both the resolution and observing frequency affect the observed morphology. In particular, the brightness contrast between the ring and the filling emission becomes progressively smoother from higher to lower frequencies. We measured an integrated spectrum of slope $\alpha =1.06\pm 0.03$, a radio power of $P_{150}=(5.4\pm 0.5)\times {10}^{25}\mathrm{W}\mathrm{H}{\mathrm{z}}^{-1}$ at 150 MHz, and a radial spectral flattening down to $\alpha \sim 0.5$ at the ring location. However, we inferred a negative (convex) spectral curvature and a peculiar spectral shape in radio colour-colour diagrams, which imply spectral flattening towards high frequencies. These properties cannot be explained by shock acceleration of thermal particles and standard ageing. Conclusions. The observed features suggest that a mixture of electrons with different spectra coexist within ORC4 volume, likely associated with shock re-acceleration of a fossil relativistic reservoir. The ring emission is plausibly dominated by higher-energy electrons re-accelerated at the shock front, while lower-energy re-accelerated electrons are more uniformly distributed across the whole source (possibly ellipsoidal) volume.