ASKAP Discovery of a Giant Odd Radio Circle at $z=0.15$

Odd radio circles (ORCs) are a recently recognized class of faint, roughly circular diffuse radio sources, typically a few hundred kiloparsecs across, whose origin is still debated. We report the serendipitous discovery of a giant ORC, ORC~J1928$-$5056, in 856~MHz continuum data from the ASKAP observations. The emission forms a single, mildly elliptical structure $4^{\prime}$ across, corresponding to $0.63$~Mpc at the redshift $z=0.1507$ of the central galaxy LEDA~140850, the brightest cluster galaxy of Abell~3639, which makes it one of the largest single ORCs known. After subtracting the radio nucleus associated with the host galaxy, the ORC has a flux density of 73~mJy integrated within an area of $\sim 12$~arcmin$^2$, i.e.\ a mean surface brightness of only $1.7$~$μ$Jy~arcsec$^{-2}$, and a rest-frame luminosity of $L_{1.4}=2\times10^{24}$~W~Hz$^{-1}$, comparable to those of the most luminous ORCs. Including its nucleus, the system reaches $L_{1.4}=6.6\times10^{24}$~W~Hz$^{-1}$, higher than that of any other ORC reported so far. The host is a massive ($M_{\ast}\sim10^{11}\,M_{\odot}$), passive elliptical galaxy with negligible star formation. It contains an AGN jet with a power-law spectrum of index $α=-0.877\pm0.089$ between 856 and 1656~MHz. Finally, we discuss the implications for ORC formation models and the observations needed to establish the nature of this system.