An emission-state-switching radio transient with a 54-minute period by Caleb, M, Lenc, E, Kaplan, DL, Murphy, T, Men, YP, Shannon, RM, Ferrario, L, Rajwade, KM, Clarke, TE, Giacintucci, S, Hurley-Walker, N, Hyman, SD, Lower, ME, McSweeney, S, Ravi, V, Barr, ED, Buchner, S, Flynn, CML, Hessels, JWT, Kramer, M, Pritchard, J, Stappers, BW