Guillaume Voisin LUX, ASTRE, CNRS, Théo Francez LUX, ASTRE

Fast radio bursts (FRBs) are bright millisecond radio events of unknown extra-galactic origin. Magnetars are one of the main contenders. Some sources, the repeaters, produce multiple events but so far generally without the characteristic periodicity that one could associate with the spin of a neutron star. We fit a geometrical model to the two main repeaters of the CHIME/FRB catalogue, namely FRB 20180814A and FRB 20180916B. Assuming the bursts originate from a magnetar's magnetosphere, we constrain the spin and magnetic parameters of the star which are encoded into burst spectro-temporal morphologies. We estimate that a very strong toroidal magnetic component together with spin periods of respectively $2.3_{-0.5}^{+0.5} ~ \rm s$ and $0.8_{-0.2}^{+0.1} ~ \rm s$ best explain the data. We argue that this points towards young magnetars with super-twisted magnetospheres.