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A0538-66 is a Be/X-ray binary (Be/XRB) hosting a 69 ms pulsar. It emittedbright X-ray outbursts with peak luminosity up to $\sim 10^{39}$ erg/s duringthe first years after its discovery in 1977. Since then, it was always seen inquiescence or during outbursts with $L_x \lesssim 4 \times 10^{37}$ erg/s. In2018 we carried out XMM-Newton observations of A0538-66 during threeconsecutive orbits when the pulsar was close to periastron. In the first twoobservations we discovered a remarkable variability, with flares of typicaldurations between $\sim$2-50 s and peak luminosities up to $\sim 4\times10^{38}$ erg/s (0.2-10 keV). Between the flares the luminosity was $\sim2\times 10^{35}$ erg/s. The flares were absent in the third observation, duringwhich A0538-66 had a steady luminosity of $2\times 10^{34}$ erg/s. In allobservations, the X-ray spectra consist of a softer component, well describedby an absorbed power law with photon index $\Gamma_1\approx 2-4$ and$N_H\approx 10^{21}$ cm$^{-2}$, plus a harder power-law component($\Gamma_2\approx 0-0.5$) dominating above $\sim$2 keV. The softer componentshows larger flux variations than the harder one, and a moderate hardeningcorrelated with the luminosity. The fast flaring activity seen in theseobservations was never observed before in A0538-66, nor, to our best knowledge,in other Be/XRBs. We explore the possibility that during our observations thesource was accreting in a regime of nearly spherically symmetric inflow. Inthis case, an atmosphere can form around the neutron star magnetosphere and theobserved variability can be explained by transitions between the accretion andsupersonic propeller regimes.

Awakening of the fast-spinning accreting Be/X-ray pulsar A0538-66