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XSS J12270-4859 (henceforth J12270) is the first low-mass X-ray binary toexhibit a transition, taking place at the end of 2012, from an X-ray activestate to a radio pulsar state. The X-ray emission based on archival XMM-Newtonobservations is highly variable, showing rapid variations (10 s) from a highX-ray luminosity mode to a low mode and back. A flaring mode has also beenobserved. X-ray pulsations have been detected during the high mode only. Inthis work we present two possible interpretations for the rapid swings betweenthe high and low modes. In the first scenario, this phenomenon can be explainedby a rapid oscillation between a propeller state and a radio-ejection pulsarstate, during which the pulsar wind prevents matter from falling onto theneutron star surface. In the second scenario, a radio pulsar is always active,the intra-binary shock is located just outside the light cylinder in the highmode, while it expands during the low mode. At variance with other transitionalpulsars, J12270 shows two instances of the low mode: a low-soft and low-hardmode. Performing an X-ray spectral analysis, we show that the harder component,present in the low-hard spectra, is probably related to the tail of the flareemission. This supports the understanding that the flare mechanism isindependent of the high-to-low mode transitions.

X-ray study of high-and-low luminosity modes and peculiar low-soft-and-hard activity in the transitional pulsar XSS J12270-4859