A MAGNETAR-LIKE EVENT FROM LS I +61°303 AND ITS NATURE AS A GAMMA-RAY BINARY

We report on the Swift Burst Alert Telescope detection of a short burst from the direction of the TeV binary LS I + 61 degrees 303, resembling those generally labeled as magnetar-like. We show that it is likely that the short burst was indeed originating from LS I + 61 degrees 303 (although we cannot totally exclude the improbable presence of a far-away, line-of-sight magnetar) and that it is a different phenomenon with respect to the previously observed ks-long flares from this system. Accepting the hypothesis that LS I + 61 degrees 303 is the first magnetar detected in a binary system, we study those implications. We find that a magnetar-composed LS I + 61 degrees 303 system would most likely be (i.e., for the usual magnetar parameters and mass-loss rate) subject to a flip-flop behavior, from a rotationally powered regime (in the apastron) to a propeller regime (in the periastron) along each of the LS I + 61 degrees 303 eccentric orbital motion. We prove that, unlike near an apastron, where an interwind shock can lead to the normally observed LS I + 61 degrees 303 behavior, during TeV emission the periastron propeller is expected to efficiently accelerate particles only to sub-TeV energies. This flip-flop scenario would explain the system's behavior when a recurrent TeV emission only appears near the apastron, the anti-correlation of the GeV and TeV emission, and the long-term TeV variability (which seems correlated to LS I + 61 degrees 303's super-orbital period), including the appearance of a low TeV state. Finally, we qualitatively put the multi-wavelength phenomenology into the context of our proposed model and make some predictions for further testing.