2012ApJ...756..188P

The properties of the short, energetic bursts recently observed from the γ-ray binary LS I +61°303 are typical of those showed by high magnetic field neutron stars (NSs) and thus provide a strong indication in favor of a NS being the compact object in the system. Here, we discuss the transitions among the states accessible to a NS in a system like LS I +61°303, such as the ejector, propeller, and accretor phases, depending on the NS spin period, magnetic field, and rate of mass captured. We show how the observed bolometric luminosity ( ≳ fewx10³⁵ erg/s) and its broadband spectral distribution indicate that the compact object is most probably close to the transition between working as an ejector all along its orbit and being powered by the propeller effect when it is close to the orbit periastron, in a so-called flip-flop state. By assessing the torques acting onto the compact object in the various states, we follow the spin evolution of the system, evaluating the time spent by the system in each of them. Even taking into account the constraint set by the observed γ-ray luminosity, we found that the total age of the system is compatible with being ~5-10 kyr, comparable to the typical spin-down ages of high-field NSs. The results obtained are discussed in the context of the various evolutionary stages expected for a NS with a high-mass companion.