Possible changes of state and relevant timescales for a neutron star in LS I +61{\deg}303

TL;DR

This paper investigates the state transitions of a neutron star in LS I +61°303, analyzing how its luminosity, spectral features, and spin evolution suggest it switches between ejector and propeller states, with an estimated age of 5-10 kyr.

Contribution

It provides a detailed analysis of the neutron star's state transitions and spin evolution in LS I +61°303, linking observational data to theoretical models of neutron star behavior.

Findings

The neutron star is likely near the transition between ejector and propeller states.

The system's age is estimated to be around 5-10 kyr.

The observed luminosity supports the flip-flop state hypothesis.

Abstract

The properties of the short, energetic bursts recently observed from the gamma-ray binary LS I +61{\deg}303, are typical of those showed by high magnetic field neutron stars, and thus provide a strong indication in favor of a neutron star being the compact object in the system. Here, we discuss the transitions among the states accessible to a neutron star in a system like LS I +61{\deg}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 (>= few x 1E35 erg/s), and its broad-band 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 gamma-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 neutron stars. The results obtained are discussed in the context of the various evolutionary stages expected for a neutron star with a high mass companion.