Associating long-term gamma-ray variability with the superorbital period of LS I 61 303

TL;DR

This study detects a superorbital gamma-ray variability in LS I 61 303, correlating it with the star's outflow dynamics, and highlights the potential of gamma-ray data to probe massive star outflows in eccentric binaries.

Contribution

It is the first to link the superorbital period of 1667 days with gamma-ray variability in LS I 61 303, revealing phase-dependent modulation and its relation to stellar outflows.

Findings

Gamma-ray emission shows sinusoidal variability matching the superorbital period.

Modulation is more prominent at orbital phases around apastron.

Variability at the orbital period appears and disappears over time.

Abstract

Gamma-ray binaries are stellar systems for which the spectral energy distribution (discounting the thermal stellar emission) peaks at high energies. Detected from radio to TeV gamma rays, the gamma-ray binary LS I 61 303 is highly variable across all frequencies. One aspect of this system's variability is the modulation of its emission with the timescale set by the ~26.4960-day orbital period. Here we show that, during the time of our observations, the gamma-ray emission of LS I 61 303 also presents a sinusoidal variability consistent with the previously-known superorbital period of 1667 days. This modulation is more prominently seen at orbital phases around apastron, whereas it does not introduce a visible change close to periastron. It is also found in the appearance and disappearance of variability at the orbital period in the power spectrum of the data. This behavior could be explained by a quasi-cyclical evolution of the equatorial outflow of the Be companion star, whose features influence the conditions for generating gamma rays. These findings open the possibility to use gamma-ray observations to study the outflows of massive stars in eccentric binary systems.