Observational and theoretical study of the interaction of relativistic winds from young pulsars with inhomogeneous stellar winds

Abstract

Gamma-ray binaries are systems that comprise a compact object orbiting a companion star and display the maximum of the non-thermal Spectral Energy Distribution (SED) in gamma rays. Currently we know five gamma-ray binaries. All of them host an early type star and a compact object of unknown nature, either a black hole or a neutron star, except for one of them where radio pulsations have been detected.

The optical emission received from gamma-ray binaries is produced by the optical star and its environment. If the optical star is a Be star, then it presents a circumstellar decretion disk, being its size traced by the Equivalent Width of the Halpha line (EW). Numerical simulations of gamma-ray binaries hosting a Be star suggest that the circumstellar decretion disk is perturbed/disrupted during the periastron passage of the compact object by the tidal forces and/or the putative pulsar wind. The circumstellar disk contribution to the optical photometry is a significant fraction of the total optical emission. The observed optical photometric flux from it will be proportional to the projected area of the optically emitting disk. Therefore, any orbital variability in the optical light curves can be associated to changes in the circumstellar disk. We conducted long-term optical photometric and EW observations of the gamma-ray binary LS I +61 303, aimed to unveil the optical superorbital variability seen at other wavelengths.

We obtained the following results from the observations: 1) The optical photometry and EW present a superorbital variability of the orbital phase of the maximum, as seen in radio and X-rays, providing an evidence of the coupling between the thermal and non-thermal emission processes in LS I +61 303. 2) The optical observations present a superorbital variability of the flux compatible with the 1667 d superorbital period. 3) This superorbital variability is attenuated or missing in some orbital phases. 4) Orbital variability in a multi-wavelength context presents lags that can be naturally interpreted considering different emitting regions. 5) The observations seem to be only compatible with an extended and (quasi)-coplanar circumstellar disk. 6) The observations are compatible with a density wave scenario, and with a very restrictive precessing-disk model.

Gamma-ray binaries hosting a massive star and a young non-accreting pulsar present strong interaction between the relativistic pulsar wind, and the wind of the stellar companion, resulting in efficient particle acceleration and in the production of non-thermal radiation, from radio to gamma rays. The study of the dynamical interaction between the winds can be conducted through numerical simulations, allowing a qualitative analysis of the radiative output of the system. Furthermore, the two-wind interaction region might suffer the impact of an inhomogeneous stellar wind (hereafter clumps), making its dynamics and hence its radiative output more complex. We conducted RHD simulations of the interaction of

relativistic winds from young pulsars with inhomogeneous stellar winds aimed to provide a plausible framework for the high-energy variability observed in gamma-ray binaries.

We obtained the following results from the numerical simulations: 1) The two-wind interaction structure is very unstable and sensitive to the tiniest perturbations, which lead to quick Rayleigh-Taylor (RT) and in particular Kelvin-Helmholtz (KH) instability growth. 2) The arrival of clumps can have a very strong impact on the whole interaction structure, which is expected to strongly affect the non-thermal radiation as well. 3) The clumps trigger violent RT/KH instabilities leading to quick changes of the shocked pulsar-wind region. 4) Clumps generate quick and global variations in the shocked pulsar wind. This can lead to strong short time-scale flux variability in the non-thermal radiation of gamma-ray binaries.