Modeling the effects of clumpy winds in the high-energy light curves of γ-ray binaries

Abstract

Context. High-mass gamma-ray binaries are powerful nonthermal galactic sources, some of them hosting a pulsar whose relativistic wind interacts with a likely inhomogeneous stellar wind. So far, modeling these sources including stellar wind inhomogeneities has been done using either simple analytical approaches or heavy numerical simulations, none of which allow for an exploration of the parameter space that is both reasonably realistic and general. Aims. Applying different semi-analytical tools together, we study the dynamics and high-energy radiation of a pulsar wind colliding with a stellar wind with different degrees of inhomogeneity to assess the related observable effects. Methods. We computed the arrival of clumps to the pulsar wind-stellar wind interaction structure using a Monte Carlo method and a phenomenological clumpy-wind model. The dynamics of the clumps that reach deep into the pulsar wind zone was computed using a semi-analytical approach. This approach allows for the characterization of the evolution of the shocked pulsar wind region in times much shorter than the orbital period. With this three-dimensional information about the emitter, we applied analytical adiabatic and radiative models to compute the variable high-energy emission produced on binary scales.Results. An inhomogeneous stellar wind induces stochastic hour-timescale variations in the geometry of the two-wind interaction structure on binary scales. Depending on the degree of stellar wind inhomogeneity, 10-100% level hour-scale variability in the X-rays and gamma rays is predicted, with the largest variations occurring roughly once per orbit. Conclusions. Our results, based on a comprehensive approach, show that present X-ray and future very-high-energy instrumentation can allow us to trace the impact of a clumpy stellar wind on the shocked pulsar wind emission in a gamma-ray binary.

Unveiling Properties of the Nonthermal X-Ray Production in the Gamma-Ray Binary LS 5039 Using the Long-term Pattern of Its Fast X-Ray Variability

“…Nevertheless, if a significant fraction of the stellar wind is in the form of relatively large clumps, the interaction rate may be ∼10 clump-pulsar wind interactions per orbit, which roughly matches the observed short-term behavior of the orbital light curves. It is worth noting that the more quantitative analysis derived by Kefala & Bosch-Ramon (2022), which adopted a more realistic clump mass distribution for the stellar wind and considered simultaneously many clumps, also predicts similar rates to those obtained here.…”

“…Nevertheless, it must be taken into account that the chaotic arrival of smaller clumps can lead to nontrivial behaviors of the X-ray light curve, with short-term variability features and their rates being dependent on the stellar wind clumping properties. In particular, Kefala & Bosch-Ramon (2022) results indicate that a few smaller clumps (compared to R c above) can impact simultaneously on the two-wind interaction region, inducing an evolution time of the perturbation longer than in the case of only one clump. We note that stellar wind inhomogeneities large enough to engulf the two-wind interaction region, even under moderate density contrasts (say a factor of ∼2), may also induce variations in the structure of the two-wind interaction region.…”

“…When inhomogeneities of different scales are present in the stellar wind, it is expected that the interaction structure will vary in space and time (Bosch-Ramon 2013; Paredes-Fortuny et al 2015;Kefala & Bosch-Ramon 2022), resulting in the observed short-term variability. Following this idea, we discuss in what follows the characteristic values of time/size scale and frequency (e.g., Bosch-Ramon 2013).…”

Inhomogeneous stellar winds in gamma-ray binaries

“…The CD structure largely diverges from the smooth-wind case and is quickly deformed as clumps penetrate the two-wind region and then get deflected, dissolve, and mix with the shocked material. Details on the derivation of the clump trajectories and dynamics, as well as results for stellar wind scenarios with different degrees of inhomogeneity, can be found in [7].…”

“…This still provides accurate enough results for our purposes and allows for a more intuitive understanding of the emission behavior. A more extensive study of such a physical scenario and a discussion on how certain parameters (e.g., the clump mass distribution and filling factor, the emitter size, and the wind momentum rate ratio) can affect the frequency of interactions and the induced variability is provided in [7].…”