Non-thermal radiation from a pulsar wind interacting with an inhomogeneous stellar wind

Abstract

Context. Binaries hosting a massive star and a non-accreting pulsar are powerful non-thermal emitters due to the interaction of the pulsar and the stellar wind. The winds of massive stars are thought to be inhomogeneous, which could have an impact on the nonthermal emission. Aims. We study numerically the impact of the presence of inhomogeneities or clumps in the stellar wind on the high-energy nonthermal radiation of high-mass binaries hosting a non-accreting pulsar. Methods. We compute the trajectories and physical properties of the streamlines in the shocked pulsar wind without clumps, with a small clump, and with a large one. This information is used to characterize the injection and the steady state distribution of non-thermal particles accelerated at shocks formed in the pulsar wind. The synchrotron and inverse Compton emission from these non-thermal particles is calculated, accounting also for the effect of gamma-ray absorption through pair creation. A specific study is done for PSR B1259-63/LS2883. Results. When stellar wind clumps perturb the two-wind interaction region, the associated non-thermal radiation in the X-ray band, of synchrotron origin, and in the GeV-TeV band, of inverse Compton origin, is affected by several equally important effects: (i) strong changes in the the plasma velocity direction that result in Doppler boosting factor variations, (ii) strengthening of the magnetic field that mainly enhances the synchrotron radiation, (iii) strengthening of the pulsar wind kinetic energy dissipation at the shock, potentially available for particle acceleration, and (iv) changes in the rate of adiabatic losses that affect the lower energy part of the non-thermal particle population. The radiation above 100 GeV detected, presumably, during the post-periastron crossing of the Be star disc in PSR B1259-63/LS2883, can be roughly reproduced assuming that the crossing of the disc is modeled as the encounter with a large inhomogeneity. Conclusions. Because of the likely diverse nature of clumps in the stellar wind, and hydrodynamical instabilities, the non-thermal radiation of high-mass binaries with a non-accreting pulsar is expected to be boosted somewhat chaotically, and present different superimposed variability patterns. Some of the observed variability in gamma rays from PSR B1259-63/LS2883 is qualitatively reproduced by our calculations.

A hydrodynamics-informed, radiation model for HESS J0632 + 057 from radio to gamma-rays

“…A next step would be to carry out more accurate radiation calculations applying post-processing to the hydrodynamical solution for HESS J0632+057, and compute consistently the particle evolution in space and energy in a way similar to those employed for instance by Dubus et al (2015) and de la Cita et al (2017). Another step forward would be to increase the resolution in the θ-coordinate when simulating the wind collision at large scales in this source.…”

X-ray mapping of the stellar wind in the binary PSR J2032+4127/MT91 213

“…Paredes-Fortuny et al 2015). These perturbations may enhance the energy dissipation of the pulsar wind at the termination shock, strengthen the post-shock magnetic field, or change the direction of motion of the shocked pulsar wind, thus giving rise to X-ray flares (de la Cita et al 2017).…”

Gamma-rays from the binary system containing PSR J2032+4127 during its periastron passage

“…However, homogeneous winds can also mix efficiently due to the instabilities which appear during wind collisions as shown in two and three dimensional hydrodynamical simulations [30,31]. In the case of very inhomogeneous winds, the stellar wind can load the pulsar wind with the barionic matter [32,33,34]. Then, both winds mix to some extent.…”