Clumpy stellar winds and high-energy emission in high-mass binaries hosting a young pulsar

Departament d’Astronomia i MeteorologiaInstitut de Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona, Spain
e-mail: vbosch@am.ub.es

Received: 10 July 2013
Accepted: 5 October 2013

Abstract

Context. High-mass binaries hosting young pulsars can be powerful gamma-ray emitters. The stellar wind of the massive star in the system, which interacts with the pulsar wind, is expected to be clumpy. Since the high-energy emission comes from the interaction of the two winds, the presence of clumps can affect the spectrum and variability of this radiation.

Aims. We look for the main effects of the presence of clumps in the stellar wind in the two-wind interaction region and in the non-thermal radiation that originates there.

Methods. A simple analytical model for the two-wind interaction dynamics was developed. The model accounts for the lifetime of clumps under the pulsar-wind impact. This time plays a very important role with regard to the evolution of the clump, the magnetic field in the clump-pulsar wind interaction region, and the non-radiative and radiative cooling of the non-thermal particles. We also computed the high-energy emission produced at the interaction of long-living clumps with the pulsar wind.

Results. For reasonable parameters, the clumps will induce small variability on the X-ray and gamma-ray radiation. Sporadically, large clumps can reach closer to the pulsar increasing the magnetic field, triggering synchrotron X-ray flares and weakening other emission components like inverse Compton. The reduction of the emitter size induced by clumps also makes non-radiative losses faster. Stellar wind clumps can also enhance instability development and matter entrainment in the shocked pulsar wind when it leaves the binary. Growth limitations of the clumps from the wind acceleration region may imply that a different origin for the largest clumps is required. The large-scale wind structures behind the observed discrete absorption components in the ultraviolet may be the source of these large clumps.

Conclusions. The presence of structure in the stellar wind can produce substantial energy-dependent variability and thus should not be neglected when studying the broadband emission from high-mass binaries hosting young pulsars.