An investigation into the fraction of particle accelerators among colliding-wind binaries

Abstract

Particle-accelerating colliding-wind binaries (PACWBs) are multiple systems made of early-type stars able to accelerate particles up to relativistic velocities. The relativistic particles can interact with different fields (magnetic or radiation) in the colliding-wind region and produce non-thermal emission. In many cases, non-thermal synchrotron radiation might be observable and thus constitute an indicator of the existence of a relativistic particle population in these multiple systems. To date, the catalogue of PACWBs includes about 40 objects spread over many stellar types and evolutionary stages, with no clear trend pointing to privileged subclasses of objects likely to accelerate particles. This paper aims at discussing critically some criteria for selecting new candidates among massive binaries. The subsequent search for non-thermal radiation in these objects is expected to lead to new detections of particle accelerators. On the basis of this discussion, some broad ideas for observation strategies are formulated. At this stage of the investigation of PACWBs, there is no clear reason to consider particle acceleration in massive binaries as an anomaly or even as a rare phenomenon. We therefore consider that several PACWBs will be detected in the forthcoming years, essentially using sensitive radio interferometers which are capable of measuring synchrotron emission from colliding-wind binaries. Prospects for high-energy detections are also briefly addressed.

Megahertz emission of massive early-type stars in the Cygnus region

“…In line with discussions in De Becker et al (2017), late-type objects are underrepresented in the catalogue of PACWBs. This comes from the lower kinetic power available in weaker wind systems, resulting in less available energy to feed NT processes in the CWR.…”

“…This is especially worth asking given the spectral classification of the star, indicating a rather weak wind in terms of kinetic power. As emphasised by De Becker et al (2017), the class of PACWBs identified so far is mainly populated by objects with stronger winds, even though the detailed requirements for efficient particle acceleration still deserve to be established. Given the above inferences from the reported separation of the known components of the system, the radio detection presented here can be indicative of a third member of the system, closer to one of the stars.…”

Non-thermal X-rays from colliding wind shock acceleration in the massive binary Eta Carinae

“…The collision of strong stellar winds in massive binary systems creates powerful shocks, which have been expected to produce high-energy cosmic-rays through Fermi acceleration at the shock interface. The accelerated particles should collide with stellar photons or ambient material, producing non-thermal emission observable in X-rays and γ-rays [3,4]. The supermassive binary star η Car drives the strongest colliding wind shock in the solar neighborhood [5,6].Observations with non-focusing high-energy observatories indicate a high energy source near η Car, but have been unable to conclusively identify η Car as the source because of their rela-of the non-thermal source in the extremely hard X-ray band, which is found to be spatially coincident with the star within several arc-seconds.…”

The high-energy emission from HD 93129A near periastron

“…If more CWBs are confirmed as non-thermal emitters at high energies, it will be possible to assess the role of CWBs as cosmic ray injectors. Moreover, it will give compelling support to the idea that massive binary systems are frequently γray emitters (Benaglia & Romero 2003;De Becker et al 2017).…”

“…Unfortunately, it is not possible to directly assess the efficiency of cosmic ray acceleration from observations. The study of massive colliding-wind binaries (CWBs) is key in understanding the non-thermal physics taking place in systems harbouring massive stars (De Becker et al 2017). The signature of relativistic particles is a non-thermal spec-E-mail: sdelpalacio@iar.unlp.edu.ar (IAR) trum, typically a power-law with additional features such as a spectral break or an exponential cutoff.…”