Michaël De Becker’s contributions mainly involve observation, especially in the area of high energies (X-rays and gamma rays). We have seen that relativistic electrons are capable of emitting synchrotron radiation in the radio range. However, they can also do something else: there is a process called Inverse Compton scattering, during which high-energy relativistic electrons transfer part of their energy to ultraviolet (UV) or visible low-energy photons. For this to be efficient there needs to be a lot of UV or visible photons in the environment of the electrons. This is very much the case because massive stars are the brightest stellar objects in these spectral domains. The relativistic electrons are therefore capable of interacting with the photons emitted by the stars of the binary system to up-scatter them to high energies, justifying the observations of Michaël De Becker, notably X-ray observations which were carried out in order to search for such emissions.

The particular case of HD195592

The binary system HD195592 presented in the article published in the journal Astronomy & Astrophysics (1), constitutes a variant of the process of cosmic particle acceleration. The hydrodynamic shock necessary in order to accelerate particles is caused by a runaway star in this case! A runaway star is a star that has been ejected from its birthplace. This star was born in a dense environment where many stars can form. In this type of environment, there is a lot of dynamical interaction and it is possible that a star or couple of stars were ejected, a little like the movement of a satellite which suddenly accelerates while passing near a planet. Another scenario could explain the flight of these stars: in a binary system, it is possible that one of the two stars explodes into a supernova and leaves its birthplace or is ejected from it.


A runaway star crosses the interstellar environment. Its stellar wind comes into contact with denser regions (the environment is not homogeneous) and it is here that the hydrodynamic shock necessary for the acceleration of particles takes place.   As the stellar wind extends quite far around the runaway star, the shock where the acceleration of particles takes place is a little further from the runaway star than in the case of most colliding-wind massive binaries. The UV radiation field experienced by the relativistic electrons is therefore less intense. This is undoubtedly insufficient to cause an Inverse Compton scattering effect. On the other hand, there is another source of radiation closer to the shock: in front of the shock there is a compression of matter with an increase in the temperature of the interstellar material in this zone. The dust that is present there then emits photons like any heated body, but in the infrared. It is these infrared photons which undergo the Inverse Compton scattering and are transformed into high-energy photons such as gamma rays.

(1) del Valle M., Romero G.E. & De Becker M., 2013, Astronomy & Astrophysics : Is the bowshock of the runaway massive star HD 195592 a Fermi source?