Cosmic Rays in super Bubbles – CRiBs

In order to gain insight into superbubble activities, we have combined our theoretical expertise on cosmic-ray transport and acceleration (in the German Bochum group) and our observational expertise on the interstellar medium and cosmic rays (in the French Saclay group) to compare the cases of the compact, few-million-year old, bursting Cygnus X bubble and the broader, ten-million-year-old Orion-Eridanus superbubble. We have taken advantage of the proximity of the latter, at about a thousand light years from the Sun, to detail its gas, magnetic field, and cosmic-ray content. To do so, we have exploited ?-ray observations of cosmic rays and multi-wavelength (radio to ?-ray) observations of the interstellar medium to get a comprehensive view of the composite structure of the superbubble. In parallel, we have developed a model to quantify how much energy cosmic rays penetrating into the superbubble would gain by travelling through the enhanced level of magnetohydrodynamic turbulence inside the bubble.

According to the theoretical calculations, the stellar and interstellar conditions inside the Orion-Eridanus superbubble should favour efficient cosmic-ray acceleration or reacceleration, but we have surprisingly found no ?-ray evidence for such activity. Compared to the case of Cygnus X, such an inefficiency is puzzling. The difference may relate to the supernova rate in Orion-Eridanus where they are less frequent and more scattered than in the Cygnus X region which contains one of the most massive stellar clusters of the Milky Way.
Another highlight is the evidence of a loss of cosmic rays in a cloud located just outside the Orion-Eridanus superbubble, along magnetic field lines that point toward the halo of the Galaxy. It is the first time measurements of the cosmic-ray flux and magnetic field have been compared in the same cloud. It opens the possibility to investigate the role of gas fountains and of their magnetic-field loops in cosmic-ray escape to the halo.

This work has triggered stimulating discussions at the Cosmic Ray International Conference. It calls for several follow-up studies, both observational and theoretical, for instance to extend the analyses to the youngest part of the Orion-Eridanus superbubble to study age effects, to study of other well-resolved superbubbles in gamma rays to broaden the sample beyond the only two cases studied so far, the joint cosmic-ray and magnetic-field study of other nearby clouds and fountains, calculations of reaccelerated cosmic-ray spectra for different types of MHD turbulence as we have shown that its wavenumber slope can significantly impact the acceleration efficiency in the weakly magnetized plasma of a superbubble.

The collaboration was very fruitful and led to the publication of 6 articles in leading international journals, several presentations in international conferences, and the completion of one bachelor thesis, one master thesis, and two PhD theses, as well as significant contributions to two other PhD theses.

A century-long standing problem in astrophysics is to explain how cosmic rays are accelerated to relativistic energies and how they diffuse in their host galaxy. Notable progress has been made on their production in supernova remnants, but the recent discovery in gamma rays, with the Fermi satellite, of a cocoon of fresh and energetic cosmic rays in the Cygnus X superbubble has disclosed an important new facet of the problem: what is the impact on particle (re-)acceleration and diffusion of the large level of turbulence generated in the superbubble medium by the numerous massive stars? Can this turbulent phase significantly modify our current views on cosmic-ray transport in the Galaxy because most cosmic-ray sources are to be found in active star-forming regions?
In this project, we propose to compare two superbubbles, the few-million-year old, bursting Cygnus X bubble and the older, less energetic Orion-Eridanus superbubble near the Sun. We propose to combine our expertise on cosmic-ray transport/acceleration, on gamma-ray observations of cosmic rays, and on multi-wavelength observations of the interstellar conditions prevailing in these bubbles, in order to test a new acceleration mechanism and to revisit the impact of the turbulent bubble medium on different observational cosmic-ray diagnostics, Galactic-wide and locally in the Local Superbubble.