Searching for the origin of the excess of cosmic rays observed in the Galactic centre region. Supernovae, stellar winds, or the Galactic centre black hole?

Cosmic rays are energetic particles that reach the Earth from outer space. Most cosmic rays are accelerated within our Galaxy, and remain trapped in the Galactic magnetic field for about 10 million years before escaping in the intergalactic space.

Cosmic rays interact with the matter that constitutes the interstellar medium. Cosmic rays characterised by particle energies in the MeV domain ionise the interstellar gas, while particles in the GeV and TeV domain (or above) collide with interstellar matter producing neutral pions that then decay into gamma rays. Therefore, gamma-ray observations, as well as measurements of the ionisation rate of the interstellar gas can be used to infer the intensity of the cosmic radiation throughout the Galaxy. The picture emerging from observations reveals that the intensity of cosmic rays is roughly constant at any location in the Galaxy. However, data from the Galactic centre region are puzzling.

On one side, gamma-ray observations of a 200 pc region surrounding the centre of the Galaxy show that the intensity of GeV and TeV cosmic rays in that region is slightly enhanced (a factor of a few) with respect to the local one. On the other hand, the ionisation rate measured in the Galactic centre region largely exceeds the local one, suggesting that the intensity of MeV cosmic rays must largely exceed (a factor of 100 or even more!) the local one.

Why is the intensity of MeV cosmic rays so large in the Galactic centre region, while that of GeV and TeV cosmic rays is not?

In order to reply to this question one should remember that the Galactic centre region is a very peculiar one. First of all, it harbours a super-massive black hole (of mass equal to several millions of solar masses) that accretes matter and undergoes occasionally through periods of intense activity. Recent gamma-ray observations suggests that the black hole could have acted, in the recent past, as a powerful particle accelerator. In addition to that, in the region surrounding the central black hole, stars are formed at a very fast rate, and the resulting enhanced rate of supernova explosions might also be accompanied by the acceleration of a very large number of energetic particles. Finally, both X-ray and gamma-ray observations indicates that matter must have been ejected in the past from the Galactic centre region. It is still a mystery whether this outflow of matter is due to the activity of the black hole or rather is a consequence of the intense star formation in the region. Since cosmic rays are attached to matter via the magnetic field, the presence of such powerful outflows will certainly affect their behaviour.

The thesis will be devoted to the development of models for the acceleration and transport of cosmic ray particles in the Galactic centre region. Different scenarios will be considered, including the acceleration of particles at the supermassive black hole, or at supernova remnant shocks.  The transport of particles in the very complex environment surrounding the black hole will also be modelled. The final goal is to search for a scenario that may accommodate all the observational constraints available (in particular in the X-ray and gamma-ray domain) and explain the puzzling difference in the behaviour of low and high energy cosmic rays.

Requirements: Master in Astrophysics, Astroparticle Physics, or related topic. Interest in both theoretical studies and in observations (e.g. fitting model results to existing data). Possibility to be involved in some data analysis, especially concerning the X-ray observations of the region surrounding the black hole.

Practicalities: the thesis work will take place at APC (Paris 7 campus), under the supervision of Stefano Gabici (thesis supervised or co-supervised: 4) and Andrea Goldwurm (thesis supervised or co-supervised: 7). The team of scientists involved in this research includes Anne Lemiere and Regis Terrier.

Funding: 50% funded on LabEx grant


Recent (past 5 years) relevant publications authored by the supervisors

[1] G. Ponti, et al., An X-ray chimney extending hundreds of parsecs above and below the Galactic Centre, Nature (2019) 567, 347

[2] S. Gabici et al., The origin of Galactic cosmic rays: challenges to the standard paradigm, Int. J. Mod. Phys. D (2019) 1930022

[3] D. Chuard, et al., Glimpses of the past activity of Sgr A inferred from X-ray echoes in Sgr C, A&A (2018) 610, A34

[4] V.H.M. Phan, G. Morlino, S. Gabici, What causes the ionization rates observed in diffuse molecular clouds? The role of cosmic ray protons and electrons, MNRAS (2018) 480, 5167

[5] H.E.S.S. Collaboration, Acceleration of petaelectronvolt protons in the Galactic Centre, Nature (2016) 531, 476


Stefano Gabici, Andrea Goldwurm






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