Master Projects with Prof. Mertsch in 2018/19


Project 1: The Fermi gamma-ray bubbles and jets from the Galactic centre

The Fermi bubbles are two giant, bi-lobular structures seen in the gamma-ray sky above 1 GeV. Their source might the supermassive black hole at the Galactic centre that could have actively accreted matter some million years ago and produced jets. Alternatively, episodes of increased star formation and supernova activity in the Galactic centre region could have produced large-scale outflows. Modelling of the high-energy emission from the bubbles will provide crucial information on the origin of the Fermi bubbles.

In this project, you will investigate the acceleration of high-energy particles in the jet and the transport throughout the Fermi bubbles. You will start by getting acquainted with the basics of acceleration and transport processes, the observational status on the Fermi bubbles and numerical techniques. Next, you will write your own code using stochastic differential equations, a technique that has never been used in this context. Comparing with observations from radio to gamma-ray energies you should be able to answer the question whether jets from the supermassive black hole can be the origin of the Fermi bubbles.

Project 2: The streaming instability and a cosmic ray conspiracy

Dark matter indirect detection, that is the search for signs of dark matter annihilation in cosmic rays, gamma-rays and neutrinos, requires good control over the distribution of charge particles in the Galaxy. Their transport is mostly a diffusion process, mediated by the interaction with turbulent magnetic fields. This turbulence can be generated by the cosmic rays themselves through the streaming instability in regions close to their sources. This would confine cosmic rays for extended periods of time and could possibly also explain a peculiar conspiracy: The spectra of protons, anti-protons and positrons are remarkably similar between 10 and 300 GeV while their sources and their transport should be very different.

You will start by learning about particle-wave interactions and the standard picture of transport without the streaming instability. Next, you will explore a number of approximations to the picture allowing for (semi-)analytical solution of the underlying set of equations. Ultimately though, the level of complexity will likely require a numerical approach and you will compute the particle spectra close to their source and after propagation. Is this model able to explain why the different particle spectra are so similar? What implications does this hold for dark matter indirect searches?

Project 3: Diffuse Galactic emission at 100 TeV as seen by HAWC

Despite more than 100 years of progress, the source of cosmic rays have not been unambiguously identified. Particularly pressing is the question of what and where the sources of the highest energy Galactic sources are. These must be accelerating cosmic rays to a few times 1015 GeV, far above the energy of terrestrial collides. Colliding with the gas in their vicinity, cosmic rays would produce gamma-rays at hundreds of TeV which are currently searched for with the HAWC gamma-ray experiment in Mexico. Theoretical predictions for large-scale diffuse emission are sparse as the transport picture that is well-tested at GeV energies needs to be adjusted and extended.

You will start with a review of the modelling of Galactic diffuse emission at GeV energies, including the underlying physical processes and the phenomenological problems. Next, you will consider effects that become more relevant at higher energies, like a smaller number of sources contributing, anisotropic diffusion and drifts in the inhomogeneous magnetic fields. We will likely focus on a few good candidate source regions, but also ask whether there is emission on much larger angular scales.