Master thesis topics

Title: From continuous-loss to catastrophic-loss approximation treatment of electromagnetic cascades: Approaching the truth

Keywords: Monte Carlo, cosmic rays, high energy photons, pair production

Abstract: 

If high-energy electrons (or/and high-energy photons) run into a high-density high-energy photon field, electromagnetic cascades are unavoidably initiated. Here electrons (and/or positrons) loose energy by inverse Compton scattering thereby increasing the incoming photon's energy significantly. These photons in turn suffer from photon-photon pair production in the high-energy radiation field, thus producing high-energy lepton-pairs in turn. In both, the latter process as well as the Klein-Nishina regime of inverse Compton scattering, a significant amount of the original photon/electron is lost (called "catastrophic losses"), whereas in most pair cascade codes using the transport equation the so-called "continuous-loss approximation" (i.e., the fractional energy loss per interaction of the incoming particle is small) is used. This works aims at high-lighting spectral effects between the "catastrophic-loss" and continuous-loss" treatment in pair cascades, and subsequently searches for corresponding correction terms for the transport equation.

For this work photon-photon pair production as well as inverse Compton scattering Monte Carlo codes are available.

Title: High-energy emission from heavy nuclei interactions in colliding wind binaries

Keywords: Massive stars, winds, cosmic rays, binary system, acceleration

Abstract: Massive star (systems) at various evolutionary stages are considered as prime candidate sources contributing to the cosmic-ray flux below the so-called "knee" of the cosmic-ray spectrum. During the so-called "Wolf-Rayet" (WR) phase massive stars show prominent high mass-loss rate supersonic winds, and are often found in binary (or multiple) star systems. Further, the wind composition is often enriched in heavy elements (mostly C, N, O). In such binary systems these winds collide to form a contact discontinuity at the location where their winds ram pressure balances, and shocks form. This allows wind particles to get accelerated via e.g., diffusive shock acceleration, when entering the shocks. While many of such WR-binary systems emit synchrotron radiation in the radio band (proving that electron acceleration to relativistic energies takes place here), so far only two of these have also been detected at gamma-ray energies (Eta Carinae, Gamma^2 Vel). These gamma rays have been proposed (e.g., Reitberger et al 2017) to be produced (in parts) by accelerated nucleons interacting inelastically with the wind material. In turn, neutral pions (amongst others) are produced in such nuclear interactions which decay to contribute to the gamma-ray flux, while the decay products of the charged pions contribute to the neutrino all-sky flux. So far these nuclear interactions used simple scaling laws to take into account heavier elements in their target/projectile. This master thesis work aims to improve on this situation by using a more realistic interaction model (e.g., the Monte Carlo event generator DPMJET III (Roesler et al 2001)). Gamma-ray and neutrino source spectra for a series of target/projectile combinations realized in massive star winds shall be calculated from the interaction model, and compared to corresponding results from the "scaling approximation". Further, these source spectra shall be implemented in a colliding wind emission model (Reimer et al 2006), which in turn allows modeling WR-binary systems at high-energies, and providing predictions for their neutrino emission.

References:

Reimer, A., Pohl, M., Reimer, O., 2006, ApJ, 644, 1118-1144

Reitberger, K., Kissmann, R., Reimer, A., 2017, ApJ, 847, 40-55 Roesler, S., Engel, R., Ranft, J., in: Advanced Monte Carlo for radiation physics, particle transport simulations and applications, Springer 2001, pp 1033-1038

Roesler, S., Engel, R., Ranft, J., in: International Cosmic Ray Conference, 2, 2001, p 438