Gamma-ray bursts with Fermi: Jet dynamics around the saturation radius and the onset of external shock emission

This research programme sets out to solve one of the major mysteries of contemporary Astrophysics, namely the physics of gamma-ray bursts (GRBs).

The aim is to confirm and develop the new paradigm of GRB emission, in which the main ingredients is an initial, strong and efficient photosphere and a subsequent, external shock emission.It is an ambitious research programme.

However, by combining my experience of the physics of the gamma-ray burst photospheres with the excellent and large data set from the Fermi Gamma-ray Space Telescope, my group will have the optimal prerequisites to be able to solve these mysteries.

In particular, we have the state-of-the art description of (i) the physics of photopsheric emission around the saturation radius (Ryde et al. 2017) (ii) the physics of the radiation mediated shocks below the photosphere (Samuelsson, Lundman \& Ryde 2021).

Moreover, we are developing the framework to investigate synchrotron emission from interaction of the GRB jet with the circumburst medium, by using polarisation measurements (Sharma et al. 2019) and synchrotron modelling (Ryde et al. 2022).

We will further combine these advantages with the development of cutting-edge numerical programmes, describing new physical scenarios for emission from shocks in dense, relativistically moving media through a collaboration with experts at the Bar-Ilan University in Israel.I was one of the first to advocate the photospheric model to describe the prompt, gamma-ray emission in GRBs and have been leading the development of it in combination with the use of it in interpreting the observations.

My successful research programme, which has been supported by the Swedish National Space Board, has received wide recognition, culminating in the 2016 Göran Gustafsson Prize in Physics, which is awarded by the Royal Swedish Academy of Sciences (KVA).An understanding of the GRB emission is important since GRBs are the largest explosions in the universe and are connected with general stellar explosions, such as supernovae and neutron star mergers, as well as, since GRBs occur at large redshifts they can be used to study questions in cosmology and the epoch of reionization.

Solving these mysteries will open up new, powerful possibilities to use gamma-ray bursts as probes of the early Universe, and as probes of the physics of gravitational wave events and of exploding stars, such as their mechanisms, structures, and jet formations.

It will also give KTH, the Royal Institute of Technology, an internationally leading role in the new era of multimessenger astrophysics of transient events.