Gamma-ray bursts are the brightest explosions in the universe. During a brief period of time, typically lasting seconds to minutes, they emit as much energy, in gamma-rays, as the Sun does during its entire life time. The gamma-ray emission originates from a so called jet, which is a collimated beam of plasma travelling close to the speed of light. The jets are believed to form when massive stars explode and when compact objects merge. Understanding gamma-ray bursts has implications for a range of important topics, including black hole formation and particle acceleration. Since they can be seen at great distances, gamma-ray bursts may also serve as excellent probes of the early universe. A long-standing problem in the field is to understand the radiative processes that give rise to the gamma-ray emission. An answer to this question is needed in order to be able to derive many of the most important physical properties of the systems. In the past progress has been hindered by lack of physical models with which to analyse data. We have recently made a significant advancement in this respect by using a code that treats all the relevant radiation processes in a relativistic outflow. Specifically, we have run the code for a range of input parameters, thus producing a grid of spectra corresponding to different physical conditions. We then fit observational data of gamma-ray bursts by interpolating in this grid. Our initial results (presented in Ahlgren at al., 2015, MNRAS, 454, 31) showed excellent fits as well as good constraints on physical parameters.
Using the allocations over the last two years we have significantly improved the model by extending the parameter space and examining different physical scenarios. The model has been used to analyse a large sample of gamma-ray bursts (Ahlgren et al., 2018, MNRAS, under review), providing novel physical constraints on the jets. An early version of this work has also been published in the Licentiate thesis by B. Ahlgren (KTH 2016). With our latest allocation we also begun the development of a third and further refined model, which we have started to test on data.
With this proposal the goal is to complete the third incarnation of our model, which we plan to make public to the community, and which will constitute a major part of the PhD thesis by B. Ahlgren. The new version of the model includes significant improvements, particularly a more extensive treatment of photon-electron interactions towards the end of the simulations. This version also introduces the magnetic field strength as a free parameter. What remains to be done is to fill in the grid and perform additional convergence tests.