Date:
Tue, 29/05/201812:30-13:30
Title: GRB Jets: From Acceleration to Dissipation to Radiation
Abstract:
The highly luminous and variable prompt emission in gamma-ray bursts (GRBs) arises in an ultra-relativistic outflow. The exact underlying radiative mechanism shaping its non-thermal spectrum is still uncertain, which makes it hard to determine the outflows's bulk Lorentz factor (LF). Observations of high-energy spectral cutoffs due to pair-production can prove to be very useful in determining the LF. However, Compton downscattering by electron-positron pairs plays an important role in setting the energy of the spectral cutoff in emitting regions with high compactness.
To understand the prompt emission mechanism, first one has to understand the composition of the outflow, which then dictates the energy dissipation mechanism that leads to radiation. In Poynting flux dominated jets the energy reservoir is the magnetic field, however, how its energy is dissipated in the flow is unclear. If the relativistic outflow resembles a striped wind, then magnetic energy can be dissipated by magnetic reconnection, where this process may be facilitated by the Kruskal-Schwarzchild instability (KSI). Interestingly, the KSI can be self-sustaining, whereby its dissipation causes acceleration of the flow which yields further dissipation.
In this talk, I'll present a coherent picture of GRB jets, i.e. from acceleration to dissipation in the outflow to radiation, while showing some new results. I'll argue that Compton downscattering by pairs is an important effect when considering spectral cutoffs. Ignoring it can under-predict the LF by as much as an order of magnitude. In regards the KSI, I'll show results from a 2D ideal-MHD simulation and argue that, at least in the 2D case, the efficiency of the KSI is limited by the Kelvin-Helmholtz instability. However, 3D and resistive MHD simulations can bring further insight.
Abstract:
The highly luminous and variable prompt emission in gamma-ray bursts (GRBs) arises in an ultra-relativistic outflow. The exact underlying radiative mechanism shaping its non-thermal spectrum is still uncertain, which makes it hard to determine the outflows's bulk Lorentz factor (LF). Observations of high-energy spectral cutoffs due to pair-production can prove to be very useful in determining the LF. However, Compton downscattering by electron-positron pairs plays an important role in setting the energy of the spectral cutoff in emitting regions with high compactness.
To understand the prompt emission mechanism, first one has to understand the composition of the outflow, which then dictates the energy dissipation mechanism that leads to radiation. In Poynting flux dominated jets the energy reservoir is the magnetic field, however, how its energy is dissipated in the flow is unclear. If the relativistic outflow resembles a striped wind, then magnetic energy can be dissipated by magnetic reconnection, where this process may be facilitated by the Kruskal-Schwarzchild instability (KSI). Interestingly, the KSI can be self-sustaining, whereby its dissipation causes acceleration of the flow which yields further dissipation.
In this talk, I'll present a coherent picture of GRB jets, i.e. from acceleration to dissipation in the outflow to radiation, while showing some new results. I'll argue that Compton downscattering by pairs is an important effect when considering spectral cutoffs. Ignoring it can under-predict the LF by as much as an order of magnitude. In regards the KSI, I'll show results from a 2D ideal-MHD simulation and argue that, at least in the 2D case, the efficiency of the KSI is limited by the Kelvin-Helmholtz instability. However, 3D and resistive MHD simulations can bring further insight.