### Question --- Describe the basic features of the "fireball" model of gamma-ray burst afterglows. What is the emission mechanism? What inputs are required? ### Answer --- ##### Describe the basic features of the "fireball" model of gamma-ray burst afterglows. What is the emission mechanism? When a [[Gamma Ray Burst|GRB]] is released from some compact object, it will leave a multi-wavelength (x-ray $\to$ radio) "afterglow" in the shocked medium. These afterglows allow us to better localize the source of the [[Gamma Ray Burst|GRB]] using various EM wavelengths *(many that have better angular resolution (smaller $\theta_{\rm min}$ , see [[Question 156]]) than radio)*. **Fireball Model**: ![[GRB_fireball.png|align:center|600]] ![[GRB_mechanism.jpg|align:center|600]] 1. Collimated and relativistic jet forms and is launched from a [[Stellar Explosions#Type Ic-BL|Type Ic-BL]] or a [[Stellar Explosions#Kilonova|Kilonova]] - Potentially related to the magnetic field prior to emission, but jet launching is highly certain and an area of active research 2. Jet shocks the surrounding medium and sweeps up this material as it expands 3. The gamma-ray photons (defining the GRB) come from "internal shocks" within the jet itself. - Gamma-rays are produced in [[Synchrotron Radiation|synchrotron radiation]] created by the shock-accelerated electrons - These photons are further [[Inverse-Compton scattering|inverse-Compton]] scattered to higher energies. - The "fireball" is opaque and ultrarelativistic at early times, but expands and eventually becomes transparent to gamma-rays in the ultra-hot plasma 4. Eventually jet collides with and shocks the [[Interstellar Medium|ISM]] ("external shocks"). - Remaining kinetic energy is dissipated here, producing lower-energy emission at later times, yielding a broad EM spectrum in the afterglow. 5. The shock front expands and picks up materials, losing energy along the way - This leads to an increased observing angle of the shock front and decrease in the observed emission/flux. 6. The afterglow emission last for ~ days - weeks - Eventually, we will see an achromatic break in the light curve with the shock front expands enough ![[GRB_shockfront_expansion.png|align:center]] ##### What inputs are required? Mostly unknown, but we need some energetic accelerating mechanism can produce an ultrarelativistic jet. - Mostly originating from the central engine (i.e. [[Stellar Explosions#Type Ic-BL|Type Ic-BL]] or [[Stellar Explosions#Kilonova|Kilonova]])).