# Ultra-relativistic double explosions

## TL;DR

This paper analyzes the fluid dynamics of relativistic double explosions, revealing conditions for a second shock to form and its potential to enhance high-energy emission, with implications for astrophysical phenomena.

## Contribution

It introduces a self-similar model for the second shock in relativistic double explosions and explores its structure in different external environments.

## Key findings

- Second shock forms if second explosion energy is ≥ 1% of the first.
- Self-similar structure exists in constant density environments.
- Second shock can temporarily exceed primary shock's Lorentz factor, boosting high-energy emission.

## Abstract

We consider fluid dynamics of relativistic double explosion - when a point explosion with energy $E_1$ is followed by a second explosion with energy $E_2$ after time $t_d$ (the second explosion could be in a form of a long lasting wind). The primary explosion creates a self-similar relativistic blast wave propagating with \Lf\ $\Gamma_1(t)$. A sufficiently strong second explosion, with total energy $E_2 \geq 10^{-2} E_1$, creates a fast second shock in the external fluid previously shocked by the primary shock. At times longer than the interval between the explosions $t_d$, yet short compared with the time when the second shock catches up the primary shock at $\sim t_d \Gamma_1^2$, the structure of the second shock is approximately self-similar. Self-similar structure of the second shock exist for the case of constant external density (in this case $\Gamma_2 \propto t^{-7/3}$), but not for the wind environment. At early times the \Lf\ of the second shock may exceed that of the primary shock and may boost the synchrotron emission of locally accelerated electrons into the Fermi LAT range.