Rarefaction acceleration of ultrarelativistic magnetized jets in gamma-ray burst sources

TL;DR

This paper investigates how magnetic energy in ultrarelativistic gamma-ray burst jets is rapidly converted into kinetic energy during sideways expansion, revealing a rarefaction acceleration mechanism that explains observed jet behaviors.

Contribution

It identifies and confirms a novel rarefaction acceleration process in relativistic jets, linking magnetic geometry to efficient energy conversion during jet expansion.

Findings

Rarefaction acceleration occurs during jet expansion after progenitor star exit.

Magnetic energy converts into kinetic energy via strong magnetosonic waves.

This process allows jet Lorentz factors to significantly exceed previous limits.

Abstract

When a magnetically-dominated super-fast magnetosonic GRB jet leaves the progenitor star the external pressure support may drop and the jet may enter the regime of ballistic expansion during which its magnetic acceleration becomes highly ineffective. However, recent numerical simulations suggested that the transition to this regime is accompanied by a sudden "burst" of acceleration. We confirm this finding and attribute the acceleration to the sideways expansion of the jet - the magnetic energy is converted into the kinetic one in the strong magnetosonic rarefaction wave, which is launched when the jet loses its external support. This type of acceleration, the rarefaction acceleration, is specific to relativistic jets because their energy budget can still be dominated by magnetic energy even in highly super-fast magnetosonic regime. Just like the collimation acceleration of externally confined magnetized jets, it is connected with the geometry of magnetic flux sufaces. In both cases, in the acceleration zone the poloidal field lines diverge faster than in the monopolar configuration. On the other hand, whereas the collimation acceleration keeps the product of jet opening angle and Lorentz factor somewhat below unity, the rarefaction acceleration allows to make it significantly larger, in agreement with the standard model of jet breaks in afterglow light curves.