GRB 060218: The nature of the optical-UV component

TL;DR

This paper models the optical-UV emission in GRB 060218 as optically thick cyclotron radiation, linking it to the thermal X-ray component and suggesting a magnetically dominated outflow possibly powered by a magnetar.

Contribution

It proposes a novel model connecting optical-UV and thermal X-ray emissions through a photosphere in a quasi-spherical outflow, highlighting the role of magnetic fields and low Lorentz factors.

Findings

Optical-UV emission is consistent with optically thick cyclotron radiation.

The outflow's magnetic field is near equipartition, indicating magnetic importance.

GRB 060218's properties are similar to ordinary GRBs but with lower Lorentz factor.

Abstract

The optical-UV component in GRB 060218 is assumed to be due to optically thick cyclotron emission. The key aspect of this model is the high temperature of the absorbing electrons. The heat input derives from nuclei accelerated in semi-relativistic internal shocks, like in ordinary gamma-ray bursts. Coulomb collisions transfer part of that energy to electrons. Inverse Compton cooling on the X-ray photons leads to electron temperatures around 100 keV. Such a high brightness temperature for the optical-UV emission implies an emitting area roughly equal to that of the thermal X-ray component. This suggests a model in which the radio, optical-UV and thermal X-ray emission are closely related: Although the optical-UV and thermal X-ray emission are two separate spectral components, it is argued that they both come from the photosphere of a quasi-spherical, continuous outflow, whose interaction with the circumstellar medium gives rise to the radio emission. The properties of GRB 060218, as measured in the co-moving frame, are similar to those of ordinary gamma-ray burst; i.e., the main difference is the much lower value of the bulk Lorentz factor in GRB 060218. The cyclotron absorption implies a magnetic field in rough equipartition with the matter energy density in the outflow. Hence, the magnetic field could have a dynamically important role, possibly with a magnetar as the central engine.