  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.
