Are GRB jets dominated by magnetic energy or hot matter?
In plain words
The jet may carry its energy mainly as magnetic field or as hot gas and light; the two pictures predict different signals.
Precise statement
Determine the magnetization $\sigma = B'^{2}/(4\pi\rho' c^{2})$, with B' the comoving magnetic field and rho' the comoving rest-mass density, of GRB jets at the dissipation radius $r \sim 1e13 \text{ to } 1e16\,\mathrm{cm}$, distinguishing Poynting-dominated ($\sigma >> 1$) from fireball ($\sigma << 1$) jets, using the strength of a thermal photospheric component, prompt gamma-ray polarization degree and early reverse-shock optical emission. Answer: $\sigma$ at the emission radius for the bulk of bursts, with evidence.
What would settle it
Time-resolved prompt gamma-ray polarimetry on a sample of bright bursts combined with early optical reverse-shock measurements.