ASTRO
In the literature: contested
Millisecond magnetar or accreting black hole as GRB engine?
In plain words
The engine could be a fast-spinning, highly magnetized neutron star or a newborn black hole swallowing a disk; each leaves different marks in the X-ray afterglow.
Precise statement
For long and short GRBs, determine whether the engine is a millisecond proto-magnetar (rotational energy at most $\sim 2e52\ \mathrm{erg}$, X-ray plateau followed by $L\sim t^{-2}$ decay) or a hyperaccreting black hole (Blandford-Znajek power with fall-back accretion L ~ t^-5/3). Answer: per-burst engine classification with energy budget, using X-ray plateaus, extended emission and late flares.
What would settle it
Beaming-corrected jet energies above $2e52\,\mathrm{erg}$ excluding magnetars for a burst subclass, and gravitational-wave detection of the post-merger remnant for short bursts.