  Recent observations suggest that gamma-ray bursts (GRBs) and their afterglows
are produced by jets of highly relativistic cannonballs (CBs), emitted in
supernova explosions. The fully ionized CBs cool to a temperature below 4500 K
within a day or two, at which point electron-proton recombination produces an
intense Lyman-$\alpha$ emission. The line energy is Doppler-shifted by the CBs'
motion to X-ray energies in the observer's frame. The measured line energies,
corrected for their cosmological redshift, imply Doppler factors in the range
600 to 1000, consistent with those estimated -in the CB model- from the
characteristics of the $\gamma$-ray bursts. All other observed properties of
the lines are also well described by the CB model. Scattering and
self-absorption of the recombination lines within the CB also produce a
wide-band flare-up in the GRB afterglow, as the observations indicate. A very
specific prediction of the CB model is that the X-ray lines ought to be narrow
and move towards lower line energies as they are observed: their current
apparently large widths would be the effect of time integration, and/or of the
blending of lines from CBs with different Doppler factors.
