  Thermonuclear runaway burning of carbon is in rare cases observed from
accreting neutron stars as day-long X-ray flares called superbursts. In the few
cases where the onset is observed, superbursts exhibit a short precursor burst
at the start. In each instance, however, the data was of insufficient quality
for spectral analysis of the precursor. Using data from the propane
anti-coincidence detector of the PCA instrument on RXTE, we perform the first
detailed time resolved spectroscopy of precursors. For a superburst from 4U
1820-30 we demonstrate the presence of photospheric radius expansion. We find
the precursor to be 1.4-2 times more energetic than other short bursts from
this source, indicating that the burning of accreted helium is insufficient to
explain the full precursor. Shock heating would be able to account for the
lacking energy. We argue that this precursor is a strong indication that the
superburst starts as a detonation, and that a shock induces the precursor.
Furthermore, we employ our technique to study the superexpansion phase of the
same superburst in greater detail.
