  We report the finding of an unusual, weak precursor to a thermonuclear X-ray
burst from the accreting millisecond pulsar SAX J1808.4-3658. The burst in
question was observed on Oct. 19, 2002 with the Rossi X-Ray Timing Explorer
(RXTE) proportional counter array (PCA). The precursor began approximately 1 s
prior to the onset of a strong radius expansion burst, lasted for about 0.4 s,
and exhibited strong oscillations at the 401 Hz spin frequency. Oscillations
are not detected in the approximately 0.5 s interval between the precursor and
the main burst. The estimated peak photon flux and energy fluence of the
precursor are about 1/25, and 1/500 that of the main burst, respectively. From
joint spectral and temporal modeling, we find that an expanding burning region
with a relatively low temperature on the spinning neutron star surface can
explain the oscillations, as well as the faintness of the precursor with
respect to the main part of the burst. We dicuss some of the implications of
our findings for the ignition and spreading of thermonuclear flames on neutron
stars.
