An Unusual Precursor Burst with Oscillations from SAX J1808.4−3658

We report the finding of an unusual, weak precursor to a thermonuclear X-rayburst from the accreting millisecond pulsar SAX J1808.4-3658. The burst inquestion was observed on Oct. 19, 2002 with the Rossi X-Ray Timing Explorer(RXTE) proportional counter array (PCA). The precursor began approximately 1 sprior 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. Oscillationsare not detected in the approximately 0.5 s interval between the precursor andthe main burst. The estimated peak photon flux and energy fluence of theprecursor are about 1/25, and 1/500 that of the main burst, respectively. Fromjoint spectral and temporal modeling, we find that an expanding burning regionwith a relatively low temperature on the spinning neutron star surface canexplain the oscillations, as well as the faintness of the precursor withrespect to the main part of the burst. We dicuss some of the implications ofour findings for the ignition and spreading of thermonuclear flames on neutronstars.Comment: 10 pages, 4 figures, accepted for publication in Ap