Spreading of Thermonuclear Flames on the Neutron Star in SAX J1808.4-3658: An Observational Tool

We analyse archival Rossi X-Ray Timing Explorer (RXTE) proportional counterarray (PCA) data of thermonuclear X-ray bursts from the 2002 outburst of theaccreting millisecond pulsar SAX J1808.4-3658. We present evidence ofnonmonotonic variations of oscillation frequency during burst rise, andcorrelations among the time evolution of the oscillation frequency, amplitude,and the inferred burning region area. We also discuss that the amplitude andburning region area evolutions are consistent with thermonuclear flamespreading on the neutron star surface. Based on this discussion, we infer thatfor the 2002 Oct. 15 thermonuclear burst, the ignition likely occured in themid-latitudes, the burning region took ~ 0.2 s to nearly encircle theequatorial region of the neutron star, and after that the lower amplitudeoscillation originated from the remaining asymmetry of the burning front in thesame hemisphere where the burst ignited. Our observational findings andtheoretical discussion indicate that studies of the evolution of burstoscillation properties during burst rise can provide a powerful tool tounderstand thermonuclear flame spreading on neutron star surfaces under extremephysical conditions.Comment: 12 pages, 1 table, 2 figures, accepted for publication in ApJ Letter