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Burst oscillations during the rising phases of thermonuclear X-ray bursts areusually believed to originate from flame spreading on the neutron star surface.However, the decrease of fractional oscillation amplitude with rise time, whichprovides a main observational support for the flame spreading model, have sofar been reported from only a few bursts. Moreover, the non-detection andintermittent detections of rise oscillations from many bursts are not yetunderstood considering the flame spreading scenario. Here, we report thedecreasing trend of fractional oscillation amplitude from an extensive analysisof a large sample of Rossi X-ray Timing Explorer Proportional Counter Arraybursts from ten neutron star low-mass X-ray binaries. This trend is 99.99%significant for the best case, which provides, to the best of our knowledge, byfar the strongest evidence of such trend. Moreover, it is important to notethat an opposite trend is not found from any of the bursts. The concave shapeof the fractional amplitude profiles for all the bursts suggestslatitude-dependent flame speeds, possibly due to the effects of the Coriolisforce. We also systematically study the roles of low fractional amplitude andlow count rate for non-detection and intermittent detections of riseoscillations, and attempt to understand them within the flame spreadingscenario. Our results support a weak turbulent viscosity for flame spreading,and imply that burst rise oscillations originate from an expanding hot spot,thus making these oscillations a more reliable tool to constrain the neutronstar equations of state.

Evidence of thermonuclear flame spreading on neutron stars from burst rise oscillations