We report the detection of pulsations at 552 Hz in the rising phase of two type-I ( thermonuclear ) X-ray bursts observed from the accreting neutron star EXO 0748 - 676 in 2007 January and December , by the Rossi X-ray Timing Explorer .
The fractional amplitude was 15 % ( rms ) .
The dynamic power density spectrum for each burst revealed an increase in frequency of \approx 1 –2 Hz while the oscillation was present .
The frequency drift , the high significance of the detections and the almost identical signal frequencies measured in two bursts separated by 11 months , confirms this signal as a burst oscillation similar to those found in 13 other sources to date .
We thus conclude that the spin frequency in EXO 0748 - 676 is within a few Hz of 552 Hz , rather than 45 Hz as was suggested from an earlier signal detection by ( 27 ) .
Consequently , Doppler broadening must significantly affect spectral features arising from the neutron star surface , so that the narrow absorption features previously reported from an XMM-Newton spectrum could not have arisen there .
The origin of both the previously reported 45 Hz oscillation and the X-ray absorption lines is now uncertain .