DETECTION OF COHERENT 7.6 HZ OSCILLATIONS DURING A BURST FROM AQUILA X-1

We report the results of timing and spectral analysis of the X-ray source Aql X-1 (X1908 + 005) using data obtained with the Einstein SSS and MPC instruments. A classic type I burst was observed from Aql X-1 in both detectors, and a coherent modulation with a period of 131.66 +/- 0.02 ms (7.6 Hz) and a pulsed fraction of 10% was detected in the SSS data (0.64-4.5 keV) during the period of enhanced emission. The signal has a random occurrence probability of less than 5 x 10(-5). The sensitivity of the MPC for high-resolution timing during the burst is greatly reduced because of the low-duty cycle (< 10%) during the burst, and the modulation is not detected. There is no evidence for a loss of coherence during the approximately 80 s when it is observable, giving f/DELTA-f > 600. The 2 sigma upper limit on the rate of change of the pulse period is 5 x 10(-5) s/s-1. We argue that an asymmetrical burst occurring on a neutron star rotating at 7.6 Hz offers a plausible explanation for the oscillation.