We report the discovery of a highly coherent oscillation in a type-I X-ray burst observed from 4U 1916 - 053 by the Rossi X-ray Timing Explorer ( RXTE ) .
The oscillation was most strongly detected \approx 1 s after the burst onset at a frequency of 269.3 Hz , and it increased in frequency over the following 4 seconds of the burst decay to a maximum of \simeq 272 Hz .
The total measured drift of 3.58 \pm 0.41 Hz ( 1 \sigma ) represents the largest fractional change in frequency ( 1.32 \pm 0.15 % ) yet observed in any burst oscillation .
If the asymptotic frequency of the oscillation is interpreted in terms of a decoupled surface burning layer , the implied neutron star spin period is around 3.7 ms .
However , the expansion of the burning layer required to explain the frequency drift during the burst is around 80 m , substantially larger than expected theoretically ( assuming rigid rotation ) .
The oscillation was not present in the persistent emission before the burst , nor in the initial rise .
When detected its amplitude was 6–12 % ( RMS ) with a roughly sinusoidal profile .
The burst containing the oscillation showed no evidence for photospheric radius expansion , while at least 5 of the other 9 bursts observed from the source by RXTE during 1996 and 1998 did .
No comparable oscillations were detected in the other bursts .
A pair of kilohertz quasi-periodic oscillations ( QPOs ) has been previously reported from this source with a mean separation of 348 \pm 12 Hz .
4U 1916 - 053 is the first example of a source where the burst oscillation frequency is significantly smaller than the frequency separation of the kHz QPOs .