2008ApJ...672L..37S

We report the discovery of burst oscillations at 414.7 Hz during a thermonuclear X-ray burst from the low-mass X-ray binary (LMXB) 4U 0614+091 with the Burst Alert Telescope (BAT) on board Swift. In a search of the BAT archive, we found two burst triggers consistent with the position of 4U 0614+091. We searched both bursts for high-frequency timing signatures and found a significant detection at 414.7 Hz during a 5 s interval in the cooling tail of the brighter burst. This result establishes the spin frequency of the neutron star in 4U 0614+091 as ~415 Hz. The oscillation had an average amplitude (rms) of 14%. These results are consistent with those known for burst oscillations seen in other LMXBs. The inferred ratio of the frequency difference between the twin kHz quasi-periodic oscillations (QPOs) and the spin frequency, Δν/ν_s, in this source is strongly inconsistent with either 0.5 or 1 and tends to support the recent suggestions by Yin et al. and Mendez & Belloni that the kHz QPO frequency difference may not have a strong connection to the neutron star spin frequency.