Discovery of the Spin Frequency of 4U 0614+09 with Swift/BAT

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) onboard 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 QPOs, and the spin frequency, ∆ν/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. Subject headings: stars: neutron—stars: rotation—stars: oscillations—X-rays: bursts—X-rays: binaries–X-rays: individual (4U 0614+091)