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

TL;DR

This paper reports the discovery of 414.7 Hz burst oscillations in the LMXB 4U 0614+091, establishing the neutron star's spin frequency and providing insights into kHz QPOs and their relation to neutron star spin.

Contribution

The study identifies the neutron star's spin frequency using burst oscillations detected with SWIFT/BAT, a novel measurement for this source.

Findings

Neutron star spin frequency is 415 Hz.

Oscillation amplitude (rms) is 14%.

Frequency difference between kHz QPOs does not strongly relate to spin.

Abstract

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 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.