An ultraviolet burst oscillation candidate from the low-mass X-ray binary EXO 0748-676

Abstract: X-ray burst oscillations are quasi-coherent periodic signals at frequencies close to the neutron star spin frequency. They are observed during thermonuclear Type I X-ray bursts from a number of low-mass X-ray binaries (LMXBs) hosting a fast-spinning, weakly magnetic neutron star. Besides measuring the spin frequencies, burst oscillations hold the potential to accurately measure neutron star mass and radius, thus providing constraints on the equation of state of matter at nuclear densities. Based on far-ultraviolet (FUV) observations of the X-ray binary EXO 0748-676 taken with the Hubble Space Telescope in 2003, we report a possible indication of ultraviolet burst oscillations at the neutron star spin frequency ($\sim$552 Hz), potentially the first such case for an LMXB. The candidate signal is observed during an $\sim$8 s interval in the rising phase of an FUV burst, which occurred $\sim$4 s after a Type I X-ray burst. Through simulations, we estimated that the probability of detecting the observed signal power from pure random noise is 3.7$\%$, decreasing to 0.3$\%$ if only the burst rise interval is considered, during which X-ray burst oscillations had already been observed in this source. The background-subtracted folded pulse profile of the candidate FUV oscillations in the (120-160 nm) band is nearly sinusoidal with a $\sim$16$\%$ pulsed fraction, corresponding to a pulsed luminosity of $\sim$8$\times$10$<sup>{33}$ erg/s. Interpreting the properties of this candidate FUV burst oscillations in the light of current models for optical-ultraviolet emission from neutron star LMXBs faces severe problems. If signals of this kind are confirmed in future observations, they might point to an unknown coherent emission process as the origin of the FUV burst oscillations observed in EXO 0748-676.