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$10${}^{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.

accepted for publication by A&A

References in corpus (32)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• Measuring the neutron star equation of state using X-ray timing
• Nuclear-Powered Millisecond Pulsars and the Maximum Spin Frequency of Neutron Stars
• Thermonuclear burst oscillations
• Coherent Emission Mechanisms in Astrophysical Plasmas
• Propagation of thermonuclear flames on rapidly rotating neutron stars: extreme weather during type I X-ray bursts
• Pulse profiles of millisecond pulsars and their Fourier amplitudes
• The Oblate Schwarzschild Approximation for Light Curves of Rapidly Rotating Neutron Stars
• The Multi-INstrument Burst ARchive (MINBAR)
• X-ray bursting neutron star atmosphere models using an exact relativistic kinetic equation for Compton scattering
• Thermonuclear X-ray bursts
• Further X-ray observations of EXO 0748-676 in quiescence: evidence for a cooling neutron star crust
• The Dynamic Radio Sky
• Discovery of a 552 Hz burst oscillation in the low-mass X-ray binary EXO 0748-676
• Biases for neutron-star mass, radius and distance measurements from Eddington-limited X-ray bursts
• Eclipse Timings of the Transient Low Mass X-ray Binary EXO0748-676. IV. The Rossi X-Ray Timing Explorer Eclipses
• Constraining the Properties of the Thermonuclear Burst Oscillation Source XTE J1814-338 Through Pulse Profile Modelling
• A uniform search for thermonuclear burst oscillations in the RXTE legacy dataset
• Multiwavelength Observations of EXO 0748-676 - I. Reprocessing of X-ray Bursts
• Multiwavelength Observations of EXO 0748--676 -- II. Emission Line Behavior
• XMM-Newton light curves of the low-mass X-ray binary EXO 0748--676: dips, eclipses, and bursts
• Multi-messenger-Athena Synergy White Paper
• The eclipsing bursting X-ray binary EXO 0748-676 revisited by XMM-Newton
• Accretion heated atmospheres of X-ray bursting neutron stars
• Optical and ultraviolet pulsed emission from an accreting millisecond pulsar
• The X-ray bursts of XTE J1739-285: a NICER sample
• Eclipse Mapping of EXO 0748-676: Evidence for a Massive Neutron Star
• Relativistic ocean $r$-modes during type-I X-ray bursts
• UV and X-ray pulse amplitude variability in the transitional millisecond pulsar PSR J1023+0038
• Nuclear-powered X-ray millisecond pulsars
• Discovery of a 584.65 Hz Burst Oscillation in the Low Mass X-Ray Binary 4U 1730--22
• Simultaneous Optical and X-ray Detection of a Thermonuclear Burst in the 2024 Outburst of EXO 0748-676