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

¹ ASI – Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma, Italy
² INAF, Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, RM, Italy
³ INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
⁴ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
⁵ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels, (Barcelona), Spain
⁶ Universitá degli Studi di Palermo, Dipartimento di Fisica e Chimica, Via Archirafi 36, 90123 Palermo, Italy
⁷ INFN, Sezione di Roma 2, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁸ Department of Electronics Information and Bioengineering, Politecnico di Milano, 20133 Milano, Italy
⁹ INAF, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹⁰ University of Oxford, Department of Physics, Astrophysics, Denys Wilkinson Building, Keble Road, OX1 3RH Oxford, United Kingdom

^⋆ Corresponding author: arianna.miraval@asi.it

Received: 23 April 2025
Accepted: 12 September 2025

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, and thus provide 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 (∼552 Hz), potentially the first such case for an LMXB. The candidate signal is observed during an ∼8 s interval in the rising phase of an FUV burst, which occurred ∼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 ∼16% pulsed fraction, corresponding to a pulsed luminosity of ∼8 × 10³³ erg s⁻¹. Interpreting the properties of these 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.