2026ApJ..1000...52M

Burst oscillations during thermonuclear X-ray bursts are powered by thermonuclear energy on the neutron star (NS) surface and typically occur close to the spin frequency of the NS. We performed a comprehensive timing analysis of all thermonuclear bursts from the newly discovered millisecond X-ray pulsar SRGA J144459.2-604207, observed with Neutron Star Interior Composition Explorer, XMM-Newton, and Nuclear Spectroscopic Telescope Array (NuSTAR) during the 2024 outburst. A total of 39 bursts were detected, allowing for a detailed search for burst oscillations, which had not been previously observed from this source. We report the discovery of burst oscillations at 447.7-448.0 Hz from SRGA J144459.2-604207 using XMM-Newton and NuSTAR data, consistent with the spin frequency of the NS. The strongest burst oscillation in the XMM-Newton data occurred with a single-trial significance of 5.1σ and maximum Z² power of ∼31. In the NuSTAR data, the strongest oscillation signal has a significance of 5.2σ and maximum Z² power of ∼32. The folded pulse profile corresponding to the strongest signal in the 0.5-10 keV band of the XMM-Newton data shows a sinusoidal shape with a fractional rms amplitude of ∼8.5%, while the measurements of the NuSTAR data (3-40 keV range) yield ∼21%. These results represent the first detection of burst oscillations in SRGA J144459.2-604207. Additionally, we report the detection of 447.6 Hz oscillations occurring just before a burst onset observed with XMM-Newton. This marks only the second instance in which burst oscillations have been observed before the burst onset.