Discovery of burst oscillations in the newly discovered millisecond X-ray pulsar SRGA J144459.2$-$604207

Abstract

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$-$604207, observed with NICER, XMM-Newton, and NuSTAR during the 2024 outburst. A total of 39 bursts were detected, allowing for a detailed search for burst oscillations, which had not been previously observed from this source. We report the discovery of burst oscillations at 447.7$-$448.0 Hz from SRGA J144459.2$-$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 $5.1\sigma$ and maximum $Z^2$ power of $\sim31$. In the NuSTAR data, the strongest oscillation signal has a significance of $5.2\sigma$ and maximum $Z^2$ power of $\sim32$. 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 $\sim8.5\%$, while the measurements of the NuSTAR data (3-40 keV range) yield $\sim21\%$. 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.