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

TL;DR

This paper reports the first detection of burst oscillations in the millisecond X-ray pulsar SRGA J144459.2$-$604207, revealing its spin frequency and providing new insights into neutron star surface phenomena during thermonuclear bursts.

Contribution

It presents the discovery of burst oscillations at the neutron star's spin frequency in SRGA J144459.2$-$604207, including oscillations before burst onset, which is a novel observation.

Findings

Burst oscillations detected at 447.7-448.0 Hz, matching the neutron star's spin.

Oscillations observed with high significance in XMM-Newton and NuSTAR data.

First detection of burst oscillations in this source, including pre-burst oscillations.

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.