SRG/ART-XC discovery of SRGAJ144459.2-604207: a well-tempered bursting accreting millisecond X-ray pulsar

TL;DR

This paper reports the discovery of a new accreting millisecond X-ray pulsar, SRGAJ144459.2-604207, with detailed timing, spectral, and burst analysis revealing its binary parameters, burst behavior, and neutron star properties.

Contribution

The study presents the first detection and detailed characterization of the pulsar SRGAJ144459.2-604207, including its orbital parameters, burst properties, and pulse profile differences during bursts.

Findings

Detected 447.8 Hz pulsations modulated by orbital motion

Observed 19 thermonuclear X-ray bursts with consistent energetics

Estimated neutron star radius of 11-12 km and source distance of 8-9 kpc

Abstract

We report on the discovery of the new accreting millisecond X-ray pulsar SRGAJ144459.2-604207 using the SRG/ART-XC data. The source was observed twice in February 2024 during the declining phase of the outburst. Timing analysis revealed a coherent signal near 447.8~Hz modulated by the Doppler effect due to the orbital motion. The derived parameters for the binary system are consistent with the circular orbit with a period of $\sim5.2$~h. The pulse profiles of the persistent emission, showing a sine-like part during half a period with a plateau in between, can well be modelled by emission from two circular spots partially eclipsed by the accretion disk. Additionally, during our 133~ks exposure observations, we detected 19 thermonuclear X-ray bursts. All bursts have similar shapes and energetics, and do not show any signs of photospheric radius expansion. The burst rate decreases linearly from one per $\sim$1.6~h at the beginning of observations to one per $\sim$2.2~h at the end and anticorrelates with the persistent flux. Spectral evolution during the bursts is consistent with the models of the neutron star atmospheres heated by accretion and imply a neutron star radius of 11--12~km and the distance to the source of 8--9~kpc. We also detected pulsations during the bursts and showed that the pulse profiles differ substantially from those observed in the persistent emission. However, we could not find a simple physical model explaining the pulse profiles detected during the bursts.