MAXI J1957+032: a new accreting millisecond X-ray pulsar in an ultra-compact binary

TL;DR

MAXI J1957+032 is a newly identified ultra-compact binary system with a fast-spinning neutron star, exhibiting coherent X-ray pulsations, a very short orbital period, and a spectrum consistent with a hard state during outburst.

Contribution

This paper reports the discovery of MAXI J1957+032 as an accreting millisecond X-ray pulsar in an ultra-compact binary, providing detailed temporal and spectral analysis.

Findings

Detected coherent 314 Hz pulsations indicating a fast-rotating neutron star.

Determined the binary's ultra-compact nature with a ~1-hour orbital period.

Characterized the spectrum as thermal plus Comptonisation without iron lines.

Abstract

The detection of coherent X-ray pulsations at ~314 Hz (3.2 ms) classifies MAXI J1957+032 as a fast-rotating, accreting neutron star. We present the temporal and spectral analysis performed using NICER observations collected during the latest outburst of the source. Doppler modulation of the X-ray pulsation revealed the ultra-compact nature of the binary system characterised by an orbital period of ~1 hour and a projected semi-major axis of 14 lt-ms. The neutron star binary mass function suggests a minimum donor mass of 1.7e-2 Msun, assuming a neutron star mass of 1.4 Msun and a binary inclination angle lower than 60 degrees. This assumption is supported by the lack of eclipses or dips in the X-ray light curve of the source. We characterised the 0.5-10 keV energy spectrum of the source in outburst as the superposition of a relatively cold black-body-like thermal emission compatible with the emission from the neutron star surface and a Comptonisation component with photon index consistent with a typical hard state. We did not find evidence for iron K-alpha lines or reflection components.