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.
@article{arxiv.2208.05807,
title = {MAXI J1957+032: a new accreting millisecond X-ray pulsar in an ultra-compact binary},
author = {A. Sanna and P. Bult and M. NG and P. S. Ray and G. K. Jaisawal and L. Burderi and T. Di Salvo and A. Riggio and D. Altamirano and T. E. Strohmayer and A. Manca and K. C. Gendreau and D. Chakrabarty and W. Iwakiri and R. Iaria},
journal= {arXiv preprint arXiv:2208.05807},
year = {2022}
}
Comments
5 pages, 4 figures. Accepted for publication in MNRAS