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

A. Sanna*, P. Bult, M. NG, P. S. Ray, G. K. Jaisawal, L. Burderi, T. Di Salvo, A. Riggio, D. Altamirano, T. E. Strohmayer, A. Manca, K. C. Gendreau, D. Chakrabarty, W. Iwakiri, R. Iaria

*Corresponding author for this work

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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.
Original languageEnglish
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Pages (from-to)L76–L80
Publication statusPublished - 2022


  • Binaries:general–stars:neutron
  • X-rays:binaries
  • Accretion: accretion disks


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