MAXI J1957+032: a new accreting millisecond X-ray pulsar in an ultra-compact binary
A. Sanna, Peter Bult, Mason Ng, Paul S. Ray, Gaurava K. Jaisawal, L. Burderi, T. Di Salvo, A. Riggio, D. Altamirano, Tod E. Strohmayer, Antonella Manca, Keith C. Gendreau, Deepto Chakrabarty, W. Iwakiri, R. Iaria
Abstract
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 characterized by an orbital period of ∼1 h and a projected semimajor axis of 14 lt-ms. The neutron star binary mass function suggests a minimum donor mass of 1.7 × 10−2 M⊙, assuming a neutron star mass of 1.4 M⊙ and a binary inclination angle lower than 60 deg. This assumption is supported by the lack of eclipses or dips in the X-ray light curve of the source. We characterized 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 Comptonization component with photon index consistent with a typical hard state. We did not find evidence for iron K α lines or reflection components.