The Discovery of the 528.6 Hz Accreting Millisecond X-Ray Pulsar MAXI J1816-195

Peter Bult;Diego Altamirano;Zaven Arzoumanian;Deepto Chakrabarty;J??r??me Chenevez;Elizabeth C. Ferrara;Keith C. Gendreau;Sebastien Guillot;Tolga G??ver;Wataru Iwakiri;Gaurava K. Jaisawal;Giulio C. Mancuso;Christian Malacaria;Mason Ng;Andrea Sanna;Tod E. Strohmayer;Zorawar Wadiasingh;Michael T. Wolff

2022-01-01

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Abstract

We present the discovery of 528.6 Hz pulsations in the new X-ray transient MAXI J1816-195. Using NICER, we observed the first recorded transient outburst from the neutron star low-mass X-ray binary MAXI J1816-195 over a period of 28 days. From a timing analysis of the 528.6 Hz pulsations, we find that the binary system is well described as a circular orbit with an orbital period of 4.8 hr and a projected semimajor axis of 0.26 lt-s for the pulsar, which constrains the mass of the donor star to 0.10-0.55 M (circle dot). Additionally, we observed 15 thermonuclear X-ray bursts showing a gradual evolution in morphology over time, and a recurrence time as short as 1.4 hr. We did not detect evidence for photospheric radius expansion, placing an upper limit on the source distance of 8.6 kpc.