We report on the discovery by the Nuclear Spectroscopic Telescope Array ( NuSTAR ) and the Neutron Star Interior Composition Explorer ( NICER ) of the accreting millisecond X-ray pulsar IGR J17591 - 2342 , detecting coherent X-ray pulsations around 527.4 Hz ( 1.9 ms ) with a clear Doppler modulation .
This implies an orbital period of \sim 8.8 hours and a projected semi-major axis of \sim 1.23 lt-s. From the binary mass function , we estimate a minimum companion mass of 0.42 M _ { \odot } , obtained assuming a neutron star mass of 1.4 M _ { \odot } and an inclination angle lower than 60 degrees , as suggested by the absence of eclipses or dips in the light-curve of the source .
The broad-band energy spectrum is dominated by Comptonisation of soft thermal seed photons with a temperature of \sim 0.7 keV by electrons heated to 21 keV .
We also detect black-body-like thermal direct emission compatible with an emission region of a few kilometers and temperature compatible with the seed source of Comptonisation .
A weak Gaussian line centered on the iron K \alpha complex can be interpreted as a signature of disc reflection .
A similar spectrum characterises the NICER spectra , measured during the outburst fading .