NuSTAR and NICER Reveal IGR J17591-2342 as a New Accreting Millisecond X-ray Pulsar
Journal Article - Open Access
NAVAL RESEARCH LAB WASHINGTON DC WASHINGTON United States
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We report 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. Coherent X-ray pulsations around 527.4 Hz 1.9 ms with a clear Doppler modulation were detected. This implies an orbital period of similar to 8.8 h and a projected semi-major axis of similar to 1.23 lt-s. With the binary mass function, we estimate a minimum companion mass of 0.42 M-circle dot, obtained assuming a neutron star mass of 1.4 M-circle dot 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 broadband energy spectrum, obtained by combining NuSTAR, Swift, and INTEGRAL observations, is dominated by Comptonisation of soft thermal seed photons with a temperature of similar to 0.7 keV by electrons heated to 21 keV. We also detect black-body-like thermal direct emission that is compatible with an emission region of a few kilometers and a temperature compatible with the seed source of Comptonisation. A weak Gaussian line centred on the iron K alpha complex can be interpreted as a signature of disc reflection. A similar spectrum characterises the NICER spectra, which was measured when the outburst faded.