NuSTAR and NICER reveal IGR J17591-2342 as a new accreting millisecond X-ray pulsar

TL;DR

This paper reports the discovery of IGR J17591-2342 as a new accreting millisecond X-ray pulsar using NuSTAR and NICER, detailing its orbital parameters, spectral characteristics, and implications for neutron star binary systems.

Contribution

The study presents the first detection and detailed characterization of IGR J17591-2342 as an accreting millisecond X-ray pulsar with specific orbital and spectral properties.

Findings

Detected coherent pulsations at 527.4 Hz with Doppler modulation.

Estimated a minimum companion mass of 0.42 solar masses.

Spectral analysis shows Comptonisation and disc reflection features.

Abstract

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 ~8.8 hours and a projected semi-major axis of ~1.23 lt-s. From the binary mass function, we estimate a minimum companion mass of 0.42 solar masses, obtained assuming a neutron star mass of 1.4 solar masses 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 ~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.