Letter to the Editor

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

¹ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu km 0.7, 09042 Monserrato, Italy
e-mail: andrea.sanna@dsf.unica.it
² ISDC, Department of Astronomy, University of Geneva, Chemin d’Écogia 16, 1290 Versoix, Switzerland
³ Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352, USA
⁴ Institut für Astronomie und Astrophysik, Eberhard Karls Universität, Sand 1, 72076 Tübingen, Germany
⁵ National Space Institute, Technical University of Denmark, Elektrovej 327-328, 2800 Lyngby, Denmark
⁶ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan
⁷ The Hakubi Center for Advanced Research, Kyoto University, Yoshida-Ushinomiya-cho, Sakyo-ku, Kyoto 606-8302, Japan
⁸ Physics & Astronomy, University of Southampton, Southampton, Hampshire SO17 1BJ, UK
⁹ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, via Archirafi 36, 90123 Palermo, Italy
¹⁰ Astrophysics Science Division, NASA’s Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹¹ INAF, Osservatorio Astronomico di Roma, Via di Frascati 33, 00044 Monteporzio Catone (Roma), Italy
¹² Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA
¹³ INAF/IASF Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy
¹⁴ IRAP, Université de Toulouse, CNRS, UPS, CNES, Toulouse, France
¹⁵ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹⁶ CNRS, IRAP, 9 avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
¹⁷ Université de Toulouse, CNES, UPS-OMP, 31028 Toulouse, France

Received: 30 August 2018
Accepted: 10 September 2018

Abstract

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 ∼8.8 h and a projected semi-major axis of ∼1.23 lt-s. With the binary mass function, we estimate a minimum companion mass of 0.42 M_⊙, obtained assuming a neutron star mass of 1.4 M_⊙ and an inclination angle lower than 60°, as suggested by the absence of eclipses or dips in the light curve of the source. The broad-band energy spectrum, obtained by combining NuSTAR, swift and INTEGRAL observations, 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 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 Kα complex can be interpreted as a signature of disc reflection. A similar spectrum characterises the NICER spectra, which was measured when the outburst faded.