Letter to the Editor

Discovery of 105 Hz coherent pulsations in the ultracompact binary IGR J16597–3704

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 Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA
³ ISDC, Department of Astronomy, University of Geneva, Chemin d’Écogia 16, 1290 Versoix, Switzerland
⁴ University of Alberta, Physics Dept., CCIS 4-181, Edmonton AB T6G 2E1, Canada
⁵ Physics and Astronomy, University of Southampton, Southampton, Hampshire SO17 1BJ, UK
⁶ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands
⁷ Department of Physics, Box 41051, Science Building, Texas Tech University, Lubbock, TX 79409-1051, USA
⁸ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, via Archirafi 36, 90123 Palermo, Italy
⁹ INAF, Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
¹⁰ INAF, Osservatorio Astronomico di Roma, Via di Frascati 33, 00044 Monteporzio Catone, Roma, Italy

Received: 8 November 2017
Accepted: 28 November 2017

Abstract

We report the discovery of X-ray pulsations at 105.2 Hz (9.5 ms) from the transient X-ray binary IGR J16597–3704 using NuSTAR and Swift. The source was discovered by INTEGRAL in the globular cluster NGC 6256 at a distance of 9.1 kpc. The X-ray pulsations show a clear Doppler modulation that implies an orbital period of ~46 min and a projected semi-major axis of ~5 lt-ms, which makes IGR J16597–3704 an ultracompact X-ray binary system. We estimated a minimum companion mass of 6.5 × 10⁻¹⁰ M_⊙, assuming a neutron star mass of 1.4 M_⊙, and an inclination angle of <75° (suggested by the absence of eclipses or dips in its light curve). The broad-band energy spectrum of the source is well described by a disk blackbody component (kT ~ 1.4 keV) plus a comptonised power-law with photon index ~2.3 and an electron temperature of ~30 keV. Radio pulsations from the source were unsuccessfully searched for with the Parkes Observatory.