IGR J17329-2731: The birth of a symbiotic X-ray binary

¹ Department of Astronomy, Université de Genèva, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
e-mail: enrico.bozzo@unige.ch
² Department of Physics and Astronomy, Michigan State University, East Lansing, MI, USA
³ Dipartimento di Fisica, Universitá degli Studi di Cagliari, SP Monserrato-Sestu km 0.7, 09042 Monserrato, Italy
⁴ Faulkes Telescope Project, School of Physics and Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK
⁵ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁶ New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
⁷ Dipartimento di Fisica e Chimica, Universitá di Palermo, via Archirafi 36, 90123 Palermo, Italy
⁹ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monteporzio Catone (RM), Italy
¹⁰ Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
¹⁰ INAF - Osservatorio Astronomico di Brera, via Emilio Bianchi 46, 23807 Merate (LC), Italy

Received: 4 January 2018
Accepted: 15 February 2018

Abstract

We report on the results of the multiwavelength campaign carried out after the discovery of the INTEGRAL transient IGR J17329-2731. The optical data collected with the SOAR telescope allowed us to identify the donor star in this system as a late M giant at a distance of 2.7_-1.2^+3.4 kpc. The data collected quasi-simultaneously with XMM–Newton and NuSTAR showed the presence of a modulation with a period of 6680 ± 3 s in the X-ray light curves of the source. This unveils that the compact object hosted in this system is a slowly rotating neutron star. The broadband X-ray spectrum showed the presence of a strong absorption (≫10²³ cm⁻²) and prominent emission lines at 6.4 keV, and 7.1 keV. These features are usually found in wind-fed systems, in which the emission lines result from the fluorescence of the X-rays from the accreting compact object on the surrounding stellar wind. The presence of a strong absorption line around ~21 keV in the spectrum suggests a cyclotron origin, thus allowing us to estimate the neutron star magnetic field as ~2.4 × 10¹² G. All evidencethus suggests IGR J17329-2731 is a symbiotic X-ray binary. As no X-ray emission was ever observed from the location of IGR J17329-2731 by INTEGRAL (or other X-ray facilities) during the past 15 yr in orbit and considering that symbiotic X-ray binaries are known to be variable but persistent X-ray sources, we concluded that INTEGRAL caught the first detectable X-ray emission from IGR J17329-2731 when the source shined as a symbiotic X-ray binary. The Swift XRT monitoring performed up to ~3 months after the discovery of the source, showed that it maintained a relatively stable X-ray flux and spectral properties.