E. Bozzo (Geneva), A. Bahramian (Michigan State), C. Ferrigno (Geneva), A. Sanna (Cagliari), J. Strader (Michigan State), F. Lewis (FT/ARI), D. M. Russell (New York University Abu Dhabi), T. di Salvo (Palermo), L. Burderi, A. Riggio (Cagliari), A. Papitto (INAF-Rome), P. Gandhi (Southamton) and P. Romano (INAF-Brera)

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+3.4−1.2 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 (>> 10^23 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 NuSTAR spectrum suggests a cyclotron origin, thus allowing us to estimate the neutron star magnetic field as ~2.4×10^12 G. All evidence thus 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.

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