New interpretation of the two hard X-ray sources IGR J17503-2636 and IGR J17507-2647

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via A. Corti 12, I-20133 Milano, Italy
² INAF, Osservatorio di Astrofisica e Scienza dello Spazio, Via P. Gobetti 101, I-40129 Bologna, Italy
³ Scuola Universitaria Superiore IUSS Pavia, Piazza della Vittoria 15, I-27100 Pavia, Italy
⁴ INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, (LC), Italy
⁵ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, D-85748 Garching, Germany
⁶ School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK

^⋆ Corresponding author; lara.sidoli@inaf.it

Received: 18 July 2024
Accepted: 21 November 2024

Abstract

We report on the results of X-ray observations (XMM–Newton, INTEGRAL and Swift) of two hard X-ray sources, IGR J17503-2636 and IGR J17507-2647, whose nature is not fully elucidated in the literature. Three XMM–Newton observations covered the field of IGR J17503-2636, in 2020 and twice in 2023. The analysis of the two XMM–Newton observations performed in September 2023, six days apart, did not detect IGR J17503-2636, allowing us to pose the most stringent 3σ upper limit on the source flux to date (∼9.5 × 10⁻¹⁴ erg cm⁻² s⁻¹, 2–10 keV, flux corrected for absorption). This value implies that the amplitude of the X-ray flux variability exceeds a factor of ∼2100, compared with the discovery outburst in 2018. A candidate X-ray periodicity at 0.335397(3) seconds has been barely detected (significance of ∼3.8σ) from IGR J17503-2636 with XMM–Newton (pulsed fraction of (10 ± 1)%). The new data, put into the context of previous literature, allow us to propose a new classification of IGR J17503-2636 as a symbiotic X-ray binary, rather than a candidate supergiant fast X-ray transient. IGR J17507-2647 was formerly reported below 10 keV only during Chandra observations performed in 2009. We report here on two XMM–Newton observations that serendipitously covered the source field in 2020 and in 2023, finding a stable X-ray emission, both in X-ray flux and spectral shape. The long-term, persistent X-ray emission has also been probed by several Swift/XRT short observations and by INTEGRAL data spanning several years. We have detected an iron line in the emission (with centroid energy in the range of 6.3–6.6 keV), never reported before in the IGR J17507-2647 spectrum. The source properties favor the identification with a cataclysmic variable.