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

TL;DR

This study reclassifies two hard X-ray sources, IGR J17503-2636 as a symbiotic X-ray binary and IGR J17507-2647 as a cataclysmic variable, based on new multi-epoch X-ray observations and spectral analysis.

Contribution

The paper provides new observational data and analysis that lead to a revised classification of two previously uncertain X-ray sources.

Findings

IGR J17503-2636 shows flux variability over 2100 times between outburst and recent observations.

A candidate X-ray pulsation at 0.335 seconds suggests a neutron star in IGR J17503-2636.

Detection of an iron line in IGR J17507-2647 supports its identification as a cataclysmic variable.

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 3sigma upper limit on the source flux to date (~9.5x10^-14 erg/cm2/s, 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 about 3.8sigma) from IGR J17503-2636 with XMM-Newton (pulsed fraction of (10+/-1) per cent). 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.