K-band spectroscopy of IGR J16358-4726 and IGR J16393-4643: two new symbiotic X-ray binaries

TL;DR

This study identifies and classifies two new symbiotic X-ray binaries using K-band spectroscopy, expanding the known population and providing insights into their orbital parameters and stellar companions.

Contribution

The paper presents the first spectral classification of two candidate counterparts, confirming them as new members of the symbiotic X-ray binary class.

Findings

Both counterparts show features of late-type stars, including strong CO absorption bands.

The systems are confirmed as new symbiotic X-ray binaries based on spectral analysis.

Increased the number of known SyXBs to eight, highlighting their significance in galactic X-ray binary populations.

Abstract

Symbiotic X-ray Binaries (SyXBs) are a recently discovered subclass of Low Mass X-ray Binaries. Their growing number makes them an important evolutionary channel of X-ray Binaries. Our goal is to perform spectral analysis and classification of the proposed counterparts to IGR J16358-4726 and IGR J16393-4643 and to establish their nature as X-ray systems. We used the ESO/UT1 ISAAC spectrograph to observe the proposed counterparts to the two sources, obtaining K-band medium resolution spectra (R = 500) with a S/N > 140. Data reduction was performed with the standard procedure. We classified them by means of comparison with published atlases. We performed SED fitting in order to refine the spectral classification. The two counterparts clearly exhibit the typical features of late-type stars, notably strong CO absorption bands in the red part of the spectrum. With information from previous X-ray studies, we classify the two systems as two new members of the SyXB class. For IGR J16393-4643, we considered the most probable counterpart to the system, although three other objects cannot be completely discarded. For this system, we compared our findings with available orbital solutions, constraining the orbital parameters and the mass of the companion star. By including two more systems, we increased to eight the number of known SyXBs, which emerges as a non-negligible category of galactic X-ray binaries.