Investigating the Optical Counterpart Candidates of Four INTEGRAL Sources localized with Chandra

TL;DR

This study uses optical spectroscopy and likelihood analysis to identify and characterize potential optical counterparts of four INTEGRAL X-ray sources, revealing diverse stellar types and highlighting challenges in confirming symbiotic star candidates.

Contribution

It provides new spectroscopic classifications of candidate counterparts and discusses the limitations of optical spectra alone in confirming symbiotic systems.

Findings

Some candidates are active late-type stars in RS CVn systems.

Likelihood analysis suggests some counterparts are more probable despite weak optical features.

Optical spectra alone may be insufficient to confirm symbiotic star nature.

Abstract

We report on the optical spectroscopic follow up observations of the candidate counterparts to four INTEGRAL sources: IGR J04069+5042, IGR J06552-1146, IGR J21188+4901 and IGR J22014+6034. The candidate counterparts were determined with Chandra, and the optical observations were performed with 1.5-m RTT-150 telescope (T\"{U}B\.{I}TAK National Observatory, Antalya, Turkey) and 2.4-m Hiltner Telescope (MDM Observatory, Kitt Peak, Arizona). Our spectroscopic results show that one of the two candidates of IGR J04069+5042 and the one observed for IGR J06552-1146 could be active late-type stars in RS CVn systems. However, according to the likelihood analysis based on Chandra and INTEGRAL, two optically weaker sources in the INTEGRAL error circle of IGR J06552-1146 have higher probabilities to be the actual counterpart. The candidate counterparts of IGR J21188+4901 are classified as an active M-type star and a late-type star. Among the optical spectra of four candidates of IGR J22014+6034, two show H\alpha emission lines, one is a late-type star and the other is a M type. The likelihood analysis favors a candidate with no distinguishing features in the optical spectrum. Two of the candidates classified as M type dwarfs are similar to some IGR candidates claimed to be symbiotic stars. However, some of the prominent features of symbiotic systems are missing in our spectra, and their NIR colors are not consistent with those expected for giants. We consider the IR colors of all IGR candidates claimed to be symbiotic systems and find that low resolution optical spectrum may not be enough for conclusive identification.