Is CGCS 5926 a symbiotic X-ray binary?

TL;DR

This study investigates whether the carbon star CGCS 5926 could be a symbiotic X-ray binary by analyzing multiwavelength data, but current evidence makes this unlikely, though not impossible, pending further observations.

Contribution

The paper provides the first multiwavelength follow-up observations of CGCS 5926, assessing its potential as a symbiotic X-ray binary and discussing the implications of X-ray nondetection.

Findings

CGCS 5926 is confirmed as a giant carbon star at about 5 kpc.

The star shows ~0.3 mag variability with a 151-day period, likely due to pulsations.

X-ray observations do not detect the source, challenging its classification as a symbiotic X-ray binary.

Abstract

We here report on multiwavelength (X-ray to optical) followup observations of carbon star CGCS 5926. These were motivated by the fact that this star is positionally coincident with a faint X-ray emitting object belonging to the ROSAT catalog of sources, thus suggesting a possible symbiotic X-ray binary (SyXB) nature for it. Our spectrophotometric optical data confirm the giant carbon star nature of the object and allow us to classify its spectral type as C(6,2). This classification places CGCS 5926 at a distance of about 5 kpc from Earth. BVRcIc photometry of the star shows that it displays a variability of about 0.3 mag on timescales of months, with the star getting bluer when its brightness increases. Our photometric data indicate a periodicity of 151 days, which we explain as due to radial pulsations of CGCS 5926 on the basis of its global characteristics. The source is not detected at X-rays with Swift/XRT down to a 0.3-10 keV band luminosity of around 3e32 erg/s. This nondetection is apparently in contrast with the ROSAT data; however we show that, even if the probability that CGCS 5926 can be a SyXB appears quite low, the present information does not completely rule out such a possibility, while it makes other interpretations even more unlikely if we assume that the ROSAT detection was real. This issue might thus be settled by future, more sensitive, observations at high energies.