Is CGCS 5926 a symbiotic X-ray binary?^⋆

¹ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy
e-mail: masetti@iasfbo.inaf.it
² INAF – Osservatorio Astronomico di Padova, Sede di Asiago, via dell’Osservatorio 8, 36012 Asiago, Italy
³ AAVSO, 49 Bay State Road, Cambridge, MA 02138, USA
⁴ Department of Physics & Astronomy, University of Leicester, Leicester, LE1 7RH, UK
⁵ School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA

Received: 15 May 2011
Accepted: 12 September 2011

Abstract

We report on multiwavelength (X-ray to optical) follow-up observations of the 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  ~5 kpc from Earth. BVR_CI_C photometry of the star shows that it displays a variability of  ~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  ≈3  ×  10³² erg s^-1. 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.