Classification of low-luminosity stellar X-ray sources in the field of the Draco dwarf spheroidal galaxy

Abstract

A previous study of the X-ray luminosity function of the X-ray sources in the field of the Draco dwarf spheroidal (dSph) galaxy indicated the presence of a population of unknown X-ray sources in the soft energy range of 0.5-2 keV. In 2015, Draco dSph was observed again in twenty-six deep XMM-Newton, observations providing an opportunity for a new study of the yet unclassified sources. We apply the classification criteria presented in our previous multi-wavelength study of the X-ray sources of the Draco dSph to the sources detected in the combined 2009 and 2015 XMM-Newton data set. These criteria are based on X-ray studies and properties of the optical, near-infrared, and mid-infrared counterparts and allows us to distinguish background active galactic nuclei~(AGNs) and galaxies from other types of X-ray sources. We present the classification of X-ray sources, for which the counterpart is identified as a stellar object based on our criteria from multi-wavelength data. We identify three new symbiotic stars in the Draco dSph with X-ray luminosities between $\sim$3.5$\times 10^{34}$ erg s${}^{-1}$ and 5.5$\times 10^{34}$ erg s${}^{-1}$. The X-ray spectral analysis shows that two of the classified symbiotic stars are $\beta$-type. This is the first identification of this class of symbiotic stars in a nearby galaxy. Eight sources are classified as Galactic M dwarfs in the field of the Draco dSph. The distances of these M dwarfs are between$\sim$140-800 pc, their X-ray luminosities are between $10^{28}-10^{29}$ erg s${}^{-1}$ and the logarithmic ratio of X-ray to bolometric luminosity, log$(\frac{L_{\text{X}}}{L_{\text{bol}}})$, is between $-3.4$ to $-2.1$. The multiple observations allowed us to investigate flare activity of the M dwarfs. Moreover, we classified three foreground sources, located at distances of the order of $\sim$1-3 kpc in the field of the Draco dSph.

Accepted, A&A, 05/2019, 19 pages, 45 figures

References in corpus (9)

• How to Constrain Your M Dwarf: measuring effective temperature, bolometric luminosity, mass, and radius
• Multi-Element Abundance Measurements from Medium-Resolution Spectra. II. Catalog of Stars in Milky Way Dwarf Satellite Galaxies
• IPHAS and the symbiotic stars. I. Selection method and first discoveries
• Carbon in Red Giants in Globular Clusters and Dwarf Spheroidal Galaxies
• A machine learning approach for identification and classification of symbiotic stars using 2MASS and WISE
• Draco, a flawless dwarf galaxy
• The Nature of the Hard-X-Ray Emitting Symbiotic Star RT Cru
• A new interpretation of the remarkable X-ray spectrum of the symbiotic star CH Cyg
• A search for stellar X-ray sourcesin the Sagittarius and Carina dwarf galaxies - I: X-ray observations

Cited by in corpus (3)

• Accretion from Winds of Red Giant Branch Stars May Reveal the Supermassive Black Hole in Leo I
• XMM-Newton study of X-ray sources in the field of Willman 1 dwarf spheroidal galaxy
• XMM-Newton study of the Sculptor dwarf spheroidal galaxy