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

Abstract

Aims. A previous study of the X-ray luminosity function of the X-ray sources in the Draco dwarf spheroidal (dSph) galaxy field indicates the presence of a population of unknown X-ray sources in the soft energy range of 0.5–2 keV. In 2015, there were twenty-six further deep XMM-Newton observations of Draco dSph, providing an opportunity for a new study of the as yet unclassified sources. Methods. We applied 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. In this work we performed X-ray spectral and timing analyses for fifteen sources in the field of Draco dSph with stellar counterparts. Results. 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 ∼3.5 × 1034 erg s−1 and 5.5 × 1034 erg s−1. The X-ray spectral analysis shows that two of the classified symbiotic stars are β-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. These M dwarfs are between ∼140 and 800 pc distant, with X-ray luminosities are between 1028 and 1029 erg s−1 and logarithmic ratios of X-ray to bolometric luminosity, log(LX/Lbol), between −3.4 and −2.1. The multiple observations allowed us to investigate flare activity of the M dwarfs. For 5 M dwarfs flare(s) are observed with a significance of > 3σ level of confidence. Moreover, we classified three foreground sources, located at distances of the order of ∼1–3 kpc in the field of the Draco dSph. Based on both the X-ray luminosities of these foreground sources (> 1030 erg s−1) and their optical counterparts (late type G or K stars), these X-ray sources are classified as candidates of contact binary systems. Conclusions. Our study of X-ray sources of the Draco dSph shows that accreting white dwarfs are the most promising X-ray population of dSphs, which is in line with theoretical expectations. The number of Galactic M dwarfs detected at our X-ray sensitivity limit is consistent with the expectation based on the space density of M dwarfs.