The X-ray properties of the dwarf Magellanic-type galaxy NGC 55

We present an analysis of the X-ray properties of the Magellanic-type galaxy NGC 55 based on two contiguous XMM–Newton observations. We detect a total of 137 X-ray sources in the field of view, down to a flux of ∼5 × 10−15-erg-cm−2-s−1 (0.3–6 keV), 42 of which are located within the optical confines of the galaxy. On the basis of X-ray colour classification and after correcting for background objects, we conclude that our source sample includes ∼20 X-ray binaries (XRBs), five supernova remnants and seven very soft sources (including two good candidate supersoft sources) associated with NGC 55. We also detect an X-ray source coincident with a previously identified globular cluster in NGC 55. Detailed spectral and timing analysis was carried out on four of the brightest X-ray sources (excluding the brightest source, which was the subject of a previous paper). One of these objects is identified with a Galactic foreground star and is a possible new RS CVn system. The other three are persistent X-ray sources with X-ray spectra well described by either a single absorbed power law (Γ∼ 2) or a multicolour disc blackbody (kTin∼ 1 keV) model. While the observed luminosities of these sources (LX∼ 1–2 × 1038-erg-s−1) and their X-ray spectra are consistent with accreting XRBs, further evidence of short-term variability is required to confirm this. Although the observed X-ray emission from NGC 55 is dominated by point sources, we do find evidence of an underlying component, which is concentrated on the bar region but has an extent of at least 6 arcmin (3 kpc) in the plane of the galaxy and ±1 arcmin (±500 pc) perpendicular to it. This emission is best fitted by a thermal plasma (mekal) (kT∼ 0.2 keV) plus power-law (Γ∼ 2) model but with high intrinsic absorption consistent with its location in the central disc of the galaxy. We interpret the soft component as diffuse thermal emission linked to regions of current star formation, whilst the hard power-law component may originate in unresolved X-ray binary sources. The intrinsic luminosity of this residual disc emission may exceed LX∼ 6 × 1038-erg-s−1 (0.3–6 keV). A comparison with other Magellanic systems confirms that, in terms of both its discrete X-ray source population and its extended emission, NGC 55 has X-ray properties which are typical of its class.