Abstract
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, 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, 5 supernova remnants and 7 very soft sources (including 2 good candidate supersoft sources) associated with NGC 55. Detailed spectral and timing analysis was carried out on 4 of the brightest X-ray sources. One of these objects is identified with a Galactic foreground star and is a possible new RS CVn system. The other three are consistent with accreting X-ray binaries, though further evidence of short term variability is required to confirm this. We also find evidence of an underlying component, which is concentrated on the bar region but has an extent of at least 6' (3 kpc) in the plane of the galaxy and +/- 1' (+/- 500 pc) perpendicular to it. This emission is best fitted by a thermal plasma plus power-law 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 ~ 6e38 ergs/s (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.
Highlights
NGC 55 is a member of the nearby Sculptor Group of galaxies, of which the other prominent members are the starburst galaxy NGC 253 and the spirals NGC 45, NGC 247, NGC 300 and NGC 7793
We performed a brief comparison of these sources to the NGC 55 source list from the Chandra observation (Obs ID 2255), produced via the standard CIAO processing tools. From this comparison we found that our XMM–Newton sources 29, 34, 55 and 56 are potentially subject to source confusion, as they could be resolved into two separate sources by the subarcsecond spatial resolution of Chandra
A total of 137 X-ray sources were detected in the NGC 55 field of which 42 were located within the optical confines of the galaxy as defined by the D25 ellipse
Summary
NGC 55 is a member of the nearby Sculptor Group of galaxies, of which the other prominent members are the starburst galaxy NGC 253 and the spirals NGC 45, NGC 247, NGC 300 and NGC 7793. The optical morphology of NGC 55 is rather asymmetric, with the brightest region displaced ∼3 arcmin from the geometrical centre of the galaxy (Robinson & van Damme 1966) This feature has been interpreted as a bar viewed near to end-on (de Vaucouleurs 1961). The first detailed X-ray information came from ROSAT PSPC and HRI observations (Read, Ponman & Strickland 1997; Roberts 1997; Schlegel, Barrett & Singh 1997; Dahlem, Weaver & Heckman 1998), revealing a total of 25 sources in and around the galaxy and evidence of localized diffuse emission.
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