X - ray observations of the starburst galaxy NGC 253 I . Point sources

We report the results of a deep spatial, spectral, and timing analysis of ROSAT HRI and PSPC observations of the edge-on starburst galaxy NGC 253. In this first paper, point-like X-ray sources detected within the galaxy and in the field are discussed. The sources are characterized by their X-ray properties (including comparisons with results from the Einstein and ASCA satellites), by correlations with other wavelength and some optical spectroscopic follow up observations. In total, 73 X-ray sources have been collected in the NGC 253 field, 32 of which are associated with the disk of the galaxy. Though 27 of these disk sources are detected with the HRI (some being resolvable with the PSPC), the remaining 5 PSPC-only detected sources are likely not to be real point sources, being instead due to fluctuations within the X-ray structure of the disk. The source close to the center of the galaxy is extended (Lx ∼ 1 × 1039 erg s−1 in the ROSAT 0.1–2.4 keV band), and is most likely associated with the nuclear starburst activity. The remaining sources have luminosities ranging from 7 × 1036 erg s−1 to 3.0 × 1038 erg s−1, yielding an integrated point source luminosity of 1×1039 erg s−1. The brightest pointlike source is located ∼ 20′′ south of the nucleus, at the border of a plume of diffuse X-ray emission. Its high X-ray luminosity, time variability and hard spectrum make it a good candidate for a black hole X-ray binary. Including four Einstein detections of X-ray transients the number of point-like X-ray sources in NGC 253 increases to 30 sources, 13 of which are time variable. These time variable sources are all brighter than 5 × 1037 erg s−1 and most likely represent X-ray binaries radiating close to or at the Eddington limit. Besides the nuclear source there is only one source above this luminosity that shows no time variability and therefore may represent a young supernova or extremely bright supernova remnant, or an unresolved cluster of several X-ray sources. The point source population of NGC 253 is compared to that of other galaxies, and it is shown that the luminosity distribution matches ROSAT results obtained for M 31 and M 33. The halo of NGC 253 is filled with diffuse, filamentary Xray emission. Seven sources are located (or projected) in this Send offprint requests to: A. Vogler ([email protected]) ? Based partially on observations performed at the European Southern Observatory, La Silla, Chile diffuse emission region. Time variability arguments, together with optical identifications, are put forward to explain 4 sources as background objects, the other 3 sources likely being spurious detections caused by local enhancements in the diffuse emission of the halo of NGC 253. The diffuse X-ray emission components of NGC 253 will be discussed in a separate paper. The sources detected in the field outside the disk of NGC 253 cover a flux range from (9− 300)× 10−15erg cm−2 s−1 in the 0.1–2.4 keV band. None of the sources in the field correlate with published lists of globular cluster candidates. Optical counterparts are proposed for 27 of them, and a few also correlate with radio sources. While two sources are identified as foreground stars, the remaining ones are mostly background active galactic nuclei.