Astron. Astrophys. 342, 101-123 (1999)

X-ray observations of the starburst galaxy NGC 253

I. Point sources in the bulge, disk and halo ^*

A. Vogler ^1,2 and W. Pietsch <sup>1

1</sup> Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, D-85740 Garching, Germany
² CEA/Saclay, DAPNIA, Service d'Astrophysique, L'Ormes des Merisiers, Bât. 709, F-91191 Gif-sur-Yvette, France

Received 8 July 1998 / Accepted 20 October 1998

Abstract

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 ( 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  erg s^-1 to  erg s^-1, yielding an integrated point source luminosity of  erg s^-1. The brightest point-like source is located 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  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 X-ray emission. Seven sources are located (or projected) in this 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 erg 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.

Key words: galaxies: individual: NGC 253 galaxies: spiral galaxies: starbust X-rays: galaxies

* Based partially on observations performed at the European Southern Observatory, La Silla, Chile

Send offprint requests to: A. Vogler (ajv@mpe.mpg.de)

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observations and data analysis
  • 2.1. Attitude corrections
  • 2.2. Image generation
    • 2.2.1. Images of the HRI observations
    • 2.2.2. Image of the PSPC observations
  • 2.3. Constructing a combined HRI/PSPC point source catalog
    • 2.3.1. HRI point source detection
    • 2.3.2. PSPC point source detection
    • 2.3.3. Combined HRI/PSPC point source catalog
  • 2.4. Time variability investigations
• 3. Results
  • 3.1. HRI and PSPC point sources in the NGC 253 field
  • 3.2. Resolving the complex nuclear emission area
    • 3.2.1. Spatial analysis of the HRI data
    • 3.2.2. Spectral analysis of the PSPC data
  • 3.3. Point sources within the NGC 253 disk
  • 3.4. Emission components of the NGC 253 disk
• 4. Discussion
  • 4.1. Comparison of the ROSAT point source catalog with Einstein
  • 4.2. The nature of the NGC 253 point sources
  • 4.3. Non-detection of SN 1940E?
  • 4.4. Possible contributions from foreground or background sources
  • 4.5. Comparison to results previously published on ROSAT and ASCA observations
  • 4.6. Comparison to other wavelengths
  • 4.7. Comparison to X-ray point sources in other spiral galaxies
  • 4.8. Point sources outside the disk of NGC 253 possibly correlated to the galaxy
• 5. Summary
• Acknowledgements
• Appendix A: Sources located outside the area covered by the ellipse of NGC 253
• References

© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998
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