Astron. Astrophys. 352, 64-84 (1999)

The soft X-ray emission components of NGC 4258 decomposed by deep ROSAT observations

A. Vogler ^1,2 and W. Pietsch <sup>2

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

Received 18 March 1999 / Accepted 29 September 1999

Abstract

Spatial, spectral, and timing analysis of deep ROSAT HRI and PSPC observations of the active spiral galaxy NGC 4258 are reported.

The diffuse emission of NGC 4258 has a 0.1-2.4 keV luminosity of  erg s^-1 (corrected for Galactic foreground absorption) and fills of the diameter. One half of the HRI count rate of this region is caused by a ridge of high surface brightness emission following the anomalous spiral arms of the galaxy. PSPC spectra of the diffuse emission suggest the superposition of two thermal emission components. A 0.2 keV component can be attributed to gas in the outer disk and eastern halo hemisphere, a second component of higher absorption and temperature (0.5 keV) most probably traces hot gas along the anomalous spiral arms.

The point source catalogue of NGC 4258 contains 15 source candidates within the optical extent of the galaxy. At the position of the nucleus, the HRI results allow for a point-like source on top of diffuse emission, and, if indeed present, the luminosity of such a source is estimated to  erg s^-1. Excluding the nuclear region and two sources which were flagged as extended, the sources have luminosities between  erg s^-1 and  erg s^-1, accounting for an integrated of  erg s^-1.

A deficiency of the 0.1-0.4 keV band emission in the outer disk of NGC 4258 is detected and can be explained by a shadowing of the diffuse soft band background, which originates from sources behind the disk of NGC 4258 and is absorbed by cold gas in the outer disk. A lower limit of  cts s^-1 arcmin^{- 2} for the diffuse 0.1-0.4 keV background is estimated.

The new results are compared to previous ROSAT and ASCA observations of NGC 4258 as well as to results from other wavelengths and to other spiral galaxies.

Key words: galaxies: individual: NGC 4258 galaxies: Seyfert galaxies: spiral X-rays: galaxies

Send offprint requests to: A. Vogler (ajv@discovery.saclay.cea.fr)

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Contents

• 1. Introduction
• 2. Observations and data analysis
  • 2.1. Observations
  • 2.2. Attitude corrections
  • 2.3. Image generation
    • 2.3.1. Images of the HRI observations
    • 2.3.2. PSPC broad band image
  • 2.4. Constructing a combined HRI/PSPC point source catalogue
    • 2.4.1. HRI point source detection
    • 2.4.2. PSPC point source detection
    • 2.4.3. Combined HRI/PSPC point source catalogue
  • 2.5. Time variability investigations
• 3. Results
  • 3.1. HRI and PSPC point sources in the NGC 4258 field
  • 3.2. Resolving the central 10 kpc region of NGC 4258
    • 3.2.1. The HRI: spatial resolution
    • 3.2.2. The PSPC: spectral capabilities
  • 3.3. Point sources in the outer disk of NGC 4258
  • 3.4. The emission components of NGC 4258
• 4. Discussion
  • 4.1. Comparison to results previously published on ROSAT and ASCA observations
  • 4.2. Superluminous sources in NGC 4258
  • 4.3. Comparison of the NGC 4258 point source population to other galaxies
  • 4.4. Non-detection of the type II SN 1981 K
  • 4.5. Comparison of the X-ray point source list to other wavelengths
  • 4.6. The anomalous spiral arms and a possible halo component
  • 4.7. A combination of the spatial resolution of the HRI with the spectral resolution of the PSPC for the diffuse emission features
  • 4.8. Comparison of the diffuse emission components of NGC 4258 to those of other spiral galaxies
  • 4.9. Shadowing of the soft X-ray background by the outer disk of NGC 4258
• 5. Summary
• Acknowledgements
• Appendix A: X-ray spectra of two QSOs aligned symmetrically to the active nucleus of NGC 4258
• References

© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999
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