Astron. Astrophys. 322, 719-729 (1997)

A ROSAT observation of the warm-absorbed soft X-ray spectrum of NGC 4051

Stefanie Komossa and Henner Fink

Max-Planck-Institut für extraterrestrische Physik, D-85740 Garching, Germany

Received 23 July 1996 / Accepted 18 November 1996

Abstract

We present and analyze a pointed ROSAT PSPC observation of the Seyfert galaxy NGC 4051. The X-ray spectrum consists of a powerlaw modified by absorption edges and an additional soft excess during the high-state in source flux. Modeling the spectrum in terms of warm absorption yields a large column density of the ionized material of = 22.7 and an ionization parameter of = 0.4. These properties are essentially constant throughout the observation, whereas the luminosity changes by more than a factor of 4. The underlying powerlaw is in its steepest observed state, with a photon index = -2.3. Consulting information from optical observations, evidence for a separate EUV bump component in NGC 4051 is provided. The impact of several parameters on the deduced properties of the ionized material is critically assessed. In particular, the influence of dust mixed with the warm gas is explored and shown to constrain the history or density of the absorber. The absorber-intrinsic optical-UV emission (and absorption) line spectrum is predicted and the possibility of a warm absorber origin of one of the observed emission line regions in NGC 4051 is investigated. Consequences for the narrow-line Seyfert 1 character of NGC 4051 are discussed.

Key words: galaxies: active individual: NGC 4051 emission lines Seyfert X-rays: galaxies

Send offprint requests to: S. Komossa (skomossa@mpe-garching.mpg.de)

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Contents

• 1. Introduction
• 2. Data reduction
• 3. Spectral analysis
  • 3.1. Simple spectral models
  • 3.2. Warm absorber models
  • 3.3. Warm absorber plus soft excess ?
• 4. Temporal analysis
  • 4.1. Flux variability
  • 4.2. Spectral variability
• 5. Discussion
  • 5.1. SED and ionization parameter
  • 5.2. Properties of the warm absorber
    • 5.2.1. Column density and abundances
    • 5.2.2. Density
    • 5.2.3. Location
    • 5.2.4. Influence of dust
    • 5.2.5. Warm-absorber intrinsic line emission and covering factor
    • 5.2.6. UV absorption lines
  • 5.3. Time variability of the spectral components and comparison with other observations
    • 5.3.1. Warm absorber
    • 5.3.2. Powerlaw component
  • 5.4. NLSy1 - character of NGC 4051
• 6. Summary and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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