Astron. Astrophys. 333, 411-416 (1998)

ASCA detection of the Fe K edge in the spectrum of the Seyfert 2 galaxy IRAS 04575-7537: a sign of a complex absorber

C. Vignali ¹, A. Comastri ², G.M. Stirpe ², M. Cappi ^{3, 4}, G.G.C. Palumbo ^{1, 3}, M. Matsuoka ⁴, G. Malaguti ³ and L. Bassani <sup>3

1</sup> Dipartimento di Astronomia, Università di Bologna, Via Zamboni 33, I-40126 Bologna, Italy
² Osservatorio Astronomico di Bologna, Via Zamboni 33, I-40126 Bologna, Italy
³ Istituto per le Tecnologie e Studio Radiazioni Extraterrestri ITeSRE/CNR, via Gobetti 101, I-40129 Bologna, Italy
⁴ The Institute of Physical and Chemical Research (RIKEN), 2-1, Hirosawa, Wako, Saitama, 351-01, Japan

Received 21 October 1997 / Accepted 15 December 1997

Abstract

ASCA X-ray spectral analysis of the Seyfert 2 galaxy IRAS 04575-7537 is presented. The main result is the presence of a significant iron edge at a rest energy of keV. The spectrum is flat ( 1.5) with a substantial absorption: 10²² cm^-2 and does not require any reflection component. There is also evidence of a narrow Fe K emission line, whose rest energy (E = keV) and equivalent width (EW 130 eV) suggest that the line originates from thick and cold matter. The intensity of the line and, in particular, the depth of the iron edge are too strong to be explained by simple transmission through the measured absorption column density. This strongly suggests that a model more complex than a single absorbed power law is needed. We propose an absorption model that we call "leaky-total absorber" which can explain the spectrum flatness, the iron emission line and the edge absorption feature and has also the advantage of having a straightforward physical interpretation in the framework of Unified models. In this model a thick absorber ( 10²³ cm^-2), possibly associated with broad line region (BLR) clouds, partially ( 36%) covers the source continuum with fixed to 1.9 as observed in Sey 1 galaxies. The escaping radiation is then absorbed by a column density 10²² cm^-2, which can be attributed to the torus, seen through its rim.

Key words: X-rays: galaxies galaxies: active galaxies: Seyfert galaxies: individual: IRAS 04575-7537 galaxies: emission lines

Send offprint requests to: C. Vignali
Correspondence to: l-vignali@astbo3.bo.astro.it

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Contents

• 1. Introduction
• 2. Optical spectrum and classification
• 3. ASCA data reduction
• 4. ASCA analysis and results
  • 4.1. Timing analysis
  • 4.2. Spectral analysis
• 5. Discussion
  • 5.1. X-ray continuum properties
  • 5.2. Origin of the iron line and edge
  • 5.3. The soft excess
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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