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Astron. Astrophys. 328, 69-77


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The variable X/$\gamma$ -ray spectrum of the Seyfert 2 galaxy NGC 7172

F. Ryde1, J. Poutanen2, R. Svensson1, S. Larsson1, and S. Ueno3

1Stockholm Observatory, S-133 36 Saltsjöbaden, Sweden
2Uppsala Astronomical Observatory, Box 515, S-751 20, Uppsala, Sweden
3Kyoto University, Sakyo-ku Kyoto, Japan

Received 3 February 1997 / Accepted 8 August 1997

Abstract

A broad band X-, gamma-ray spectral study of the Seyfert 2 galaxy NGC 7172 is presented. We use our ASCA observations from May 1995 and combine these with the CGRO OSSE data from March 1995. The only Seyfert 2 galaxy previously to have been modelled over such a broad spectral range is NGC 4945.

The ASCA GIS data alone, can be acceptably described by a model consisting of a power law, with a photon index $ \Gamma =
1.58 \pm 0.12$, affected by absorption due to intervening neutral matter along the line of sight, with an equivalent hydrogen column density $N_{\rm H}= (8.2^{+0.6} _{-0.5}) \times 10^{22}$cm-2. An Fe K$\alpha$ emission line is not required by the fits. The observed flux in the 2-10 keV range is $F_{2-10} =
(4.75\pm 0.09) \times 10^{-11}$ erg cm-2 s-1, which corresponds to a small increase since the Ginga measurement in October 1989. The power law found is flatter than what is expected in the unified model of Seyfert galaxies.

Combining these ASCA data with the CGRO OSSE data from March 1995, the above conclusions do not change. In this case, we also find that the most probable model for the data is an absorbed power law, but being affected by a high energy exponential cut-off. The power law is still flat with $\Gamma =
1.54 \pm 0.13$, while $N_{\rm H}= (8.1 \pm 0.6 )
\times 10^{22}$ cm-2. The OSSE data do not contradict the flat spectrum found from using the ASCA data alone, which could have been an artifact of the narrow spectral range covered by this single instrument.

Even though it is not possible to completely reject an alternative explanation to the observed flat spectrum in terms of a transmission or a reflection model, the spectral index of the underlying power law of NGC 7172 appears actually to have varied from 1.85 to 1.5 since the Ginga observations in 1989. The e-folding energy is relatively well constrained and lies at 140 +310 -70 keV, using the CGRO OSSE viewing period with the highest quality data. We note, however, that the CGRO OSSE spectral shape appears to be variable on a time scale of weeks.

Key words: galaxies: individual: NGC 7172 - galaxies: Seyfert - X-rays: galaxies

Send offprint requests to: F. Ryde


© European Southern Observatory (ESO) 1997

Online publication: October 30, 1997
Last change: March 24, 1998
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