THE X/GAMMA-RAY SPECTRAL PROPERTIES OF NGC 7172

 by 

 Felix Ryde, Juri Poutanen, Roland Svensson, Stefan Larsson and Shiro Ueno 


   We present a combined, non-simultaneous, ASCA GIS and CGRO OSSE spectrum
   of the Seyfert 2 galaxy,  NGC 7172, and make broad band  spectral  fits.
   The only Seyfert 2 galaxy  previously  studied over such a broad band is
   NGC 4945.  We find that the most probable  model for the data is a power
   law with an exponential  cut-off being  affected by a neutral  absorber.
   The best fit  parameters  are found to be  $\Gamma = 1.47 \pm  0.15$ and
   $\NH = (7.8 \pm 0.6) \cdot  10^{22}$  cm$^{-2}$.  The spectral  index of
   the underlying power law of NGC 7172 has therefore  varied from $1.8$ to
   $1.5$ since the {\it Ginga} observations in 1989.  For this simple model
   the  e-folding  energy at $88  ^{+65}  _{-28}$  keV is  relatively  well
   constrained.  The observed  flux in the $2-10$ keV range is  $F_{2-10} =
   4.7 \cdot 10^{-11}$ erg cm$^{-2}$ s$^{-1}$, which corresponds to a small
   increase since the {\it{Ginga}} measurement in October 1989.