THE VARIABLE X GAMMA-RAY SPECTRUM OF THE SEYFERT-2 GALAXY NGC-7172/

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 comb ine these with the CGRO OSSE data from March 1995. The only Seyfert 2 galaxy previously to have been modelled over such a broad spectral ran ge is NGC 4945. The ASCA GIS data alone, can be acceptably described b y a model consisting of a power law, with a photon index Gamma = 1.58 +/- 0.12, affected by absorption due to intervening neutral matter alo ng the line of sight, with an equivalent hydrogen column density N-H = (8.2(-0.5)(+0.6)) x 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 F2-1 0 = (4.75 +/- 0.09) x 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 Se yfert galaxies. Combining these ASCA data with the CGRO OSSE data from March 1995, the above conclusions do not change. In this case, we als o 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 powe r law is still flat with Gamma = 1.54 +/- 0.13, while N-H = (8.1 +/- 0 .6) x 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 artifa ct of the narrow spectral range covered by this single instrument. Eve n though it is not possible to completely reject an alternative explan ation to the observed flat spectrum in terms of a transmission or a re flection 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 constrai ned and lies at 140(-70)(+310) keV, using the CGRO OSSE viewing period with the highest quality data. We note, however, that the CGRO OSSE s pectral shape appears to be variable on a time scale of weeks.